5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo...

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1 Redes de Computadores LEIC-A, MEIC-A 5 – Data Link Layer Prof. Paulo Lobato Correia IST, DEEC –Área Científica de Telecomunicações RC –Prof. Paulo Lobato Correia 2 Objectives Understand principles behind data link layer services: Link layer addressing Error detection and correction Sharing a broadcast channel: multiple access Reliable data transfer, flow control (done) Instantiation and implementation of various link layer technologies

Transcript of 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo...

Page 1: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

1

Redes de Computadores

LEIC-A MEIC-A

5 ndash Data Link Layer

Prof Paulo Lobato CorreiaIST DEEC ndash Aacuterea Cientiacutefica de Telecomunicaccedilotildees

RC ndash Prof Paulo Lobato Correia 2

Objectives

Understand principles behind data link layer services

Link layer addressing

Error detection and correction

Sharing a broadcast channel multiple access

Reliable data transfer flow control (done)

Instantiation and implementation of

various link layer technologies

2

RC ndash Prof Paulo Lobato Correia 3

Outline

Introduction and services

Link-layer addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 4

Link Layer Introduction

Some terminology

Hosts and routers are nodes

Communication channels that

connect adjacent nodes along

communication path are links Wired links

Wireless links

LANs

Layer-2 packet is a frameIt encapsulates a datagram

Data link layer has the responsibility of transferring datagrams

from one node to the adjacent node over a link

3

RC ndash Prof Paulo Lobato Correia 5

Link Layer Context

Datagram transferred by different link protocols over different links Eg Ethernet on first link frame relay on intermediate links 80211 on

last link

Each link protocol provides different services Eg may or may not provide reliable data transfer over link

Transportation analogy Trip from Oeiras to Rennes

bus Oeiras to Lisboa

plane Lisboa to Paris

train Paris to Rennes

Tourist = datagram

Transport segment = communication link

Transportation mode = link layer protocol

Travel agent = routing algorithm

RC ndash Prof Paulo Lobato Correia 6

Link Layer Services

Framing link access

Encapsulate datagram into frame adding header trailer

Channel access if shared medium

ldquoMACrdquo addresses used in frame headers to identify source destination

bull Different from IP address

Reliable delivery between adjacent nodes

Similar solutions to those adopted in transport layer

Seldom used on low bit-error links (fiber some twisted pair)

Wireless links high error rates

Q Why both link-level and end-end reliability

4

RC ndash Prof Paulo Lobato Correia 7

Link Layer Services

Flow control

Pacing between adjacent sending and receiving nodes

Error detection

Errors caused by signal attenuation and noise

Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

Error correction

Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex

With half duplex nodes at both ends of link can transmit but not at same time

RC ndash Prof Paulo Lobato Correia 8

Link Layer

The link layer deals with data transfer between a host and a router or between two routers belonging to the same network

Data link layer tasks

5

RC ndash Prof Paulo Lobato Correia 9

Hop-to-hop Delivery

RC ndash Prof Paulo Lobato Correia 10

Where is the Link Layer Implemented

In each and every host

Link layer implemented in the ldquoadaptorrdquo (aka network interface

card - NIC)

Ethernet card PCMCI card 80211 card

Implements link physical layer

Attaches into hostrsquos system buses

Combination of hardware software firmware

controller

physicaltransmission

cpu memory

host

bus

(eg PCI)

network adapter

card

host schematic

applicationtransportnetwork

link

linkphysical

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 2: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

2

RC ndash Prof Paulo Lobato Correia 3

Outline

Introduction and services

Link-layer addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 4

Link Layer Introduction

Some terminology

Hosts and routers are nodes

Communication channels that

connect adjacent nodes along

communication path are links Wired links

Wireless links

LANs

Layer-2 packet is a frameIt encapsulates a datagram

Data link layer has the responsibility of transferring datagrams

from one node to the adjacent node over a link

3

RC ndash Prof Paulo Lobato Correia 5

Link Layer Context

Datagram transferred by different link protocols over different links Eg Ethernet on first link frame relay on intermediate links 80211 on

last link

Each link protocol provides different services Eg may or may not provide reliable data transfer over link

Transportation analogy Trip from Oeiras to Rennes

bus Oeiras to Lisboa

plane Lisboa to Paris

train Paris to Rennes

Tourist = datagram

Transport segment = communication link

Transportation mode = link layer protocol

Travel agent = routing algorithm

RC ndash Prof Paulo Lobato Correia 6

Link Layer Services

Framing link access

Encapsulate datagram into frame adding header trailer

Channel access if shared medium

ldquoMACrdquo addresses used in frame headers to identify source destination

bull Different from IP address

Reliable delivery between adjacent nodes

Similar solutions to those adopted in transport layer

Seldom used on low bit-error links (fiber some twisted pair)

Wireless links high error rates

Q Why both link-level and end-end reliability

4

RC ndash Prof Paulo Lobato Correia 7

Link Layer Services

Flow control

Pacing between adjacent sending and receiving nodes

Error detection

Errors caused by signal attenuation and noise

Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

Error correction

Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex

With half duplex nodes at both ends of link can transmit but not at same time

RC ndash Prof Paulo Lobato Correia 8

Link Layer

The link layer deals with data transfer between a host and a router or between two routers belonging to the same network

Data link layer tasks

5

RC ndash Prof Paulo Lobato Correia 9

Hop-to-hop Delivery

RC ndash Prof Paulo Lobato Correia 10

Where is the Link Layer Implemented

In each and every host

Link layer implemented in the ldquoadaptorrdquo (aka network interface

card - NIC)

Ethernet card PCMCI card 80211 card

Implements link physical layer

Attaches into hostrsquos system buses

Combination of hardware software firmware

controller

physicaltransmission

cpu memory

host

bus

(eg PCI)

network adapter

card

host schematic

applicationtransportnetwork

link

linkphysical

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 3: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

3

RC ndash Prof Paulo Lobato Correia 5

Link Layer Context

Datagram transferred by different link protocols over different links Eg Ethernet on first link frame relay on intermediate links 80211 on

last link

Each link protocol provides different services Eg may or may not provide reliable data transfer over link

Transportation analogy Trip from Oeiras to Rennes

bus Oeiras to Lisboa

plane Lisboa to Paris

train Paris to Rennes

Tourist = datagram

Transport segment = communication link

Transportation mode = link layer protocol

Travel agent = routing algorithm

RC ndash Prof Paulo Lobato Correia 6

Link Layer Services

Framing link access

Encapsulate datagram into frame adding header trailer

Channel access if shared medium

ldquoMACrdquo addresses used in frame headers to identify source destination

bull Different from IP address

Reliable delivery between adjacent nodes

Similar solutions to those adopted in transport layer

Seldom used on low bit-error links (fiber some twisted pair)

Wireless links high error rates

Q Why both link-level and end-end reliability

4

RC ndash Prof Paulo Lobato Correia 7

Link Layer Services

Flow control

Pacing between adjacent sending and receiving nodes

Error detection

Errors caused by signal attenuation and noise

Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

Error correction

Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex

With half duplex nodes at both ends of link can transmit but not at same time

RC ndash Prof Paulo Lobato Correia 8

Link Layer

The link layer deals with data transfer between a host and a router or between two routers belonging to the same network

