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Introduction 5

Figurę 1.2 The architecture of a PLMN.

1.2.2.1 Mobile Station

GSM-PLMN contains as many MSs as possible, available in various styles and power classes. In particular, the handheld and portable sta-tions need to be distinguished.

1.2.2.2 Subscriber Identity Module

GSM distinguishes between the identity of the subscriber and that of the mobile equipment. The SIM determines the directory number and the calls billed to a subscriber. The SIM is a database on the user side. Physically, it consists of a chip, which the user must insert into the GSM telephone before it can be used. To make its handling easier, the SIM has the format of a credit card or is inserted as a plug-in SIM. The SIM communicates directly with the VLR and indirectly with the HLR.

1.2.2.3 Base Transceiver Station

A large number of BTSs take care of the radio-related tasks and provide the connectivity between the network and the mobile station via the Air-interface.

1.2.2.4 Base Station Controller

The BTSs of an area (e.g., the size of a medium-size town) are con-nected to the BSC via an interface called the Abis-interface. The