GSM Architecture: Understanding the 2G Network

GSM, or Global System for Mobile Communications, is a globally accepted standard for mobile communication. It ensures interoperability and seamless roaming across different countries and networks. The GSM system architecture marks a significant milestone in the evolution of mobile communication networks.

Introduced in the early 1990s, the GSM architecture provides voice calls, text messages, and data services over a cellular mobile network. It’s also known as 2G, or Second Generation technology.

2G GSM System Architecture

Let’s dive into an overview of the 2G architecture, examining the network interfaces that formed the foundation of cellular mobile communication networks.

There are various GSM versions, including GSM900, EGSM900, GSM1800, and GSM1900, which use different frequency bands for operation. The GSM system supports FDMA/TDMA access schemes. For example, the GSM 900 system supports the 890 to 915 MHz band for the uplink and the 935 to 960 MHz band for the downlink, providing 25 MHz in each direction.

This 25 MHz bandwidth is divided into 124 data carriers and 1 guard carrier, each having a 200 KHz band. All these RF carriers carry eight time slots each.

As we know, the GSM network is composed of three basic parts or subsystems:

• Mobile Station (MS)
• Base Station Subsystem (BSS)
• Network and Switching Subsystem (NSS)

The BSS is often referred to as the GSM access network, while the NSS is known as the GSM core network.

Mobile Station

The Mobile Station is the GSM mobile phone equipment, housing the DSP, RF chip, and SIM (Subscriber Identity Module). The SIM is all that’s needed to access the GSM network services.

The SIM contains all subscriber-related information, the network to which the subscriber is subscribed, and encryption-related information. During the network entry process, the Mobile subscriber, or Mobile Station, is assigned one channel frequency and one time slot by the Base Station Subsystem. These resources (frequency, time) are used by the mobile phone to communicate with the BSS.

Base Station Subsystem

The Base Station Subsystem houses the Base Transceiver Station (BTS) and the Base Station Controller (BSC). This subsystem handles radio control functions and provides the GSM air interface for GSM mobile phones to connect with the GSM network.

To provide GSM service, a region or city is divided into various cells. The cell size usually ranges from about 100 meters to about 35 kilometers. The BTS coverage is limited to this cell. Many BTSs cover an entire region in this way.

All these BTSs are interfaced with one BSC in various ways, such as mesh, star, etc. This BSC manages radio frequency assignments to the mobile phones and handles handoffs within the BSS (i.e., between one BTS and another BTS).

Network Subsystem (NSS)

This subsystem provides the interface between the cellular system and the circuit-switched telephone network (PSTN). It performs switching and operation & maintenance-related functions. The NSS handles call processing functions such as call setup, switching, tear down, and handover between BSCs. It also manages security and authentication-related functions.

There are various network elements in this subsystem, as mentioned in the GSM network architecture above. They are explained below and are basically database elements: HLR, VLR, AUC, and EIR.

There are different interfaces in GSM, such as Air, Abis, A, and SS7.

• The Air interface is used between the Mobile Station and BTS.
• The Abis interface is used between the BTS and BSC.
• The A interface is used between the BSC and MSC.
• The SS7 interface is used between the MSC and PSTN.

Architecture of GSM at BSS side

The GSM BSS resides between the MS and NSS. It consists of one BSC (Base Station Controller) and one or more BTSs (Base Transceiver Stations).

The functions of the BSS are specific to radio access techniques, including communication with MSs (Mobile Stations), radio resource management, etc. Radio equipment of the BSS usually covers one or more cells.

The figure depicts a gsm architecture diagram with various network interfaces between the BSS and other gsm system components/subsystems.

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

The BSC controls one or multiple BTSs. The major functions of the BSC are to control and manage radio resources of the GSM system, control BTSs, handle intercell handover scenarios, power control, and so on.

2G Architecture at NSS side

The NSS is the 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 to as the CN (Core Network).

To perform the above tasks, it has two entities: databases and switches. The database elements are HLR, VLR, AUC, and EIR as follows.

• HLR (Home Location Register): Stores permanent and temporary subscriber-related information.
• VLR (Visitor Location Register): Stores visitor subscriber-related information about their facilities, the network they are subscribed to, and their 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): Helps in security by keeping track of equipment types available in the Mobile Station or Terminal.

Switches within the NSS include the MSC (Mobile Switching Center) and GMSC (Gateway Mobile Switching Center), which provides switching services. As shown, the GMSC is interfaced with PSTN, ISDN, and PSDN (i.e., the 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 a vital role in supporting essential voice and basic data services in areas with limited infrastructure or resources.