Data link layer tasks

5

RC ndash Prof Paulo Lobato Correia 9

Hop-to-hop Delivery

RC ndash Prof Paulo Lobato Correia 10

Where is the Link Layer Implemented

In each and every host

Link layer implemented in the ldquoadaptorrdquo (aka network interface

card - NIC)

Ethernet card PCMCI card 80211 card

Implements link physical layer

Attaches into hostrsquos system buses

Combination of hardware software firmware

controller

physicaltransmission

cpu memory

host

bus

(eg PCI)

network adapter

card

host schematic

applicationtransportnetwork

link

linkphysical

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 4: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

4

RC ndash Prof Paulo Lobato Correia 7

Link Layer Services

Flow control

Pacing between adjacent sending and receiving nodes

Error detection

Errors caused by signal attenuation and noise

Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

Error correction

Receiver identifies and corrects bit error(s) without resorting to retransmission

Half-duplex and full-duplex

With half duplex nodes at both ends of link can transmit but not at same time

RC ndash Prof Paulo Lobato Correia 8

Link Layer

The link layer deals with data transfer between a host and a router or between two routers belonging to the same network

Data link layer tasks

5

RC ndash Prof Paulo Lobato Correia 9

Hop-to-hop Delivery

RC ndash Prof Paulo Lobato Correia 10

Where is the Link Layer Implemented

In each and every host

Link layer implemented in the ldquoadaptorrdquo (aka network interface

card - NIC)

Ethernet card PCMCI card 80211 card

Implements link physical layer

Attaches into hostrsquos system buses

Combination of hardware software firmware

controller

physicaltransmission

cpu memory

host

bus

(eg PCI)

network adapter

card

host schematic

applicationtransportnetwork

link

linkphysical

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 5: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

5

RC ndash Prof Paulo Lobato Correia 9

Hop-to-hop Delivery

RC ndash Prof Paulo Lobato Correia 10

Where is the Link Layer Implemented

In each and every host

Link layer implemented in the ldquoadaptorrdquo (aka network interface

card - NIC)

Ethernet card PCMCI card 80211 card

Implements link physical layer

Attaches into hostrsquos system buses

Combination of hardware software firmware

controller

physicaltransmission

cpu memory

host

bus

(eg PCI)

network adapter

card

host schematic

applicationtransportnetwork

link

linkphysical

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 6: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

6

RC ndash Prof Paulo Lobato Correia 11

Adaptors Communicating

Sending side

Encapsulates datagram in frame

Adds error checking bits flow control etc

Receiving side

Looks for errors flow control etc

Extracts datagram and passes it to the upper layer at the receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

RC ndash Prof Paulo Lobato Correia 12

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

IEEE 80211 Wireless LANs

Link-layer switches

Framing

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 7: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

7

RC ndash Prof Paulo Lobato Correia 13

Link-Layer Addressing

IP address ndash Network Layer

Used to get datagram to the destination IP subnet

32-bit IP address

Hierarchical

Not portable

MAC (or LAN or physical) address ndash Data Link Layer

Function get frame from one interface to another physically-

connected interface (in the same network)

48 bit MAC address (for most LANs)

bull Burned in NIC ROM Sometimes adjustable by software

Portable

RC ndash Prof Paulo Lobato Correia 17

MAC Addresses

Each adapter on a LAN has a unique MAC address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 8: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

8

RC ndash Prof Paulo Lobato Correia 19

To deliver frames the physical (MAC) address of the host needs to be known

ARP allows to get the MAC address from the IP address

Address Resolution Protocol (ARP)

RC ndash Prof Paulo Lobato Correia 25

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 9: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

9

RC ndash Prof Paulo Lobato Correia 26

Origin Cause PreventionLine break Storms accidents

White noise Electron motion Raise signal level

Impulsive noise Lightening voltage changes car ignition

Isolate or move wires

Cross-talk Guard bands too smallWires too close

Increase guard band or isolate wires

Eco Bad quality connections Fix or adjust equipment

Lossattenuation Signal intensity decreases with distance

Use repeaters or regenerators

Intermodulation noise

Combination of signals with different origins

Isolate or move wires

Jitter Phase changes in the signals Adjust the equipments

Harmonic distortion

Non-linear amplification in frequency Adjust the equipments

Why are there Transmission Errors

RC ndash Prof Paulo Lobato Correia 27

Error Examples

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 10: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

10

RC ndash Prof Paulo Lobato Correia 28

Error Types

Error types

Isolated

Burst

RC ndash Prof Paulo Lobato Correia 29

Error Detection

Error detection

Uses redundancy ndash bits are added to allow error detection in the receptor

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 11: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

11

RC ndash Prof Paulo Lobato Correia 30

Error Detection

Methods

Vertical redundancy check (VRC) ndash one parity bit is added to each data unit to ensure that the total number of 1s is even (or odd)

Longitudinal redundancy check (LRC) or Two Dimensional Bit Parity ndash a block of bits is divided into lines and a redundant line is added to the complete block

Cyclic redundancy check (CRC) ndash a set of additional bits computed from the division by a generator polynomial known by the sender and the receiver

RC ndash Prof Paulo Lobato Correia 31

Error Detection VRC

Does not detect an even number of bits in error

A substantial overhead is introduced example 18 = 125

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 12: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

12

RC ndash Prof Paulo Lobato Correia 32

Error Detection LRC ndash Two Dimensional Bit Parity

RC ndash Prof Paulo Lobato Correia 33

Detect and correct single bit errors

0 0

Error Detection LRC ndash Two Dimensional Bit Parity

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 13: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

13

RC ndash Prof Paulo Lobato Correia 34

Error Detection

EDC ndash Error Detection and Correction bits (redundancy)

D ndash Data protected by error checking may include header fields

Error detection is not 100 reliable

Protocol may miss some errors but rarely

Larger EDC field yields better detection and correction

RC ndash Prof Paulo Lobato Correia 36

Cyclic Redundancy Check (CRC)

Widely used in practice (Ethernet 80211 WiFi ATM)

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 14: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

14

RC ndash Prof Paulo Lobato Correia 37

Error Detection CRC

Uses a generator polynomial G(x) of degree n

A message with m bits is viewed as a polynomial M(x) of degree lower than m

CRC computation

Dividend is xnM(x)

Divisor is G(x)

Modulo 2 division (exclusive or) produces a remainder R(x) of degree lower than n

The transmitted message is T(x) = xnM(x) + R(x)

Note that T(x) is divisible by G(x)

Error detection

If the received message is divisible by G(x) then no error are detected

RC ndash Prof Paulo Lobato Correia 46

CRC Example

M(x) = x7 + x4 + x3 + x1

(mensagem)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

x3 M(x) = x10 + x7 + x6 + x4

(dividendo)

R(x) = x2 + 1(resto)

T(x) = x3 M(x) + R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem transmitida ndash incluindo CRC)

1101 (n = 3)

1001 1010 (m=8)

