GLOBAL SYSTEM FOR MOBILE COMMUNICATION

- An Introduction

CONTENTSGo Mobile - Go CellularEvolution Of GSMMain Features Of GSMServices Provided By GSMGSM IdentitiesGSM Network ArchitectureNetwork InterfacesNetwork ProtocolsThe Air Interface

Go Mobile - Go CellularLimited Frequency SpectrumIn wireless communication a pair of RF (Radio Frequency) needed per active call.For a 5 MHz band for a operator, only 25 channels available for use.This way service can be offered to 200 subscribers only.To overcome this limitation, zones of coverage are created, which are known as CELLS.

Evolution Of GSM1982 : the Groupe Spcial Mobile (GSM) formed within the Conference of European Posts and Telegraphs (CEPT) to study and develop a pan-European public land mobile system. 1989 : GSM responsibility transferred to the European Telecommunication Standards Institute (ETSI).1990 : Phase I of the GSM specifications published.1991 : Commercial service started.1994 : Phase II specifications released.GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.

Main Features Of GSMCellular StructureFrequency ReuseHand - OffsRemote Monitoring and Control Of Cell - SitesEase Of Use to the End - UserSeamless Mobility

Services Provided By GSMBearer ServicesGSM users can send and receive data, at rates up to 9.6 kbps, to users on POTS), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Group 3 facsimile, as described in ITU-T recommendation T.30, supported by use of an appropriate fax adapter.SMS

Tele servicesBasic TelephonyEmergency ServiceSupplementary ServicesCall ForwardingCall barring of incoming or outgoing callsCaller IdentificationCall waitingMulti party conferencing

GSM IdentitiesIMSI (International Mobile Subscriber Identity)

Mobile Mobile Network HLR Mobile Subscriber Country Code Code identification Number MSISDN (Mobile Station ISD Number)

Country Code National HLR Subscriber Number Destination Code

MSRN (Mobile Station Roaming Number)

Country National Characteristic of the MSC in Code Destination Code which the subscriber is located

IMEI (International Mobile Equipment Identity)

Type Approval Final Serial Number(6) Spare(1) Code(6) Assembly Code(6)

CGI (Cell Global Identity)

Mobile Country Mobile Location Area Cell Identity Code Network Code Code

LAI (Location Area Identity)

GSM Network Architecture

MS (Mobile Station)Mobile Equipment + Subscriber Identity Module (SIM)Mobile Equipment uniquely identified by IMEIMobile clamps on to GSM network through a BTSSIM card contains:Serial NumberIMSIAuthentication KeyPIN and PUK numbersNetwork Code etc.

BSS (Base Station Subsystem)

BTS (Base Transceiver Station) + BSC (Base Station Controller)BTS:Houses the radio transceiversHandles radio link protocols with the Mobile StationConnects to BSC via an Abis interface (2Mbps)Transmits and receives voice over air interface at 13 kbps with the Mobile StationCommands mobiles to set Tx. Power, hand-offs.BSC:Connects the BTS to the Mobile Switching Center (MSC)Manages the radio resources for one or more BTSHandles radio-channel setup, frequency hopping, hand-offs and release of channels.Connects to the MSC on A interface.Contains entire database for all cell parameters of the associated BTS.

TRAU (Transcoder / Rate Adaptation Unit)MSC based on ISDN switching.ISDN has rate of 64 Kbps.Mobile communication is at 13 Kbps.TRAU converts data rates between 13 Kbps GSM rate to 64 Kbps standard ISDN rate.

OMC (Operations and Maintenance Center)The central monitoring and remote maintenance center for all network elements.Provides remote access to all network elements.Monitors the performance of each network element.There are two typesOMC-S: Deals with the switch, i.e. the MSCOMC-R: Deals with the radio network.

NSS (Network Sub System)MSC (Mobile Switching Center)Acts as the Switching Node for the GSM networkRegistration and authentication of the subscribersLocation UpdatingInter BSC Hand offsCall routing to roaming subscribers

HLR (Home Location Register)Contains all administrative information of each subscriber of GSM networkContains the current location of the mobile, in form of the signaling address of the VLR associated with the mobile station.

VLR (Visitor Location Register)Contains selected administrative information from the HLR of the subscribers active in its geographical area.The information is the one required for call control and provision of the subscribed services.

EIR (Equipment Identity Register)It is the mobile equipment database which has a series of IMEI.MSC uses this database to classify the Mobile Station.IMEI are classified into three categories.White List : This contains the IMEI of approved mobiles.Grey List : This contains the IMEI of mobiles for which further evaluation is required.Black list : IMEI of barred mobiles.

Authentication Center (AuC)Used for verification of SIM by the MSCSecret data and verification process algorithm stored at AuC

Network Interfaces

Network Protocols

The network subsystem uses the Mobile Application Part (MAP), built on top of the SS7 protocol.

Signaling protocol structured into three general layers, depending on the interface.

Layer 1 is the Physical Layer.

Layer 2 is the Data Link Layer.

Across the Um interface, this layer is a modified version of the LAPD protocol used in ISDN, called LAPDm.

Across the A interface, the Message Transfer Part Layer 2 (MTP2) of SS7 is used.

Layer 3 is itself divided into 3 sub layers:

Radio Resource Management: Controls the setup, maintenance and termination of radio and fixed channels, including hand-offs.

Mobility Management: Manages the location updating and registration procedures, as well as security and authentication.

Connection Management: Handles general call control, manages Supplementary services and SMS.

Signaling between different entities of NSS (HLR, VLR) accomplished through Mobile Application Part (MAP).

The Air InterfaceSeparate Bands for Uplink and DownlinkUplink (MS to BTS) : 890 915 MHzDownlink (BTS to MS) : 935 960 MHzGSM uses combination of Time and Frequency Division Multiplexing (TDMA/FDMA)FDMA achieved by dividing 25 MHz bandwidth into 124 frequency channels for GSM900Each channel of 200KHzTDMA is achieved by dividing each of this channel in time so as to handle 8 mobiles per channel.

A Final ThoughtWe know what we are, but not know what we may be.Hamlet, Act IV, Scene V