FC Layers.pdf

7/21/2019 FC Layers.pdf

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Lesson 3

FC Layers

OverviewLike nearly all modern networks, Fibre Channel (FC) is designed with a modular, layeredarchitecture. This architecture is designed to carry other protocols, as well as new nativeprotocols. A layered architecture provides benefits for both vendors and users because itenhances the clarity and flexibility of the architecture. This lesson describes the five layers of the FC layered model, and the upper layer protocols that FC supports.

ObjectivesUpon completing this lesson, you will be able to describe the Fibre Channel layered model,data constructs, SCSI-FCP read and write operations, and Link Services. This includes beingable to meet these objectives:

Describe the FC layered model

Describe the Fibre Channel Data constructs

Describe SCSI-FCP protocol operations

Describe Link Services


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2007 Cisco Systems, Inc. Appendix A: The Fibre Channel Protocol AA-43

2007 Cisco Systems, Inc. All rights reserved. ICSNS v3.07-5

Fibre Channel Layers (Cont.)

NTFS CIFS NFS DAFS ...

Fibre Channel

SCSI IP VI HiPPI ...

FC-0 Physical interface

FC-1 Encoding

FC-2 Framing and flow control

FC-3 Common Services

Upper-layer protocols

FC-4

FC-FS

FC-PI

The five layers of FC are:

FC-0: Physical interface, transmission and signaling

FC-1: 8b/10b encode/decode, link control, ordered set specifications

FC-2: Framing, flow control, exchange/sequence management

FC-3: Application-specific layer for fabric services

FC-4: Upper-layer protocol mapping specification

The lower three layers (FC-0, FC-1, and FC-2) are collectively known as the FC Physical Layer(FC-PH), even though they also implement logical functions such as framing and flow control.The FC-3 layer provides a framework for implementing new SAN-wide services, while the FC-4 layer interfaces with the ULPs and maps them to the FC.

The FC-PH specification was the original document that defined layers FC-0, FC-1, and FC-2.The final version of the FC-PH specification was FC-PH-3. However, FC-PH was thensuperceded by two additional documents:

Fibre Channel Physical Interface (FC-PI) defines FC-0.

Fibre Channel Framing and Signaling (FC-FS) defines FC-1 and FC-2


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AA-44 Implementing Cisco Storage Networking Solutions (ICSNS) v3.0 2007 Cisco Systems, Inc.



FC-1 Encoding



ULPsFC-4

FC-FS

FC-PI

FC-0: Physical Interface

FC-0 specifies the physical characteristics of the data link:

Cables and connectorsTransmitter and receiver functions

Signaling protocol

FC-0: Physical InterfaceThe FC-0 layer specifies the characteristics of the physical links. FC-0 is responsible for:

Specifying signaling protocols for transmitting and receiving a signal at different transferrates

Specifying data rates and maximum transmission distances


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Optical Media Types

TwinAx, QuadAx 33m

59m

300m

500m

10km

30m100m

30km is supported by some manufacturers

150m

300m

10km

70m

150m

10km

CoaxialShielded Twisted Pair

Multimode 62.5

Multimode 50

Single-mode 9

4Gb

1Gb2Gb

4Gb

2Gb1Gb

1Gb2Gb

4Gb Optical Cables

Electrical Cables

Optical media types:

Multimode fiber uses a 780nm short-wave laser.

Single-mode fiber uses a 1300nm long-wave laser.

Maximum link distances vary by data rate.

The current specification states a minimum 2m distance for optical fiber. This is to allow for abuild-up of photons that occur in the first 2m of cable after the laser fires, and, in multimodecables, to eliminate problems associated with some modes of light which, due to their steep

angle of reflection, do not travel very far down the cable.


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FC-1: Encoding (Cont.)

Each 8-bit data byte is encoded into a 10-bitcharacter before it is transmitted over the link:

Transmitter

Parallel Input Serial Output Media Output

8b/10bEncoder

Tx Byte

Parallel/SerialConverter (SERDES)

The preceding diagram shows the FC-1 encoding process:

The data from the transmitting I/O bus is encoded using the 8b/10b encoding scheme.

