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GSM Architecture

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Component groups of a GSM network

The principle component groups of a GSM network are: The Mobile Station (MS)

The MS consists of two parts, the Mobile Equipment (ME) and an electronic ‘smart card’ called a Subscriber Identity module (SIM).The ME is the hardware used by the subscriber to access the network. The hardwarehas an identity number associated with it, which is unique for that particular device andpermanently stored in it. This identity number is called the International MobileEquipment Identity (IMEI) and enables the network operator to identify mobile equipmentwhich may be causing problems on the system.The SIM is a card which plugs into the ME. This card identifies the MS subscriber andalso provides other information regarding the service that subscriber should receive. Thesubscriber is identified by an identity number called the International Mobile SubscriberIdentity (IMSI).

The Base Station System (BSS)

This is the part of the network which provides the radio interconnection from the MS to the land-based switching equipment.

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Mobile Station MOBILE STATION (MS) :-

The mobile station (MS) represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system.

The SIM may be a removable module, while the equipment identity is not linked to a particular subscriber.

Functions of a Mobile Station :- Radio transmission termination. Radio Channel Management. Speech Encoding/Decoding Radio Link error Protection. Flow control of data. Mobility Management.

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SIMThe SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI)This number identifies the MS subscriber. It is only transmitted over the air duringinitialization. The IMSI identifies the subscriber within the GSM network while the MSISDN is the actual telephone number a caller (possibly in another network)uses to reach that person.Security is provided by the use of an authentication key (explained later inthis section) and by the transmission of a temporary subscriber identity(TMSI) across the radio interface where possible to avoid using the permanentIMSI identity.

Temporary Mobile Subscriber Identity (TMSI)This number identifies the subscriber, it is periodically changed by the systemmanagement to protect the subscriber from being identified by someoneattempting to monitor the radio interface. Location Area Identity (LAI)Identifies the current location of the subscriber. Subscriber Authentication Key (Ki)This is used to authenticate the SIM card. Mobile Station International Services Digital Network (MSISDN)This is the telephone number of the mobile subscriber. It is comprised of a countrycode, a network code and a subscriber number.

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BASE STATION SYSTEM (BSS) :- (BSC+BTS)

The BSS consists of three major hardware components:

1) The Base Transceiver Station – BTS

The BTS contains the RF components that provide the air interface for a particular cell .This is the part of the GSM network which communicates with the MS. The antenna is included as part of the BTS.

2) The Base Station Controller – BSC

The BSC as its name implies provides the control for the BSS. The BSC communicates directly with the MSC. The BSC may control single or multiple BTS. The BSC switching matrix also allows the BSC to perform “handovers” between radio channels on BTSs, under its control, without involving the MSC.

3) The Transcoder – XCDR

The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbit/s PCM), into the form specified by GSM specifications for transmission over theair interface, that is, between the BSS and MS (64 kbit/s to 16 kbit/s and vice versa)

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Characteristics of the Base Station System (BSS) are :

The BSS is responsible for communicating with mobile stations in cell areas.One BSC controls one or more BTS’s and can perform inter-BTS and intra-BTS handover. As we have mentioned, a BSC may control several BTSs, the maximum number of BTSs which may be controlled by one BSC is not specified by GSM.

- Each PLMN Should have at least one HLR- One VLR is connected to several MSC ,but one MSC has one VLR

Characteristics of the Base Station System (BSS) are : The BSS is responsible for communicating with mobile stations in cell areas. One BSC controls one or more BTS’s and can perform inter-BTS and intra-BTS handover. The Transcoder – XCDR The Transcoder is used to compact the signals from the MS so that they are more efficiently sent over the terrestrial interfaces.

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MOBILE SWITCHING CENTER (MSC) :-

Mobile services switching center

(MSC) performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs. Each MSC provides service to MSs located within a defined geographic coverage area,the network typically contains more than one MSC. One MSC is capable of supporting a regional capital with approximately one million inhabitants. An MSC of this size will be contained in about half a dozen racks.

The functions carried out by the MSC are listed below:

Call ProcessingIncludes control of data/voice call setup, inter-BSS and inter-MSC handovers andcontrol of mobility management (subscriber validation and location). Operations and Maintenance SupportIncludes database management, traffic metering and measurement, and aman–machine interface. Internetwork InterworkingManages the interface between the GSM network and the PSTN. BillingCollects call billing data.

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HLR

HOME LOCATION REGISTER (HLR) :-

The Home Location Register (HLR)

Contains the identities of mobile subscribers, their service parameters, and their location information.HLR deals with tasks that are independent of Subscriber location.

Function of the HLR include:-

Identity of mobile subscriber Location information for call routing. HLR stores Ki parameter (for each subscriber) which is also stored in

SIM. HLR contains IMSI,MSISDN,VLR add.,Subscriber data.

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VISITOR LOCATION REGISTER (VLR) :-

The Visitor Location Register (VLR) contains the subscriber parameters and location information

for all mobile subscribers currently located in the geographical area controlled by that VLR.

Functions of the VLR include :-

Identity of mobile subscriber. Any temporary mobile subscriber identity (TMSI) allocation. Location area where the mobile station is registered Copy of the subscriber data from the HLR.- Each PLMN Should have at least one HLR- One VLR is connected to several MSC ,but one MSC has one VLR

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Authentication Centre (AUC)

HLR

MSC

Authentication is a process to verify the subscriber SIM.

Secret data & verification algorithm are stored in to the AUC.

AUC & HLR combined to authenticate the subscribers.

Subscriber authentication can be done on every call, if required.

AUC

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EIREQUIPMENT IDENTITY REGISTER (EIR) : The Equipment Identity Register (EIR) is accessed during the equipment validation procedure when a mobile station accesses the system .It contains the identity of mobile station equipment which may be valid, suspect, or known to be fraudulent.

This contains :

White or Valid list – list of valid MS equipment identities.

Grey or Monitored list – list of suspected mobiles under observation.

Black or prohibited list – list of mobiles for which service is barred

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OPERATION AND MAINTENANCE CENTER (OMC) :-

The Operation and maintenance Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS). It allows the network provider to operate, administer , and monitor the functioning of the BSS.

This enables the network provider to configure and maintain the network from a central location.Includes database management, traffic metering and measurement,man–machine interface.

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Mobility Management

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OTHER NETWORK ELEMENTS

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Other optional network elements that the MSC can interface include:

Internetwork Interworking

The IWF provides the function to enable the GSM system to interface with the variousforms of public and private data networks currently available.

Echo Cancellor

An EC is used on the PSTN side of the MSC for all voice circuits. Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition, even on short distance PSTN circuit connections.During a normal PSTN land to land call, no echo is apparent because the delay is tooshort and the user is unable to distinguish between the echo and the normal telephone“side tone”. However, without the EC and with the GSM round trip delay added, the effectwould be very irritating to the MS subscriber, disrupting speech and concentration.

Gateway MSC

The GMSC routes calls out of the network and is the point of access for calls entering the network from outside.GMSC--------PSTN+ISDN (other N/W).

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Network Management Centre

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NMC