100 1101 0000

101

10011010101

10011010000 11011101 11111001

1001110110001101

10111101

1100110100100000

01000000

10001101101

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 15: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

15

RC ndash Prof Paulo Lobato Correia 47

CRC Example (2)

x3M(x)+R(x) = x10 + x7 + x6 + x4 + x2 + 1(mensagem recebida ndash incluindo CRC)

G(x) = x3 + x2 + 1(polinoacutemio gerador)

Rrsquo(x) = 0(natildeo haacute erros ou os erros natildeo satildeo detectados)

1101 (n = 3)

1001 1010 101

000

10011010101 11011101 11111001

1001110110001101

10111101

1100110100110000

01100000

11011101000

RC ndash Prof Paulo Lobato Correia 48

CRC Properties

Let T(x) + E(x) be the received message with E(x) being the error

pattern

The error pattern is detected if and only if E(x) is not divisible by G(x)

Single errors are detected is G(x) has more than one term

Odd number of errors are detected if G(x) has x + 1 as a factor

Burst errors of length smaller or equal to n are detected if G(x) has

degree n and includes the term 1 (ie x0)

Burst errors of length n + 1 are detected with probability 1 2 n ndash 1 if G(x)

has degree n and includes the term 1

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 16: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

16

RC ndash Prof Paulo Lobato Correia 49

CRC Hardware Implementation

CRC is easily implemented in hardware by using shift registers

G(x) = x5 + x4 + x2 + 1

XOR

Input data

x5 x4 x3 x2 x 1Message M=1010001101

Input when calculating CRC

1010001101 00000

CRC =01110 (to be calculated)

(Input when calculating CRC

1010001101 01110)

RC ndash Prof Paulo Lobato Correia 50

Forward Error Correction

Forward error correction (FEC) uses codes with enough redundancy to allow

the detection and correction of errors on the receptor without requiring the

message retransmission

Examples

Hamming code detects and corrects isolated bit errors

There are more sophisticated techniques often used such as Reed-Solomon codes

FEC is often used in environments with long propagation delays (example satellite

transmissions)

There are hardware implementations of FEC techniques (example V34 modem)

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 17: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

17

RC ndash Prof Paulo Lobato Correia 51

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 52

Multiple Access Links and Protocols

Two types of ldquolinksrdquo

Point-to-point

PPP for dial-up access

Point-to-point link between Ethernet switch and host

Broadcast (shared wire or medium)

Old-fashioned Ethernet

Upstream HFC

80211 wireless LAN

Shared wire (eg cabled Ethernet)

Shared RF(eg 80211 WiFi)

Shared RF(satellite)

Humans at acocktail party

(shared air acoustical)

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 18: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

18

RC ndash Prof Paulo Lobato Correia 53

Multiple Access Protocols

Single shared broadcast channel

Two or more simultaneous transmissions by nodes ndash interference

Collision if node receives two or more signals at the same time

Multiple access protocol

Distributed algorithm that determines how nodes share channel ie

determine when node can transmit

Communication about channel sharing must use the channel itself

No out-of-band channel for coordination

RC ndash Prof Paulo Lobato Correia 54

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Fully decentralized

No special node to coordinate transmissions

No synchronization of clocks slots

4 Simple

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 19: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

19

RC ndash Prof Paulo Lobato Correia 55

MAC Protocols a Taxonomy

Three broad classes

Fixed Channel Partitioning

Divide channel into smaller ldquopiecesrdquo (time slots frequencies codes)

Allocate piece to node for exclusive use

Random Access

Channel not divided allow collisions

ldquoRecoverrdquo from collisions

Dynamic Allocation (ldquoTaking turnsrdquo)

Nodes take turns (but nodes with more to send can take longer turns)

PollSelect Token passing

RC ndash Prof Paulo Lobato Correia 56

Fixed Channel Partitioning FDMA

FDMA ndash Frequency Division Multiple Access

Channel spectrum divided into frequency bands

Each station assigned a fixed frequency band

Unused transmission time in frequency bands go idle

frequ

enc

y b

ands

FDM cable

Example of 6-station LAN

134 have packets

Frequency bands 256 are idle

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 20: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

20

RC ndash Prof Paulo Lobato Correia 57

FDMA

FDMA ndash Frequency Division Multiple Access

RC ndash Prof Paulo Lobato Correia 58

Fixed Channel Partitioning TDMA

TDMA ndash Time Division Multiple Access

Access to channel in roundsldquo

Each station gets fixed length slot (length = packet transmission time) in each round

Unused slots go idle

1 3 4 1 3 4

6-slotframe

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 21: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

21

RC ndash Prof Paulo Lobato Correia 59

TDMA

TDMA ndash Time Division Multiple Access

RC ndash Prof Paulo Lobato Correia 60

Fixed Channel Partitioning CDMA

CDMA ndash Code Division Multiple Access

Each user has access to the complete frequency band during all the time

Users are distinguished by using different codes

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 22: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

22

RC ndash Prof Paulo Lobato Correia 61

CDMA

RC ndash Prof Paulo Lobato Correia 62

Random Access Protocols

When node has packet to send

Transmit at full channel data rate R

No a priori coordination among nodes

Two or more transmitting nodes rarr ldquocollisionrdquo

Random access MAC protocol specifies

How to detect collisions

How to recover from collisions (eg via delayed retransmissions)

Examples of random access MAC protocols

ALOHA

Slotted ALOHA

CSMA CSMACD CSMACA

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 23: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

23

RC ndash Prof Paulo Lobato Correia 63

ALOHA

Created at the University of Hawaii in 1970

Aloha rarr simple no synchronization

Hosts transmit at channel rate

When frame is ready transmit immediately

Collision if two or more frames overlap (frames are lost)

If frame is lost schedule retransmission for a future instant

(randomly chosen)

t

retransmissionretransmission

collision duration

RC ndash Prof Paulo Lobato Correia 66

ALOHA Efficiency

Collisions

Frame sent at t0 collides with any other frames sent in [t0-LR t0+LR]

P (success by given node) = P (node transmits) P (no other transmission in [t0-LR t0+LR] )

t0 - LR t0 t0 + LR

( ) GekPsuccessP

sdotminus=== 20Vulnerability period

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 24: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

24

RC ndash Prof Paulo Lobato Correia 68

Slotted ALOHA

Assumptions

All frames of same size L

Time is divided into equal size slots (time to transmit 1 frame ndash LR)

Nodes start to transmit only at slot beginning

Nodes are synchronized

If 2 or more nodes transmit in a slot all nodes detect the collision

Operation

When node obtains fresh frame transmits in next slot

If no collision node can send new frame in next slot

If collision node retransmits frame in each subsequent slot with probability p until success

RC ndash Prof Paulo Lobato Correia 69

Slotted ALOHA

Pros

Single active node can continuously transmit at full channel rate

Highly decentralized only slot durations need to be in sync in every node

Simple

Cons

Collisions wasting slots

Idle slots

Nodes may be able to detect collision in less than time needed to transmit packet

Clock synchronization

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 25: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