The parallel data is converted to serial format.

The serial data is transmitted across a link.

The receive end decodes the serial data and forwards it to the receivers I/O bus.


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FC-1: Encoding (Cont.)

8b/10b Encoding Scheme: Transmission Characters always have either:

6 ones and 4 zeros = Positive disparity4 ones and 6 zeros = Negative disparity

5 ones and 5 zeros = Neutral disparity

0101000110

1010110110

-

+

110111110xDF

6 31 = D31.6

DisparityEncodedBitsByte

Transmission characters always have either:

Positive disparity: 6 ones and 4 zeros

Negative disparity: 4 ones and 6 zeros

Neutral disparity: 5 ones and 5 zeros

The 8b/10b scheme defines multiple transmission characters for each 8-bit data byte. Becausethe encoder can choose between multiple 10-bit representations for each 8-bit byte, it canbalance the number of ones and zeros in the data stream. The imbalance between the number of 1s and 0sknown as the running disparityis continually reevaluated. To balance the number

of ones and zeros, every transmitted byte is encoded into one of two possible 10-bitrepresentations depending on the current running disparity.

In the FC-1 specification, every 10-bit character is represented using a special notation:

Dxx.y: Used for data characters that map to 8-bit characters; xx is the decimal value of thelowest 5 bits and y is the decimal value of the highest bits.

Kxx.y: Used for special control characters; xx and y are defined as for data characters.


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FC-2: Framing and Flow Control

FC-2 defines the structure, organization, anddelivery of the data:

Constructs and manages frames

Inserts addressing andheader information

Manages flow control

Frame error detection


FC-1 Encoding



ULPsFC-4

FC-FS

FC-PI

FC-2: Framing and Flow ControlThe FC-2 layer specifies how the data is packaged for transmission:

To transport a stream of data from one port to another, the data must be packaged intodiscrete packets, or frames. At the receiving end, the data must then be extracted from theframes.

A frame carries identification information, such as addressing information and informationthat identifies how the network should serve, deliver and respond to this particular frame.

The FC-2 layer is responsible for inserting this information into each frame. The flow of frames across the network must be also be controlled so that the sending port

does not send data faster than the receiving port can receive it. The FC-2 layer performsthis flow control function by initiating or preventing transfer of frames.

FC-2 is also responsible for detecting frame-level errors.

FC-2 is the workhorse of FC. It is the most complex of the FC layers.


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FC-3: Common Services

FC-3 is an expandable layer that is designed tosupport services, such as:

Name server Secure key server

Management server

Time server

Future services?Compression

Encryption

Link multiplexing


FC-1 Encoding



ULPsFC-4

FC-FS

FC-PI

FC-3: Common ServicesThe FC-3 layer defines a generalized structure for implementing new services:

Generic services are functions that can span multiple ports and can be applied to multipleupper-layer protocols.

FC defines generic fabric services that include a name server, a secure key distributionserver, a management server, and a time server.

Examples of potential future services include data compression, encryption services, ormultiplexing multiple links to form one aggregated virtual link.

Generic services use the Fibre Channel Common Transport (FC-CT) protocol tocommunicate and distribute functions between switches.


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FC-4: Upper-Layer Protocol Interfaces

FC-4 maps upper-layer protocols to the FC protocol:SCSI, HiPPI, ESCON/FICON for storage

IP, VI, ATM and others

Allows multiple protocolsto be transported over thesame physical interface


FC-1 Encoding



ULPsFC-4

FC-FS

FC-PI

FC-4: Upper-Layer Protocol InterfacesThe FC-4 layer defines how the ULPs map to the lower layers of FC. It allows multipleprotocols to be transported over the same physical interface. FC-4 makes sure that the ULP dataor commands are broken down appropriately and packaged correctly into FC frames.

When a sender transmits a block of data:

The FC-4 layer receives the data from the ULP drivers and passes the data down to the FC-3 layer.

