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4 GSM Networks: Protocols, Terminology, and Implementation

Figurę 1.1 The radio coverage of an area by single cells.

•    The network has to be kept informed of the approximate location of the mobile station, even without a cali in progress, to be able to deliver an incoming cali to that mobile station.

•    The second and third items require extensive communication between the mobile station and the network, as well as between the various network elements. That communication is referred to as signaling and goes far beyond the extent of signaling that fixed networks use. The extension of Communications reąuires a cellular network to be of modular or hierarchical structure. A single central Computer could not process the amount of information involved.

1.2.2 An Overview on the GSM Subsystems

A GSM network comprises several elements: the mobile station (MS), the subscriber identity module (SIM), the base transceiver station (BTS), the base station controller (BSC), the transcoding ratę and adaptation unit (TRAU), the mobile services switching center (MSC), the home location register (HLR), the visitor location register (VLR), and the eąuipment identity register (EIR). Together, they form a public land mobile network (PLMN). Figurę 1.2 pro-vides an overview of the GSM subsystems.