Mobile Network Protocols of GSM and GPRS

Structure of the GSM Network

The user terminals of the mobile networks are the Mobile Stations (MS). An MS consists of the mobile phone itself (Mobile Equipment – ME) and a smart card identifying the subscriber (Subscriber Identity Module – SIM). Each SIM card has a worldwide unique identifier, the International Mobile Subscriber Identity (IMSI). The IMSI consists of a Mobile Country Code (MCC), a Mobile Network Code (MNC), and a Mobile Subscriber Identity Number (MSIN). Each MS has a telephone number referred to as Mobile Station ISDN Number (MSISDN).

A GSM network can be functionally divided into three subsystems:

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  Base Station Subsystem (BSS),

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  Switching and Management Subsystem (SMSS),

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  Operation and Maintenance Subsystem (OMSS).

The BSS is the physical equipment used to give radio coverage to cells and contain the equipment needed to communicate with the MSs at the radio interface. The SMSS includes the functions required to handle signaling protocols by which calls and mobility of the subscribers are controlled. The OMSS performs functions related to system security (authentication and mobile equipment identity check) as well as it is in charge of remote operations and maintenance of the PLMN (ETSI, 1998). The description of the Operation and Maintenance Center (OMC) is out of scope of this chapter. The SMSS extended with the Authentication Center (AuC) and the Equipment Identity Register (EIR) from the OMSS is often referred to as Network and Switching Subsystem (NSS), thus we will use this term as well (Figure 1).

Figure 1.

GSM architecture and protocols

Base Station Subsystem

The Base Station Subsystem provides connections between the MSs and the GSM network through the radio interface. A BSS consists of the following elements:

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  Base Transciever Stations (BTS),

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  a Base Station Controller (BSC),

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  a Transmission and Rate Adaptation Unit (TRAU).

A BTS is the radio transmitter and receiver itself. A BSS can have up to 16 BTXs, controlled by a BSC. Apart from controlling the BTSs, the BSC performs a switching function between the traffic channels incoming from the mobile switch and the radio channels. Since the bit speed of the speech transmission is different at the radio interface and in the core network, we need a TRAU to perform the necessary conversion.