The FC-3 layer performs any required manipulation on the data, such as compressing orencrypting the data, or it simply passes the data directly to the FC-2 layer.

The FC-2 layer then packages the data into frames and passes it down to the FC-1 layer.

The FC-1 layer encodes each byte into 10-bit characters using the 8b/10b scheme andpasses the data down to the FC-0 layer.

The FC-0 layer encodes the data into physical signals and transmits those signals across thelink.

Only the FC-4 layer needs to know which ULP is used. The protocol-independence of the FC-3through FC-0 layers allows FC to be easily adapted to new ULPs.


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FC-4: Upper-Layer Protocol Interfaces (Cont.)

95% of FC network utilization is for SCSI applications

FCP-SCSI

FC-SB-2 FC-LE FC-IP FC-FP F C-BB-2

SCSI-3 FICON,ESCONIEEE

802.2 LLC IPHiPPI ATM,

SONET


FC-1 Encoding


FC-2 Framing and Flow Control

The Small Computer System Interface (SCSI) command set is widely used among storagedevices. Even though SCSI bus technology is not suitable for SANs, the SCSI command set iswell-suited for many types of storage applications.

The use of the SCSI command set enables the use of inexpensive SCSI disks and SCSI tapedrives in FC SAN storage devices. SCSI-FCP also enables compatibility with existingoperating systems and legacy storage applications. In fact, most operating systems andapplications are not aware of the FC SANFC devices appear to the host and its applicationsas SCSI devices.

The mapping of the SCSI protocol to FC is called SCSI-Fibre Channel Protocol (SCSI-FCP), or

sometimes simply FCP. SCSI-FCP is the ULP command set used on most FC SANs. SCSI-FCP provides the command set for reading and writing data to and from storage devices.

The fact that FC supports a wide range of protocols allows FC to meet the needs of diverseapplications and integrate with heterogeneous platforms. FC supports the following existingULP protocols:

The Enterprise Systems Connection (ESCON) protocol is a storage interconnect used inIBM mainframe computing environments. The Fibre Connection (FICON) protocol allowsESCON assets to be used within an FC SAN infrastructure. The FC-SB-2 standard mapsFICON to FC-2.

The IEEE 802.2 standard defines the generic logical link control (LLC) layer in the OSI

Reference Model. The FC-LE standard helps map IEEE 802.2-based protocols to FC. IP is the protocol that drives the Internet. FC-IP allows FC to carry the IP protocol. Servers

can use IP to communicate with each other over the SAN.

High Performance Parallel Interface (HiPPI) connects devices at short distances and highspeeds. HiPPI is used primarily to connect supercomputers and to provide high-speedbackbones for LANs. The FC-FP standard maps HiPPI to FC.

The FC-BB-2 standard enables FC to exchange data with ATM and Synchronous OpticalNetwork (SONET) networks for long-haul transport of FC data.


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Fibre Channel Data ConstructsThis section describes the Fibre Channel Data constructs: Frames, Sequences, Exchanges, andOrdered Sets.


Fibre Channel Data Constructs

FCSequence

Word

Frame

Exchange

Initiator Target

FC

HBA

The preceding graphic shows a transaction between a host (initiator) and a storage device(target):

The smallest unit of data is a word. Words consist of 32 bits (4 bytes) of data that areencoded into a 40-bit form by the 8b/10b encoding process.

Words are packaged into frames. An FC frame is equivalent to an IP packet.

A sequence is a series of frames sent from one node to another node. Sequences areunidirectionalin other words, a sequence is a set of frames that are issued by one node.

An exchange is a series of sequences sent between tow nodes. The exchange is themechanism used by two ports to identify and manage a discrete transaction. The exchangedefines an entire transaction, such as a SCSI read or write request. An exchange is openedwhenever a transaction is started between two ports and is closed when the transactionends. An FC exchange is equivalent to a TCP session.