25

RC ndash Prof Paulo Lobato Correia 71

Aloha Efficiency

10-3

10-2

10-1

100

101

102

0

005

01

015

02

025

03

035

04

Aloha

Sloted-AlohaE

ffic

iency

S

Offered load G

184

37

RC ndash Prof Paulo Lobato Correia 72

Carrier Sense Multiple Access (CSMA)

CSMA

Listen before transmit

If channel sensed idle rarr transmit entire frame

If channel sensed busy rarr defer transmission

Human analogy donrsquot interrupt others

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 26: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

26

RC ndash Prof Paulo Lobato Correia 73

CSMA Collisions

Collisions can still occur Propagation delay means two

nodes may not hear each otherrsquos

transmission

Collision

Entire packet transmission time

wasted

spatial layout of nodes

Note

Role of distance amp propagation delay

in determining collision probability

RC ndash Prof Paulo Lobato Correia 74

CSMACD (Collision Detection)

CSMACD

Carrier sensing deferral as in CSMA

Collisions detected within short time

Colliding transmissions aborted reducing channel wastage

Collision detection

Easy in wired LANs measure signal strengths compare transmitted received signals

Difficult in wireless LANs received signal strength overwhelmed by local transmission strength

Human analogy the polite conversationalist

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 27: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

27

RC ndash Prof Paulo Lobato Correia 75

CSMACD Collision Detection

RC ndash Prof Paulo Lobato Correia 76

Carrier Sense Multiple Access (CSMA)

Listen to the channel before transmission

If channel is busy postpone frame transmission

If channel is free immediately start frame transmission

Persistent CSMA

When channel is busy the station transmits as soon as it becomes idle

Non-persistent CSMA

When channel is busy the station schedules the frame transmission for a future moment randomly chosen

CSMACD

Stations involved in a collision stop their transmission as soon as the collision is detected

CSMACA

There may be ldquohiddenldquo stationshellip

obstacle

Collision

A

B

C

D

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 28: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

28

RC ndash Prof Paulo Lobato Correia 77

Dynamic Allocation MAC Protocols

Fixed channel partitioning MAC protocols

Share channel efficiently and fairly at high load

Inefficient at low load

bull Delay in channel access 1N bandwidth allocated

even if only 1 active node

Random access MAC protocols

Efficient at low load single node can fully utilize channel

High load collision overhead

Dynamic allocation (ldquotaking turnsrdquo) protocols

Look for best of both worlds

RC ndash Prof Paulo Lobato Correia 79

Dynamic Allocation Poll Select

A central (primary) computer controls the activity of the others

ACK Acknowledge

NAK Not Acknowledge

NAKrarr no information to send

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 29: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

29

RC ndash Prof Paulo Lobato Correia 81

Dynamic Allocation Token Passing

RC ndash Prof Paulo Lobato Correia 82

Summary of MAC Protocols

Channel partitioning by time frequency or code

Frequency Division Time Division Code Division

Random access (dynamic)

ALOHA S-ALOHA CSMA CSMACD

Carrier sensing easy in some technologies (wire) hard in others

(wireless)

CSMACD used in Ethernet

CSMACA used in 80211

Dynamic allocation (ldquotaking turnsrdquo)

Polling from central site token passing

Bluetooth FDDI IBM Token Ring

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 30: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

30

RC ndash Prof Paulo Lobato Correia 83

MAC Protocols

Comparing dynamic allocation with random access protocols

Dynamic allocation protocols allow a better channel usage at high loads

Random access protocols impose lower delays at low loads

Dynamic allocation protocols require central management or token management

Random access protocols need to be controlled against unstable behavior

RC ndash Prof Paulo Lobato Correia 84

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 31: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

31

RC ndash Prof Paulo Lobato Correia 85

IEEE 8023 ndash Ethernet

ldquoDominantrdquo wired LAN technology

Cheap lt $10 for NIC

First widely used LAN technology

Simpler cheaper than token LANs and ATM

Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

RC ndash Prof Paulo Lobato Correia 86

Star Topology

Bus topology popular through mid 90s All nodes in same collision domain (can collide with each other)

Today star topology prevails Active switch in center

Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes do not collide with each other)

bus coaxial cable

switch

star

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 32: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

32

RC ndash Prof Paulo Lobato Correia 87

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver and sender clock rates

RC ndash Prof Paulo Lobato Correia 88

Addresses 6 bytes

If adapter receives frame with matching destination address (or

broadcast address ndash eg ARP packet) it passes data in frame to

the network layer

Otherwise adapter discards frame

Type indicates higher layer protocol

Mostly IP but others possible eg Novell IPX AppleTalk

Ethernet Frame Structure

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 33: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

33

RC ndash Prof Paulo Lobato Correia 89

Data

MTU 46 to 1500 bytes

CRC

x32+ x26+ x23+ x22+ x16+ x12+ x11+ x10+ x8+ x7+ x5+ x4+ x2+ x+1

Checked at receiver ndash if error is detected the frame is dropped

Ethernet Frame Structure

RC ndash Prof Paulo Lobato Correia 90

Ethernet Unreliable Connectionless

Connectionless

No handshaking between sending and receiving NICs

Unreliable

Receiving NIC doesnrsquot send ACKs or NACKs to sending NIC

Stream of datagrams passed to network layer can have gaps

(missing datagrams)

Gaps will be filled in if application is using TCP

Otherwise application will see the gaps

Ethernetrsquos MAC protocol

Unslotted CSMACD

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 34: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

34

RC ndash Prof Paulo Lobato Correia 91

Ethernet CSMACD Algorithm

1 NIC receives datagram from network layer rarr creates frame

2 If channel idle (96 bit times) starts frame transmission

If channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission

rarr success

4 If NIC detects another transmission while transmitting

rarr aborts and sends jam signal (reinforce collision)

5 After aborting NIC enters exponential backoff

rarr After mth collision NIC chooses K at random from 012hellip2m-1

rarr NIC waits K512 bit times then returns to Step 2

RC ndash Prof Paulo Lobato Correia 92

Ethernetrsquos CSMACD

Jam Signal

Make sure all other transmitters are aware of collision rarr 48 bits

Bit time

01 micros for 10 Mbps Ethernet

Exponential Backoff

Goal adapt retransmission attempts to estimated load

Heavy load random wait will be longer

1st collision choose K from 01 delay is K 512 bit transmission times

After 2nd collision choose K from 0123hellip

After 10 collisions choose K from 01234hellip1023

(K=1023 rarr wait time is about 50 msec with 10 Mbps Ethernet)

httpmediapearsoncmgcomawaw_kurose_network_2appletscsmacdcsmacdhtml

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 35: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

35

RC ndash Prof Paulo Lobato Correia 94

8023 Ethernet Standards Link amp Physical Layers

Many different Ethernet standards

Common MAC protocol and frame format

Different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10 Gbps

Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisted pair) physical layer

RC ndash Prof Paulo Lobato Correia 95

Manchester encoding

Used in 10BaseT

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other

No need for a centralized global clock among nodes

This is physical-layer material

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 36: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

36

RC ndash Prof Paulo Lobato Correia 96

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 97

Hubs

hellip physical-layer (ldquodumbrdquo) repeaters

Bits coming in one link go out in all other links at same rate

All nodes connected to hub can collide with one another

No frame buffering

No CSMACD at hub host NICs detect collisions

twisted pair

hub

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 37: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

