Gsm & Ss7 &Osi Layers

8/8/2019 Gsm & Ss7 &Osi Layers

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GSM NETWORKS

INTRODUCTION

MS AND SIMSS7

-Varun Ranjit Singh



EVOLUTION OF GSM

GSM (Groupe Spe'ciale Mobile) began in 1982, a committee

under CEPT. The task of GSM was to define a new standard for mobile comm

in the 900MHz range. CEPT evolved into ETSI, but this didnt affect GSM In 1991, the first GSM system was introduced, GSM was

changed to mean Global System for Mobile Communication. Derivatives of GSM also started to appear the same year, Digital

Cellular System (1800 MHz) – DCS 1800. In the US, DCS 1800 was adapted to the 1900MHz range and

was called the Personal Communication System, PCS 1900. GSM has proved to be a major commercial success for system

manufacturers and network operators Current Tecnologies not under GSM and are growing are CDMA,

DECT, PHS.



Factors for success of GSM

Libralization of the monoply of telecomm in europe during the

1990's and the resulting competition, which lead to lower pricesand more “market”.

The knowledge-base and professional approach of GSM,together active support from the industry.

Lack of competition, US and Japan started defining mobilecommunication standards after GSM had been well established.



System Architecture

GSM utilizes a Cellular structure, the basic ideas for that are:

Frequency reuse: Divide the available freq range, and assign apart of the frequency spectrum. Reduce range of the BaseStation (BS) so that the same frequency can be reused.

Attenuation: Alternatives were discarded because attenuationis ver high. Hence Ranges are limited to 5kms.

Diadvatages Cost of Infrastructure.

Handover Tracking – when Mobile Station (MS) is in motion Processing due to Signaling overheads



GSM Subsystem

Public Land Mobile Network (PLMN) – consists of the whole GSMsubsystem.

Mobile Station (MS) – a PLMN contains as many MSs as possible.

Available in various styles and power classes

Subscriber Identity Module (SIM) – stores identity of Subscriber. TheSIM is a chip, and communicates directly with VLR and indirectly withHLR

Base Tranciever Station (BTS) - large no. of BTSs take care of theradio related tasks and provide connectivity using Air-interface betweenn/w and MS.

Base Station Controller (BSC) – BTSs of an area are connected to theBSC via an Abis-interface. BSC takes care of central functions andcontrol of the subsystem, BSS.

BSS comprises of BSC and connected BTSs.

Transcoding Rate and Adaptation Unit (TRAU) – data compression forbetter bandwidth management. TRAU is part of the BSS



GSM Subsystem contd...

Mobile Service Switching Center (MSC) – large no. of BSCs are

connected to the MSC via Air Interface.helps in routing ofincoming and outgoing messages and assignmernt of userchannels under A interface

Home Location Register (HLR) – a repository that stores the dataof large no. subscribers, each PLM needs atleast one HLR

Visitor Location Register (VLR) – contains a part of subscribersdata, but only of sunscribers in the VLRs area

Equipment Identity Number (EIR) – a database which stores IMEInumbers of stolen mobiles



SIM

SIMs are of two types – [no change in functionality except size]

ID-1 SIM (credit card size) Plug-in SIM (1cm square)

Except for emergency calls the SIM needs to be used.

A mobile equipment only becomes a MS once the SIM isinserted.

SIMs major task is to store data such as contacts etc

GSM identifies the subscriber by his SIM and not the equipmenthence, the SIM can be used in multiple equipments.



Mobile Station

All functionality known from the BTS TRX (tranciever) like GMSK

upto Channel coding are implemented in the MS.

MS specific functionality such as DTMF Economical battery usage

MS is not peer only to BS but communicates directly to MSC via Mobility Management (MM). VLR via Call Control (CC).

Plus it has to provide transparent interface (TAF, Terminaladaptation function) for data and fax connections.

5 power classes were defined for GSM 900, 20W is nowoutdated, 8W is now the most powerful rating.



Mandatory features of a MS

DTMF capability

SMS capability

Ciphering Algorithms such as A5/1 and A5/2 (what are they usedfor?)

Display capability of SMS, dialled no.s and available PLMN

Support emergency calls without SIM

Burned on IMEI



Signaling

Signaling is the language of telecommunication that machines

and computers use to communicate with each other.

Signaling is required between various elements of the GSMnetwork

PCM (Pulse Code Modulation), is used for signaling data andpayload.

A 2Mbps PCM link can be effeciently used by using TDMA with32 channels with each capable of carrying 64kbps.

Sinaling System Number 7 or SS7 is used in GSM networks



SS7

SS7 provides in OSI layers 1 to 3 for signaling traffic on all NSSinterface and A-interface.

User parts of SS7: SCCP (Signaling Connection Control Part) TCAP (Transaction Control Application Part) /MAP (Mobile

Application Part).

SS7 Network consists of Directly connected Signaling Points (SPs). SPs that are connected through Signaling Transfer Points (STPs). A combination of SPs and STPs.

An SP is a network node that has user parts (e.g SCCP) that allowsprocessing of messages addressed to it. (MSC, BSC, PSTN fall in thiscategory)

Functionality of STP is typically related to that of the SP, but with

additional capability of being able to relay SS7 messages



Message Transfer Part (MTP)

SS7 without its user parts is OSI Layer 1 to 3. Those 3 layers are

represented by MTP. Parts of SCCP are also part of Layer 3.