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Fibre Channel Frames

Payload CRC EOF

SOF

Header

14

624

052802112

14

1 = 5374 = 2148

OptionalHeaders Data or commands

FillBytes

016064

WordsBytes

= 5280-3 = 2112

051202048

WordsBytes

Fibre Channel FramesThe maximum total length of an FC frame is 2148 bytes, or 537 words. This consists of:

A 4-byte SOF delimiter

A 24-byte header

A data payload that can vary from 0 to 2112 bytes

A 4-byte (32-bit) CRC that is used to detect bit-level errors in the payload

A 4-byte EOF delimiter

The frame payload consists of 3 elements:

The payload itself, containing data or commands, can be up to 2112 bytes.

The first 64 bytes of the payload can be used to incorporate optional headers. This wouldreduce the data payload size to 2048 bytes (2KB).

The payload ends with 1-3 fill bytes. This is necessary because the smallest unit of datarecognized by FC is a 4-byte word. However, the ULP is not aware of this FC requirement,and the data payload for a frame might not end on a word boundary. FC therefore adds upto 3 fill bytes to the end of the payloadas many as are needed to ensure that the payloadends on a word boundary.


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Frame Channel Frames (Cont.)

FC protocol trace FLOGI example:

Ordered Sets (OS) R-Rdy Idle

Frames (F)

The screen image displays an FC protocol trace. A single FC frameFabric Login (FLOGI)is displayed in the right-hand window. Each word in the frame is depicted on a separate line,beginning with the SOF Frame Delimiter (SOFi3) and ending with the EOF Frame Delimiter(EOFt). The display shows the 6 words in the frame header, 29 words in the payload, and the32-bit CRC.


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Frame Headers

Payload CRC EOF

SOF

Header

Bit

24 23 16 15 8 7 031Word

R_CTLCS_CTL

TYPESEQ_ID

OX_IDDF_CTL

RX_IDSEQ_CNT

F_CTL

Parameter

S_IDD_ID0

12345

Frame HeadersThese are the header fields of an FC frame:

R_CTL (Routing Control, 8 bytes): Frame type and function; used by the switch to routeframes

CS_CTL (Class Specific Control, 8 bytes): Class specific control information for Class 1, 4& 6

D_ID (Destination ID, 24 bytes): 24-bit address of the destination port

S_ID Source ID, 24 bytes): 24-bit address of the source port TYPE (Data Structure Type, 8 bytes): Type of Information Unit & ULP carried by this

frame

F_CTL (Frame Control, 24 bytes): Specifies number of fill bytes and sequence controlinformation

SEQ_ID (Sequence ID, 8 bytes): Unique ID for each sequence

SEQ_CNT (Sequence Count, 16 bytes): Frame count identifying each frame in thesequence

DF_CTL (Data Frame Control, 8 bytes): Information about optional headers

OX_ID (Originator ID, 16 bytes): Unique ID set by the exchange originator RX_ID (Receiver ID, 16 bytes): Unique ID set by the exchange responder

Parameter (Parameter or Offset, 32 bytes): Used for multi-purpose parameters, such asbuffer offset.


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Ordered Sets

Transmission Word

Ordered SetK28.5, Dxx.y, Dxx.y, Dxx.y

Data WordDxx.y, Dxx.y, Dxx.y, Dxx.y

Primitive SequenceFrame Delimiter Control SignalFill Word

Primitive Signal

Start-of-FrameEnd-of-Frame

IdleArbitrate

Receiver ReadyVirtual Circuit ReadyCloseOpenDynamic Half-DuplexMarkSynchronize

Non-Operational StateOffline StateLink ResetLink Reset ResponseLoop InitializationLoop Port BypassLoop Port Enable

Ordered Sets are FC words (5 bytes) that are used for link-level functions. They are usedbecause they are fast and light, and because commands sometimes need to be exchanged beforedevices have been assigned FC addresses.

The first byte of an Ordered Set is always the K28.5 character, which defines the word as anOrdered Set. The second byte identifies the Ordered Set type, and the last two bytes can be usedto transmit other parameters.

There are three types of Ordered Sets:

Frame Delimiters are used to mark the beginning and end of frames.

Primitive Signals are used to initiate, synchronize, and terminate communication sessions,and to maintain synchronization when no other information is being transmitted on the link.The two types of Primitive Signals are fill words and control signals.

