(usually having 120-degree sectors). The major functions of the base station are transmission of Ater' Figure 1.4 GSM architecture SECOND-GENERATION MOBILE NETWORKS (GSM) signals in the desired format, coding and decoding of the signals, countering the effects of multi-path transmission by using equalisation algorithms, encryption of the data streams, measurements of quality and received signal power, and operation and management of the base station equipment itself. Base Station Controller (BSC) This controls the radio subsystem, especially the base stations. The major functions of the base station controller include management of the radio resources and handover. It is also responsible for control of the power transmitted, and it manages the O&M and its signalling, security configurations and alarms. 1.4.2 Network Subsystem (NSS) The network subsystem acts as an interface between the GSM network and the public net- works, PSTN/ISDN. The main components of the NSS are MSC, HLR, VLR, AUC, and EIR. Mobile Switching Centre (MSC) The MSC (or switch as it is generally called) is the single most important element of the NSS as it is responsible for the switching functions that are necessary for interconnections between mobile users and other mobile and fixed network users. For this purpose, MSC makes use of the three major components of the NSS, i.e. HLR, VLR and AUC. Home Location Register (HLR) The HLR contains the information related to each mobile subscriber, such as the type of subscription, services that the user can use, the subscriber's current location and the mobile equipment status. The database in the HLR remains intact and unchanged until the termination of the subscription. Visitor Location Register (VLR) The VLR comes into action once the subscriber enters the coverage region. Unlike the HLR, the VLR is dynamic in nature and interacts with the HLR when recoding the data of a particular mobile subscriber. When the subscriber moves to another region, the database of the subscriber is also shifted to the V