Global System for Mobile Communication

Mobile networks implemented in different countries were usually incompatible. This incompatibility made impracticable the roaming of mobile users across international borders. In order to get around this system incompatibility, the Confe´rence Europe´enne des Postes et Te´le´communications (CEPT) created the Groupe Spe´cial Mobile1 committee in 1982. In 1989, the European Telecommunications Standard Institute (ETSI) took over the responsibility for the maintenance and evolution of GSM specifications. In 2000, this responsibility was transferred to 3GPP. With GSM, the transfer of data can be carried out over circuit-switched

GSM Architecture
The GSM network is composed of three subsystems: the Base Station Subsystem (BSS), the Network Subsystem (NSS), and the Operation Subsystem (OSS).

Mobile Station
The Mobile Station (MS) is a device that transmits and receives radio signals within a cell site. The mobile station is composed of the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). The unique International Mobile Equipment Identity (IMEI) stored in the ME identifies uniquely the device when attached to the mobile network.



Base Transceiver Station
The Base Transceiver Station (BTS) implements the air communications interface with all active MSs located under its coverage area (cell site). This includes signal modulation/demodulation, signal equalizing, and error coding. Several BTSs are connected to a single Base Station Controller (BSC).

Base Station Controller
The BSC supplies a set of functions for managing connections of BTSs under its control. Functions enable operations such as handover, cell site configuration, management of radio resources, and tuning of BTS radio frequency power levels. In addition, the BSC realizes a first concentration of circuits towards the MSC. In a typical GSM network, the BSC controls over 70 BTSs.

Mobile Switching Center and Visitor Location Register
The Mobile Switching Center (MSC) performs the communications switching functions of the system and is responsible for call set-up, release, and routing. It also provides functions for service billing and for interfacing other networks.
The Visitor Location Register (VLR) contains dynamic information about users who are attached to the mobile network including the user’s geographical location. The VLR is usually integrated to the MSC.
Through the MSC, the mobile network communicates with other networks such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), Circuit Switched Public Data Network (CSPDN), and Packet Switched Public Data Network (PSPDN).

Home Location Register
The Home Location Register (HLR) is a network element containing subscription details for each subscriber. An HLR is typically capable of managing information for hundreds of thousands of subscribers. In a GSM network, signaling is based on the Signaling System Number 7 (SS7) protocol.
The use of SS7 is complemented by the use of the Mobile Application Part (MAP) protocol for mobile specific signaling. In particular, MAP is used for the exchange of location and subscriber information between the HLR and other network elements such as the MSC. For each subscriber, the HLR maintains the mapping between the International Mobile Subscriber Identity (IMSI) and the Mobile Station ISDN Number (MSISDN).