37

RC ndash Prof Paulo Lobato Correia 98

Switch

Link-layer deviceSmarter than hubs takes active role

Stores forwards Ethernet frames

Examines incoming framersquos MAC address- Selectively forwards frame to one or more outgoing links when frame is to be forwarded on segment- Uses CSMACD to access segment

Transparent

Hosts are unaware of presence of switches

Plug-and-play self-learning

Switches do not need to be configured

RC ndash Prof Paulo Lobato Correia 99

SwitchAllows Multiple Simultaneous Transmissions

Hosts have dedicated direct

connection to switch

Switches buffer packets

Ethernet protocol used on each

incoming link but no collisions

full duplex

Each link is its own collision domain

Switching A-to-Arsquo and B-to-Brsquo

simultaneously without collisions

Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 38: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

38

RC ndash Prof Paulo Lobato Correia 100

Switch Table

Q How does switch know that Arsquo is reachablevia interface 4 and Brsquo is reachable via interface 5

A Each switch has a switch table Entries contain(MAC address of host interface to reach host time stamp)

Looks like a routing table

Q How are entries created maintained in switch table

Something like a routing protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 23

45

6

RC ndash Prof Paulo Lobato Correia 101

Switch Self-Learning

Switch learns which hosts can be reached through which interfaces

When frame is received switch ldquolearnsrdquo location of sender rarrincoming LAN segment

Records senderlocation pair in the switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 39: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

39

RC ndash Prof Paulo Lobato Correia 102

Switch Frame FilteringForwarding

When frame received

1 Record link associated with sending host

2 Index switch table using MAC destination address

3 if entry found for destinationthen

if destination on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

else flood

forward on all but the interface on which the frame arrived

RC ndash Prof Paulo Lobato Correia 103

Self-learning Forwarding Example

A

Arsquo

B

Brsquo

C

Crsquo

1 23

45

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo

Frame destination unknown

flood

Arsquo A

Destination is a known location

Arsquo 4 60

selective send

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 40: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

40

RC ndash Prof Paulo Lobato Correia 104

Interconnecting Switches

Switches can be connected together

A

B

Q Sending from A to G How does S1 know to forward frame destined to G via S4 and S3

A Self learning (Works exactly the same as in single-switch case)

S1

C D

E

FS2

S4

S3

H

I

G

RC ndash Prof Paulo Lobato Correia 105

Self-learning Multi-switch Example

Suppose C sends frame to I then I responds to C

Q Show switch tables and packet forwarding in S1 S2 S3 S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G

1

2

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 41: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

41

RC ndash Prof Paulo Lobato Correia 106

Self-learning Example

Bridge 1

Estaccedilatildeo 1

Bridge 2

Estaccedilatildeo 2

Bridge 3

LAN A

LAN B LAN D

LAN E

LAN CP

P P P PP P P

Bridge 1 aprende o endereccedilo da estaccedilatildeo 1a partir do endereccedilo origem de P

P

P

flooding de P

R R R R RRR

Pflooding de P

R R

Bridge 2 aprende oendereccedilo da estaccedilatildeo 1

destino = 2 origem = 1pacote P destino = 1 origem = 2pacote R

1 12 2

Bridge 1 aprende o endereccediloda estaccedilatildeo 2

Bridge 2 aprende o endereccedilo da estaccedilatildeo 2a partir do endereccedilo origem de R

RC ndash Prof Paulo Lobato Correia 107

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 42: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

42

RC ndash Prof Paulo Lobato Correia 109

Redundant Topology

Networks introduce redundant links between switches or bridges to overcome the failure of links

These connections introduce physical loops into the network

Bridging loops are created so if one link fails another can take over the function of traffic forwarding

RC ndash Prof Paulo Lobato Correia 110

Redundant Topology

Switches flood traffic out all ports when the traffic is sent to a destination not yet known

Broadcast and multicast traffic is forwarded out on every port (except the port on which the traffic arrived)

This traffic can be caught in a loop

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 43: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

43

RC ndash Prof Paulo Lobato Correia 111

Redundant Topology

In Layer 2 header there is no Time To Live (TTL)

A frame sent into a Layer 2 looped topology of switches can loop foreverhellip (At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0)

This creates a dilemma

A physical topology that contains switching or bridging loops is necessary for reliability yet a switched network cannot have loops

RC ndash Prof Paulo Lobato Correia 112

Redundant Topology and Spanning Tree

Solution

Allow physical loops but create a loop free logical topology

Traffic destined to Cat-5 from any host attached to Cat-4 will travel through Cat-1 and Cat-2

This happens even if there is a direct physical connection between Cat-5 and Cat-4

The loop free logical topology created is called a tree and it has a star or extended star logical topology the spanning tree of the network

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 44: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

44

RC ndash Prof Paulo Lobato Correia 113

Spanning-Tree Protocol

Ethernet bridges and switches can implement the IEEE 8021D Spanning-Tree Protocol and use the spanning-tree algorithm to construct a loop free shortest path network

RC ndash Prof Paulo Lobato Correia 114

Spanning-Tree Protocol

Shortest path is based on cumulative link costs

Link costs are based on the speed of the link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 45: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

45

RC ndash Prof Paulo Lobato Correia 115

Spanning-Tree Protocol

The Spanning-Tree Protocol establishes a root node called the

root bridge

The Spanning-Tree Protocol constructs a topology that has one path for reaching every network node

The resulting tree originates from the root bridge

Redundant links that are not part of the shortest path tree are blocked

RC ndash Prof Paulo Lobato Correia 117

Spanning-Tree Protocol

The message that a switch sends allowing the formation of a loop free logical topology is called a Bridge Protocol Data Unit (BPDU)

BPDUs continue to be received on blocked ports

This ensures that if an active path or device fails a new spanning tree can be calculated

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 46: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

46

RC ndash Prof Paulo Lobato Correia 118

Spanning-tree Operation

When the network has stabilized it has converged and there is one spanning tree per network

As a result for every switched network the following elements exist

One root bridge per network

Unused blocked ports

One root port per non root bridge

One designated port per network segment

RC ndash Prof Paulo Lobato Correia 119

Selecting the Root Bridge

When a switch is turned on the spanning-tree algorithm is used to identify the root bridge

BPDUs are sent out with the Bridge ID (BID)

The BID consists of a bridge priority that defaults to 32768 and the switch base MAC address

By default BPDUs are sent every two seconds

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 47: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

47

RC ndash Prof Paulo Lobato Correia 120

Selecting the Root Bridge

When a switch first starts up it assumes it is the root switch and sends ldquoinferiorrdquo BPDUs

These BPDUs contain the switch MAC address in both the root and sender BID

RC ndash Prof Paulo Lobato Correia 121

Selecting the Root Bridge

All switches see the BIDs sent

As a switch receives a BPDU with a lower root BID it replaces that in the BPDUs that are sent out

All bridges see these and decide that the bridge with the smallest BID value will be the root bridge

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 48: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

