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What is GSM Architecture | Network architecture of GSM (2G)

GSM is a globally accepted standard for mobile communication, which ensures interoperability and seamless roaming across different countries and networks. The GSM (Global System for Mobile Communications) system architecture is a milestone in the evolution of mobile communication networks. It has been introduced in early 1990s. The GSM architecture offers voice calls, text messages and data services over a cellular mobile network. It is also known as 2G or Second Generation technology.

2G GSM System Architecture

Let us understand overview of 2G architecture with its network interfaces that is used as foundation of cellular mobile communication network. There are various GSM versions viz. GSM900, EGSM900, GSM1800 and GSM 1900 which uses different frequency bands for their operation.

GSM system supports FDMA/TDMA access scheme. GSM 900 system supports 890 to 915 MHz band in the uplink 935 to 960 MHz band in the downlink providing 25 MHz in each direction. This 25 MHz bandwidth is divided into 124 data carriers and 1 guard carrier each having 200 KHz band. All these RF carriers carry eight time slots each.

gsm architecture,2G architecture diagram

As we know GSM network is composed of three basic parts or subsystems viz. Mobile Station (MS), Base Station Subsystem (BSS) and Network and Switching Subsystem (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. Mobile subscriber or Mobile Station has been assigned one channel frequency and one time slot during network entry process by Base Station Subsystem. These resources (freq, time) are used by mobile phone to communicate with BSS.

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.

Architecture of GSM at BSS 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 diagram with various network interfaces between BSS and other gsm system components/subsystems.

GSM system architecture at BSS side

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 >>.

2G architecture at NSS side

The NSS is core of the 2G 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.

Network architecture of GSM at NSS side

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.

Conclusion: Overall, the 2G GSM system architecture laid the foundation for subsequent generations of mobile networks, offering improved voice quality, international roaming capabilities and data services (SMS, MMS, Mobile internet access, social media interactions, banking and mobile payments, mobile apps, conference calling etc.). As technology advanced, subsequent generations like 3G, 4G LTE and 5G NR (New Radio) have emerged to provide faster data speeds and enhanced capabilities. But 2G GSM continues to play vital role in supporting essential voice and basic data services in the areas with limited infrastructure or resources.

2G GSM Technology related links

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