Primitive Sequences are similar to Primitive Signals, but are transmitted repeatedly until aresponse is received. They are used for link and loop initialization.


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SCSI-FCP OperationsThis section provides a brief overview of SCSI-FCP protocol operations.


ULP Information Transfer

FCP_RSP 5

FCP_DATA 3

FCP_CMD1 Sequence 1

Target

Sequence 3

Sequence 2

SCSI-FCP Read Operation

Initiator Fabric

Frame2

Frames

4

Frame 6IU 3

IU 1

IU 2Exchange

The preceding diagram illustrates a SCSI-FCP read operation:

1. The initiator node generates a SCSI read request (FCP_CMD), which is packaged as IU 1.

2. The initiator FC-2 layer converts IU 1 to a single command chunk and sends it across thefabric as a single frame. This constitutes Sequence 1.

3. The target node processes IU 1, retrieves the requested data (FCP_DATA) from storageand packages the data as IU 2.

4. The target FC-2 layer converts IU 2 to one or more data chunks and sends them across thefabric. This constitutes Sequence 2.

5. The target node then generates a status command (FCP_RSP) that informs the initiator thatthe requested data transmission is complete. The status command is packaged as IU 3.

6. The target FC-2 layer converts IU 3 to a single command chunk and sends it across thefabric. This constitutes Sequence 3.

At this point, the I/O operation is complete. The collection of three sequences constitutes asingle exchange.


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ULP Information Transfer (Cont.)

FCP_RSP 7

FCP_DATA5

FCP_XFR_ RDY 3

FCP_CMD1 Sequence 1

Target

SCSI-FCP Write Operation

Initiator

Exchange

Fabric

Sequence 3

Sequence 2

Sequence 4

Frame2

Frame 4

Frames

6

Frame 8

IU 1

IU 2

IU 3

IU 4

The preceding diagram illustrates a SCSI-FCP write operation:

1. The initiator node generates a SCSI write request (FCP_CMD), which is packaged by theFC-4 layer as IU 1.

2. The initiator FC-2 layer converts IU 1 to a single command chunk and sends it across thefabric as a single frame. This constitutes Sequence 1.

3. The target node responds with a SCSI write request response (FCP_XFR_RDY), which ispackaged as IU 2. The write request response is required for synchronization between the

initiator and target.4. The target FC-2 layer converts IU 2 to a single command chunk and sends it across the

fabric. This constitutes Sequence 2.

5. The initiator node retrieves the data (FCP_DATA) from its ULP buffers and packages it asIU 3.

6. The initiator FC-2 layer converts IU 3 to one or more data chunks and sends them acrossthe fabric. This constitutes Sequence 3.

7. The target then generates a status command (FCP_RSP) to confirm the end of theexchange. The command is packaged as IU 4.

8. The target FC-2 layer converts IU 4 to a single command chunk and sends it across thefabric. This constitutes Sequence 4.

The collection of four Sequences constitutes a single Exchange.


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Link ServicesThis section describes what a Link Services command is, the role of Link Services, and howLink Services differ from Ordered Sets.


Fibre Channel Link Services

ULP-independent Fibre Channel commands

Implement session management functions: Fabric and port login

Address resolution

Error recovery

Defined within the FC-CT framework Thus carried within a frame construct

N_Port N_Port

ABTS

BA_ACC

Link Services are upper-layer protocol (ULP) independent FC commands. Link Services areused to implement control functions used in session management, such as fabric and port login,address resolution, and error recovery. Link Services are defined within the Fibre ChannelCommon Transport (FC-CT) framework.

Link Services are transparent to ULPs. In other words, Link Services frames are generated bythe initiator N_Port, not by the ULP driver. Upon receiving a Link Services command, thetarget N_Port processes and discards all Link Services frames.

The preceding diagram shows an example of a Link Services exchange:

The N_Port on the left has sent an ABTS Link Services command to attempt to terminatethe current FC sequence.