48

RC ndash Prof Paulo Lobato Correia 122

Selecting the Root Bridge

A network administrator may want to influence the decision by setting the switch priority to a smaller value than the default which will make the BID smaller

This should only be implemented when the traffic flow on the network is well understood

RC ndash Prof Paulo Lobato Correia 130

Conceitos baacutesicos spanning tree (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Porta raiz

Porta raizPorta raiz

Bridge raiz

Path costRoot path cost

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

Bridge

Designada

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 49: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

49

RC ndash Prof Paulo Lobato Correia 131

Conceitos baacutesicos spanning tree (II)

Bridge ID ndash cada bridge eacute identificada por um endereccedilo que conteacutem

2 octetos de prioridade configuraacutevel pelo gestor da rede +

6 octetos fixos (um dos endereccedilos MAC das portas da bridge ou qualquer outro endereccedilo uacutenico de 48 bits)

A prioridade tem precedecircncia sobre o campo de octetos fixos

Bridge raiz (Root Bridge) ndash bridge que estaacute na raiz da spanning tree bridge com menor Bridge ID

Path cost ndash custo associado a cada porta da bridge (pode ser configurado pelo gestor da rede)

RC ndash Prof Paulo Lobato Correia 132

Conceitos baacutesicos spanning tree (III) Bridge designada (Designated Bridge) ndash bridge que numa LAN eacute

responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa a bridge raiz eacute a bridge designada em todas as LANs a que estaacute ligada

Porta designada (Designated Port) ndash porta que numa LAN eacute responsaacutevel pelo envio de pacotes da LAN para a bridge raiz e vice-versa (uma das portas da bridge designada)

Bridge

Ethernet

Bridge

Bridge

pacotes deparaa bridge raiz

bridge designada

porta designada

Bridge

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 50: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

50

RC ndash Prof Paulo Lobato Correia 133

Conceitos baacutesicos spanning tree (IV)

Porta raiz (Root Port) ndash porta que numa bridge eacute responsaacutevel pela recepccedilatildeotransmissatildeo de pacotes depara a bridge raiz

Ethernet

pacotes deparaa bridge raiz

porta raiz

Bridge

Ethernet

Ethernet Ethernet

RC ndash Prof Paulo Lobato Correia 134

Conceitos baacutesicos spanning tree (V)

Cada bridge tem associado um custo do percurso para a raiz(Root Path Cost) igual agrave soma dos custos das portas que transmitem pacotes em direcccedilatildeo agrave bridge raiz (portas raiz) no percurso de menor custo para a bridge raiz

Bridge

Ethernet

Bridge

pacotes deparaa bridge raiz

portaraiz

Bridge

Bridge raiz

Ethernet

EthernetEthernet

100

20

30

20

20

portadesignada porta

raiz

Ethernet Ethernet

portaraiz

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 51: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

51

RC ndash Prof Paulo Lobato Correia 135

Conceitos baacutesicos spanning tree (VI)

A porta raiz eacute em cada bridge a porta que fornece o melhor percurso (de menor custo) para a raiz

A porta designada eacute em cada LAN a porta que fornece o melhor percurso para a raiz

As portas activas em cada bridge satildeo

a porta raiz + as portas designadas

As restantes portas ficam inactivas (blocked)

RC ndash Prof Paulo Lobato Correia 136

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (I)

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 52: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

52

RC ndash Prof Paulo Lobato Correia 137

16Eth 6

47Eth 5

93Eth 4

47Eth 3

12Eth 2

12Eth1

Bridges designadas

Exemplo ndash spanning tree (II)

RC ndash Prof Paulo Lobato Correia 138

138

BPDUs (Bridge Protocol Data Units)

Para construir e manter a spanning tree as bridges trocam mensagens especiais entre si designadas por Bridge Protocol Data Units (BPDUs)

Existem dois tipos Configuration e Topology Change Notification

bull Destination ndash endereccedilo multicast atribuiacutedo a todas as bridgesbull Source ndash endereccedilo MAC da porta que enviou a BPDUbull DSAP = SSAP = 01000010 = 42 hex

destination source DSAP SSAP BPDU

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 53: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

53

RC ndash Prof Paulo Lobato Correia 139

Configuration BPDUs

A configuraccedilatildeo da spanning tree eacute feita pelas Conf - BPDUs (mensagens de configuraccedilatildeo)

bull Campos mais importantes

Root ID estimativa actual do endereccedilo da bridge raiz

Root Path Cost estimativa actual do custo para a bridge raiz

Bridge ID endereccedilo da bridge que envia a mensagem de configuraccedilatildeo

Port ID endereccedilo da porta que envia a mensagem de configuraccedilatildeo

RC ndash Prof Paulo Lobato Correia 140

Manutenccedilatildeo da spanning tree

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

1501515015

151098 151098 152023 153018

Convenccedilatildeo para Conf-BPDUs

Root Bridge Root Path Cost Bridge ID

Peridiocidade das Conf-BPDUs = hello time

hello time recomendado 2 segundos

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 54: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

54

RC ndash Prof Paulo Lobato Correia 141

Ordenaccedilatildeo das mensagens de configuraccedilatildeo

Uma mensagem de configuraccedilatildeo C1 diz-se melhor que outra C2 se

o Root ID de C1 for inferior ao de C2

sendo os Root ID idecircnticos o Root Path Cost de C1 for inferior ao de C2

sendo idecircnticos o Root ID e o Root Path Cost o Bridge ID de C1 for inferior aode C2

sendo idecircnticos o Root ID o Root Path Cost e o Bridge ID o Port ID de C1 forinferior ao de C2

Root ID Root Path Cost Bridge ID Port ID

18 27 32 2

18 27 32 4

18 27 43 1

18 35 23 3

23 31 45 2

RC ndash Prof Paulo Lobato Correia 142

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (I)

Cada bridge assume inicialmente que eacute a bridge raiz (faz Root Path Cost = 0) envia mensagens de configuraccedilatildeo em todas as suas portas

92092

92092

92092

92092 92092

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 55: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

55

RC ndash Prof Paulo Lobato Correia 143

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (II)

4112111

411390

81081

4119125 4112315

41

4

12 + 1

melhores mensagens recebidas na Bridge 92 ateacute um dado instante

estimativas da Bridge 92

Bridge raiz =

Porta raiz =

Custo para a raiz =

RC ndash Prof Paulo Lobato Correia 144

Bridge 92

5

4

32

1

Bridge

Bridge

Bridge Bridge

Bridge

BridgeBridge

Construccedilatildeo da spanning tree (III)

4112111

41139081081

411392

4112315

mensagens enviadas pela Bridge 92 - 411392

411392

4119125

porta inactiva

porta inactiva

porta raiz

porta activa

porta activa

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 56: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

56

RC ndash Prof Paulo Lobato Correia 145

Construccedilatildeo da spanning tree (IV)

Bridge ID = 18

Bridge ID = 83

Bridge ID = 21Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 2Cost = 20

Port ID = 1Cost = 10

Port ID = 1Cost = 10

Port ID = 3Cost = 30

Port ID = 2Cost = 20

Eth 1

Eth 2

Eth 3

A bridge 83 envia 83083 em Eth1 Eth2 e Eth3

A bridge 21 envia 21021 em Eth2 e Eth3

A bridge 83 envia 212083 em Eth1 (porta raiz = 2)

