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GSM Architecture | GSM network architecture

The GSM architecture is categorized into MS, BSS and NSS. This page describes GSM network architecture on BSS and NSS sides. The GSM architecture network interfaces between elements are also mentioned. MS, BSS and NSS stand for Mobile Station, Base Station Subsystem and Network and Switching Subsystem respectively.

Introduction:
GSM is the short form of Global System for Mobile Communications. It is called 2G or Second Generation technology. There are various GSM standards such as GSM900, EGSM900, GSM1800 and GSM 1900 which uses different frequency bands for their operation. GSM uses TDMA/FDMA access technique. GSM 900 uses uplink frequency band from 890 to 915 MHz and downlink frequency band from 935 to 960 MHz. It uses bandwidth of 200 KHz.

GSM Network Architecture

gsm architecture

As we know GSM network is composed of three basic parts or subsystems viz. MS, BSS and NSS. BSS is referred as GSM access network and NSS is referred as GSM core network.

Mobile Station- This Mobile station is GSM mobile phone equipment which houses DSP,RF chip and SIM(subscriber Identity Module). This SIM is enough to carry to avail the service of GSM network. SIM contains subscriber related all the information, network with which subscriber is subscribed with and encryption related information.

Base station Subsystem- Base station subsystem houses Base Transceiver station-BTS and Base station controller-BSC. This subsystem take care of radio control related functions and provides GSM air interface for GSM mobile phones to connect with GSM network. To provide GSM service, region/city on earth is divided into various cells. The cell size is usually about 100m to about 35 km. BTS coverage is limited to this cell. Like this many BTSs cover entire region. All this BTSs are interfaced with one BSC in various ways mesh, star etc. This BSC takes care of radio frequency assignments to the mobile phones, takes care of handoff within BSS i.e. between one BTS and the other BTS.

Network Subsystem (NSS) - This subsystem provides interface between cellular system and circuit switched telephone network i.e. PSTN. It performs switching and operation & maintenance related functions. NSS takes care of call processing functions such as call setup, switching, tear down and also hand over between BSCs. NSS takes care of security and authentication related functions. There are various network elements in this subsystem as mentioned in GSM network architecture above. They are explained below. These are basically database elements viz. HLR, VLR, AUC and EIR as explained below.

There are different interfaces in GSM such as Air, Abis, A and SS7. Air interface is used between Mobile station and BTS. Abis interface is used between BTS and BSC. A interface is used between BSC and MSC. SS7 interface is used between MSC and PSTN.

GSM Architecture on BSS side

GSM architecture BSS side and NSS side

The GSM BSS resides between MS and NSS. It consists of one BSC (Base Station Controller) and one or more BTSs (Base Transceiver Stations). The functions of BSS are specific to radio access techniques which include communication with MSs (Mobile Stations), radio resource management etc. Radio equipments of BSS usually cover one or more cells. The figure depicts gsm architecture with various network interfaces between BSS and other gsm system components/subsystems.

BTS serves single cell and works based on instructions from BSC. The major functions of BTS include channel coding, encryption/decryption, transcoding and rate adoption, modulation, RF frequency conversion, RF signal amplification and so on. The interface between BTS and BSC is proprietary to their manufacturers.

BSC controls one or multiple BTSs. The major functions of BSC are to control and manage radio resources of GSM system, controlling of BTSs, take care of intercell handover scenarios, power control and so on. Refer Gsm Network Interfaces >>.

GSM NSS network architecture

GSM network architecture NSS

The NSS is core of the GSM architecture. It performs major functions of the system such as mobility management, subscriber data management, cell handling, switching, authentication, equipment validation etc. Due to its core functionalities, it is also referred as CN (Core Network). To perform above tasks, it has two entities viz. databases and switches. The database elements are HLR, VLR, AUC and EIR are as follows.

HLR-Home Location Register, it stores permanent and temporary subscriber related information.
VLR- Visitor Location Register, it stores visitor subscriber related information about its facilities, the network it is subscribed to, and its home location and so on.
AUC- Authentication center, used to authenticate activities in the system. It holds encryption (A5 key) and authentication keys (A3 key)in both HLR and VLR.
EIR- Equipment Identification Register, it helps in security as it keeps track of equipment type available in Mobile Station or Terminal.

Switches within NSS include MSC (Mobile Switching Center) and GMSC (Gateway Mobile Switching Center) which provides switching services. As shown, GMSC is interfaced with PSTN, ISDN and PSDN (i.e. internet) in order to provide different services.



Resources on GSM basics

This GSM tutorial covers following sub topics in order to understand gsm basics.
Gsm tutorial  Frame  Burst types  PHY  gsm stack  speech processing  ARFCN  Channel types  GSM Security  Network Entry  Mobile basics  GSM Mobile originated call flow  GSM Mobile Terminated call flow  GSM downlink packet switched data call/a>  GSM uplink packet switched data call 

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