The N_Port on the right receives the ABTS request and responds with the BA_ACC LinkServices command to indicate that the N_Port has successfully processed the ABTSrequest.

Note Ordered Sets are not Link Services. Ordered Sets are short, one-word (four-byte)commands that can carry, at the most, two bytes of parameters, whereas Link Servicescommands consist of one or more FC frames. Ordered Sets are typically used at thephysical layer to perform basic link management functions. Link Services comprise a higher-level command set that is essential to performing session management and error recovery.


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Types of Link Services

Basic Link Services:Implement a handful of basic control functions

Must be supportedSingle frame (can be inserted into a sequence)

Extended Link Services:Perform a variety of control functions at the FC-2 layer New commands are added regularly to provide additional controlfunctionsOnly a few must be supported

Always sent as a separate exchange

Types of Link ServicesThere are two types of link services:

Basic Link Services implement just a handful of basic control functions. All Basic LinkServices must be supported by all FC devices. Each Basic Link Service command istransmitted as a single frameeither as a single-frame sequence or as a frame inserted intoa longer sequence.

Extended Link Services are used to perform a variety of control functions between ports at

the FC-2 layer. Extended Link Services are added regularly by the ANSI committee toprovide additional control functions. Only a few Extended Link Services are required bythe FC specification; most are optional. The number and functionality of Extended LinkServices are constantly evolving. Extended Link Services are always sent as a separateexchange.


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Basic Link Services

Pre-empt Dedicated Connection (Class 1 or 6)PRMT

Remove Dedicated Connection (Class 1 or 6)RMC

Basic Link Service RejectBA_RJT

Basic Link Service AcceptBA_ACC

Abort SequenceABTS

No OperationNOP

Basic Link ServicesThe Basic Link Services are as follows:

NOP performs no specific operation but can be used to carry control information in theheader, such as initiating a Class 1 or Class 6 connection.

ABTS is used to abort the current sequence or an entire exchange. Only the ABTScommand has a reply: BA_ACC if accepted and BA_RJT if rejected.

BA_ACC is the normal response to an ABTS command

BA_RJT is an abnormal response to an ABTS command if the ABTS request cannot beprocessed. There are several reasons why this may occur, including an invalid OX_ID orRX_ID.

RMC is used to request an immediate Class 1 or 6 disconnection. The recipient will abortany current sequences and exchanges resulting in possible frame loss and will send a finalACK frame.

PRMT is used to terminate a Class 1 or 6 connection if it is required for a higher priorityconnection.


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Extended Link Services

Required byFibre Channelstandard

Supported byall SANimplementations

Used for FC Ping and FC TraceECHO

Extended Link Services AcceptLS_ACC

Extended Link Services RejectLS_RJT

State Change RegistrationSCR

Registered State Change NotificationRSCN

State Change NotificationSCN

Process LogoutPRLO

Process LoginPRLI

Port LogoutLOGO

Port LoginPLOGI

Fabric LoginFLOGI

Examples:

Extended Link ServicesThe Fibre Channel standard requires support for only a few Extended Link Services:

FLOGI is used by an N_Port to log in to an F_Port and obtain a Fibre Channel address.

PLOGI is used by one N_port to log in to another N_Port, open a session, and exchangeservice parameters.

LOGO is used to terminate a login session and free its associated resources.

LS_ACC notifies the sender of an Extended Link Service command that the command hasbeen accepted.

LS_RJT notifies the sender of an Extended Link Service command that the command wasnot accepted. The command might have been rejected because the port was busy at thetime, the command was invalid or malformed, or the command is not supported by thereceiver. The reason code in the payload gives a more precise reason for the rejection.


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SummaryThis topic summarizes the key points that were discussed in this lesson.


Summary

Fibre Channel has a layered model that is similar in somerespects to the OSI model.

Fibre Channel constructs include words, frames, sequences andexchanges.

The Fibre Channel protocol is broken down into control and dataunits.

The control units in Fibre Channel are called Ordered Sets.

Link Services are additional communications from port to portused to relay logins, state changes, error situations and other administration messages.

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