MTP performs: Provides all functionality to provide for a reliable transport of

signaling data to various user parts. Takes necessary measures to ensure that the connection is

maintained and prevent loss of data.

MTP can be partitioned into 3 layers where

MTP1 is responsible for transmission of single bits. MTP2 defines the basic frame structure for all message types.

FLAG Ack Length Information field (op) FCS* FLAG



Message Types

Definition of SS7 message types is another functionality of MTP2

In Layer 2, 3 different types are defined FISU, Fill-in Signal Unit. Length = 0

LSSU, Link Status Signal Unit. Length = 1 or 2

MSU, Message Signal Unit. Length > 2

Length is the length of the optional data field.



FISU

Its used to supervise the link status when no traffic is available

Both sides poll each other in this idle state. FSN, BSN, FIB andBIB dont change their value during polling.

FSN: Forward Sequence Number. BSN: Backward Sequence Number. FIB: Forward Indicator Bit BIB: backward Indicator Bit.

FISU can also be used to acknowledge reciept of an MSU8 bit 7 1 7 1 6 (LI=0) 2* 16 8

FLAG BSN BIB FSN FIB LI FCS FLAG



LSSU

LSSU is used only to take the link into and out of service and during error

situations.(e.g. Overload)

To exchange status information between two SPs or STPs.

Status can be 2 octet long but it occupies 1byte, of which only thefirst 3 bits contain the actual status message. The recipeint of the

LSSU doesnt confirm its receipt

SIO: Out of alignment – start of link alignment SIN: Normal alignment – conn. brought into service (8.2s)

SIE: Emergency alignment – A conn brought into service (500ms) SIOS: Out of service – In case of error, the link is taken out of service SIPO: when layer 2 detects an error in layer 2 SIB: Signal overload, Acks cant be sent, link failure follows

8 7 1 7 1 6 (LI =1or2) 3+5 (spare) 16 8

FLAG BSN BIB FSN FIB LI FCS FLAGStatus



MSU

Used for any type of data transfer between 2 network nodes

Only SS7 message to carry traffic data, used by all user parts(SCCP, ISUP, OMAP) as a platform particularly fo that task

The Information field consists of the Service Information Octet (SIO), SIO is furture partitioned into

SubService Field (SSF) and Service Indicator (SI) with 4 bitseach. 2 MSBs of SSF are necessary to describe NetworkIndicator (NI).

NI distinguishes b/w national and international messages SI indicates to which user part the MSU (data in SIF)

belongs.

Signaling Information Field (SIF) Unlike FISU and LSSU, MSU has to be ack'd to the peer entity

whenever an MSU is received



Addressing and Routing

MSUs arent necessarily exchanged b/w two adjacent SP/STP.

SS7 uses so called Point Codes (PCs) for routing and addressingMSUs

PCs are Unique IDs within an SS7 network Exactly one PC – Signaling Point Code (SPC) is assigned to

every SP and STP.

An MSU has a routing label that contains OPC (Originating PC)and DPC (Destination PC)

The routing label is part of SIF. (Note: LSSU and FISU donthave a routing label as they are exchanged only b/w adj

nodes)14 14 4 = 32 bits (4bytes)

DPC OPC SLS



Bringing Layer 3 into Service

After the test time is over and no errors were detected, layer 2 isconsidered to be in service, Layer3 then initiates further tests

A Signaling Link Test Message (SLTM) is used for testing.

If Signaling Link Test Acknowledge is received by the sender from the

recipient then Layer3 is considered to be in Service .

FLOW diagram

SS7 (idle) -> LSSU–SIO (2 way) -> LSSU-SIOS ->LSSU-SIE/SIN ->

(tests) -> MSU/SLTM -> MSU/SLTA (in service).



Error Detection and Correction

Layer 2 is responsible for it. FSN, BSN, FCS.

All messages not acknowledged within a certain time frame areretransmitted. Retransmit when NACK received.

FSN and FIN form the send sequence no, FSN is

incremented when a MSU is sent. If a FISU or LSSU is sentits not inc.

Similarly for BSN,BIN.

All Acks can be sent in one message by sending thecorresponding last received correct FSN/BSN

In case of transmission error invert FIN/BIN.



SS7 Network Management and Test

Major task in the operation of a big network is its management oradministration.

SS7 has dedicated user parts in layer3 that automatically detect errorsituations and able to correct them autonomously.

Errors can be classified in 3 classes

Overload on single SS7 line Outage/bringing into service an SP/STP Outage/bringing into service a link between SP/STP.

To differentiate between NM and NT, SI sends 00 and 01 respectively.



Error Cases

Overload Situation: the affected STP informs its neighbors about the limited availability. The info

is sent in TFC/TFR messages. Alternate routes are used by neighbors. The changeover procedure (COO

message) is used for rerouting Once the link is up again it informs its neighbors using TFA link. The change back sequence is executed (CBD messages)

Outage/Bringing SP/STP into service All neighbors are informed immediately. TFP message is sent to all affected SPs, STPs

Gsm & Ss7 &Osi Layers

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