A bridge 18 envia 18018 em Eth1 e Eth3

A bridge 21 envia 182021 em Eth2 (porta raiz = 2)

A bridge 83 envia 181083 em Eth2 (porta raiz = 1)

O algoritmo convergiu

A bridge raiz envia periodicamente 18018

A bridge 83 retransmite com 181083 em Eth2

bridge raiz

RC ndash Prof Paulo Lobato Correia 146

Avarias nas bridges ou nas LANs (I)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

Bridge raiz

15015message age = 0

15015message age = 0

151098message age = 0

151098message age = 0

152023message age = 0

153018message age = 0

protocol identifierversion

message type = 0reserved

root IDroot path cost

bridge IDport ID

message agemax age

hello timeforward delay

TCTCA

Conf-BPDU

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 57: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

57

RC ndash Prof Paulo Lobato Correia 147

Avarias nas bridges ou nas LANs (II)

Bridge ID = 15

Bridge ID = 98 Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

40

10 10

70

10

1020

2030

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

151098 age = 0 151098 age = 0

151098 age = 5 151098 age = 5

151098 age = max age 151098 age = max age

hellip hellip bull max age recomendado 20 segundos

Bridge 98 avaria

RC ndash Prof Paulo Lobato Correia 148

Avarias nas bridges ou nas LANs (III)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

77077message age = 0

77077message age = 0

Bridge 77 presume ser raiz

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 58: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

58

RC ndash Prof Paulo Lobato Correia 149

Avarias nas bridges ou nas LANs (IV)

Bridge ID = 15

Bridge ID = 23

Bridge ID = 44

Bridge ID = 18Bridge ID = 77

50 10

10 20

10

10 10

10

2020

30

15015message age = 0

15015message age = 0

152023message age = 0

153018message age = 0

153077message age = 0

152023message age = 0

Bridge 23 passa a designada nesta LAN

porta raiz

RC ndash Prof Paulo Lobato Correia 150

Existecircncia de ciclos temporaacuterios

Apoacutes alteraccedilatildeo da topologia da rede

pode existir perda temporaacuteria de conectividade se uma porta que estava inactiva na topologia antiga ainda natildeo se apercebeu que deveraacute estar activo na nova topologia

podem existir ciclos temporaacuterios se uma porta que estava activa na topologia antiga ainda natildeo se apercebeu que deveraacute estar inactiva na nova topologia

Para minimizar a probabilidade de se formarem ciclos temporaacuterios as bridges satildeo obrigadas a esperar algum tempo antes de permitirem que uma das suas portas passe do estado inactivo para o estado activo o tempo de espera eacute funccedilatildeo do paracircmetro forward delay

Bridge

Bridge

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 59: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

59

RC ndash Prof Paulo Lobato Correia 151

Tempo de vida das entradas das tabelas de encaminhamento

Tempo de vida demasiado longo ndash pode haver um nuacutemero exagerado de pacotes perdidos quando a estaccedilatildeo muda de localizaccedilatildeo

Tempo de vida demasiado curto ndash o traacutefego na rede pode ser exagerado devido ao processo de flooding

Existem dois tempos de vida

Longo usado por omissatildeo (valor recomendado = 5 minutos)

Curto usado quando a spanning tree estaacute em reconfiguraccedilatildeo (valor recomendado = 15 segundos) - exige processo de notificaccedilatildeo de alteraccedilotildees da topologia da rede

RC ndash Prof Paulo Lobato Correia 155

Formato mensagens de configuraccedilatildeo

protocol identifier

version

message type

reserved

root ID

cost of path to root

bridge ID

port ID

message age

max age

hello time

forward delay

TCTCA

2

1

1

1

8

4

8

2

2

2

2

2

octetos

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 60: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

60

RC ndash Prof Paulo Lobato Correia 160

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

RC ndash Prof Paulo Lobato Correia 161

Switches vs Routers

Both store-and-forward devices

Routers network layer devices (examine network layer headers)

Switches are link layer devices

Routers maintain routing tables implement routing algorithms

Switches maintain switch tables implement filtering learning algorithms

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 61: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

61

RC ndash Prof Paulo Lobato Correia 162

Outline

Link-layer Addressing

Introduction and services

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 163

Elements of a Wireless Network

Network infrastructure

Wireless hosts Laptop Smartphone Tablet hellip

Run applications

May be stationary (non-mobile) or mobile

Wireless does not always mean mobility

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 62: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

62

RC ndash Prof Paulo Lobato Correia 164

Elements of a Wireless Network

Network infrastructure

Base station

Typically connected to the

wired network

Relay responsible for

sending packets between

wired network and wireless

host(s) in its ldquoareardquo

Eg cell towers 80211

access points

RC ndash Prof Paulo Lobato Correia 165

Elements of a Wireless Network

Network infrastructure

Wireless link

Typically used to connect

mobile(s) to base station

Also used as backbone link

Multiple access protocol coordinates link access

Various data rates

transmission distance

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 63: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

63

RC ndash Prof Paulo Lobato Correia 166

Elements of a Wireless Network

Network infrastructure

Infrastructure mode

Base station connects

mobiles into wired network

Handoff mobile changes

base station providing

connection into wired

network

RC ndash Prof Paulo Lobato Correia 167

Elements of a Wireless Network

Ad hoc mode

No base stations

Nodes can only transmit to

other nodes within link

coverage

Nodes organize themselves

into a network route among

themselves

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 64: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

64

RC ndash Prof Paulo Lobato Correia 169

Wireless Communications

Wireless communications are needed to Allow communications while moving

Allow communications in places where it is difficult or impossible to implement a cabled infrastructure

Allow broadcasting

Allow the fast implementation with low initial cost of a communications system

However Less controlled operation environment more subject to interference

noise unauthorized detection

Often provides lower transmission rates

The frequency bands are easier to reuse in guided media

Reflexions Dispersion DifractionShadow areas

RC ndash Prof Paulo Lobato Correia 170

Wireless Link Characteristics

Differences from wired link

Interference from other sources- Standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) - Devices (motors) interfere as well

Multipath propagationRadio signal reflects off objects and ground arriving at destination at slightly different times

Decreased signal strengthRadio signal attenuates as it propagates through matter (path loss)

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 65: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

65

RC ndash Prof Paulo Lobato Correia 171

Wireless Link Characteristics

Signal-to-noise ratio (SNR)

Larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

SNR versus BER tradeoffs

Given physical layer

increase power -gt increase SNR-gt decrease BER

Given SNR choose physical layer that meets BER requirement giving highest throughput

bull SNR may change with mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

RC ndash Prof Paulo Lobato Correia 172

Wireless Network Characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other

B C hear each other

A C can not hear each other

rarrrarrrarrrarr A C unaware of their interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuation B A hear each other

B C hear each other

A C can not hear each other interfering at B

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 66: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

66

RC ndash Prof Paulo Lobato Correia 176

IEEE 80211 Wireless LAN

80211b

24 - 5 GHz unlicensed spectrum up to 11 Mbps

80211a

5 - 6 GHz range up to 54 Mbps

80211g

24 - 5 GHz range up to 54 Mbps

80211n (multiple antennae)

24 - 5 GHz range (40 MHz channel) up to 150 Mbps

80211ac ndash under development (multiple antennae)

24 - 5 GHz range (80 or 160 MHz channel) gt 500 Mbps (up to 7 Gbps)

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

RC ndash Prof Paulo Lobato Correia 177

Characteristics of Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m ndash 4 Km

Long-rangeoutdoor

5Km ndash 20 Km

056

384

1

4

5-11

54

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

80211ag point-to-point

200 80211n

Da

ta r

ate

(M

bp

s) data

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 67: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

67

RC ndash Prof Paulo Lobato Correia 179

80211 Channels Association

80211b 24GHz - 2485GHz spectrum divided into 11 channels at different

frequencies

AP administrator chooses frequency for AP

Interference possible Channel can be same as that chosen by neighboring AP

Host ndash must associate with an AP Scans channels listening for beacon frames containing APrsquos

name (SSID) and MAC address

Selects AP to associate with

May perform authentication

Will typically run DHCP to get IP address in APrsquos subnet

RC ndash Prof Paulo Lobato Correia 181

IEEE 80211 Multiple Access

Avoid collisions 2 or more nodes transmitting at same time

80211 CSMA - sense before transmitting

Donrsquot collide with ongoing transmission by other node

80211 no collision detection

Difficult to receive (sense collisions) when transmitting due to weak received

signals (fading)

Canrsquot sense all collisions in any case hidden terminal fading

Goal avoid collisions CSMACA (Collision Avoidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 68: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

68

RC ndash Prof Paulo Lobato Correia 182

IEEE 80211 MAC Protocol CSMACA

80211 sender

1 If sense channel idle for DIFS then

- Transmit entire frame (no CD)

2 If sense channel busy then

- Start random backoff time

- Timer counts down while channel idle

- Transmit when timer expires

- If no ACK increase random backoff interval then repeat from 2

80211 receiver

- If frame received OK

- Return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

RC ndash Prof Paulo Lobato Correia 183

Avoiding Collisions (more)

IdeaAllow sender to ldquoreserverdquo channel rather than random access of data frames rarr avoid collisions of long data frames

Sender first transmits small request-to-send (RTS) packets to BS using CSMA

RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS

CTS heard by all nodes

Sender transmits data frame

Other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 69: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

69

RC ndash Prof Paulo Lobato Correia 184

Collision Avoidance RTS-CTS

APA B

time

DATA (A)

reservation collision

defer

RC ndash Prof Paulo Lobato Correia 185

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

80211 Frame Addressing

Address 2 MAC addressof wireless host or AP transmitting this frame(sender addr)

Address 1 MAC addressof wireless host or AP to receive this frame(wireless destination addr)

Address 3 MAC addressof router interface to which AP is attached (router addr)

Address 4 used only in ad hoc mode

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 70: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

70

RC ndash Prof Paulo Lobato Correia 186

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

80211 frame

R1 MAC addr H1 MAC addr

dest address source address

8023 frame

80211 Frame Addressing

RC ndash Prof Paulo Lobato Correia 187

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

80211 Frame more

Duration of reserved transmission time (RTSCTS)

Frame seq

Frame type(RTS CTS ACK data)

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 71: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

71

RC ndash Prof Paulo Lobato Correia 189

80211 Advanced Capabilities

Rate Adaptation

Base station mobile - Dynamically change transmission rate (physical layer modulation technique) as mobile moves and SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station

2 When BER becomes too high switch to lower transmission rate but with lower BER

RC ndash Prof Paulo Lobato Correia 190

80211 Advanced Capabilities

Power Management

Node-to-AP ldquoI am going to sleep until next beacon framerdquo

AP knows not to transmit frames to this node

Node wakes up before next beacon frame

Beacon frame contains list of mobiles with AP-to-mobile frames

waiting to be sent

Node will stay awake if AP-to-mobile frames are to be sent

Otherwise sleep again until next beacon frame

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 72: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

72

RC ndash Prof Paulo Lobato Correia 191

Outline

Introduction and services

Link-layer Addressing

Error detection and correction

Multiple access protocols

Ethernet

Link-layer switches

IEEE 80211 Wireless LANs

Framing

RC ndash Prof Paulo Lobato Correia 192

Framing

Data Link layer

Frames include a header with synchronization addressing and control (type and frame number) information

Frames also include a trailer with error control and synchronization information

IP PacketData link layer

Network layer

Physical layer

IP datagram

DL -H DL -TIP datagram

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 73: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

73

RC ndash Prof Paulo Lobato Correia 193

Framing

Synchronization information is needed to know where each transmission begins and ends

1

2

3

4

5

6

X X

OLAacute BOM DIA

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGOE

QUANTO CUSTA O ARTIGO

OLAacute BQUANTOVOU CHOM DIA CUSTAEGAR D O ARTIE TARDGO E

VOU CHEGAR DE TARDE

RC ndash Prof Paulo Lobato Correia 194

Framing ldquoStuffingrdquo

Network layer data may contain binary patterns equal to those used for synchronization (flags) purposes in the data link layer

There is the need to avoid taking those IP data as a data link flag

To that purpose if the flag pattern appears in the IP data it should be preceded in the data link frame by a stuffing pattern to avoid any confusion

ESC escape byteByte

Stuffing

Used in the PPP

protocol

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 74: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

74

RC ndash Prof Paulo Lobato Correia 195

Byte Stuffing

ldquoData transparencyrdquo requirement data field must be allowed to

include flag pattern lt01111110gt

Q is received lt01111110gt data or flag

Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte

Receiver

Two 01111110 bytes in a row discard first byte continue data reception

Single 01111110 flag byte

RC ndash Prof Paulo Lobato Correia 196

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 75: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

75

RC ndash Prof Paulo Lobato Correia 197

Example

Frame delimitation using the flag 0111 1110

Bit stuffing consists in

Add an extra 0 (b) after any sequence of five consecutive 1s in the data

field coming from the network layer (a) to avoid being taken for the flag

In the receptor the inverse operation takes place (c)

Framing ldquoStuffingrdquo

Bit

Stuffing

RC ndash Prof Paulo Lobato Correia 204

Summary

Principles behind data link layer services

Link layer addressing

Error detection correction

Sharing a broadcast channel multiple access

Instantiation and implementation of various link layer technologies

Ethernet

IEEE 80211 Wireless LANs

Switched LANS

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip

Page 76: 5 –Data Link Layer · 5 RC –Prof. Paulo Lobato Correia 9 Hop-to-hop Delivery RC –Prof. Paulo Lobato Correia 10 Where is the Link Layer Implemented? In each and every host; Link

76

RC ndash Prof Paulo Lobato Correia 205

Summary

Journey down protocol stack complete (except PHY)

Solid understanding of networking principles and practice

hellip could stop here hellip but lots of interesting topics

Multimedia

Security

Network management hellip