p into 47.2.3 Location registration a number of small areas called cells, with one radio base station (BS) positioned to give radio coverage of each cell. Each base station is When a mobile is not engaged in a call, it tunes to the signalling connected by a fixed link to a mobile services switching centre channel of the cell in which it is located and monitors the L-M (MSC), which is generally a digital telephone exchange with special signalling information. As the mobile moves around the network, software to handle the mobility aspects of its users. Most cellular from time to time it will need to retune to the signalling channel of networks consist of a number of MSCs each with their own BSs, and another cell when the signal from the current cell falls below an interconnected by means of fixed links. The MSCs interconnect to acceptable threshold. the public switched telephone network (PSTN) for both outgoing When the mobile retunes in this way, it reads the overhead infor- calls to, and incoming calls from fixed telephones. Figure 47.1 mation broadcast by the new cell and updates the operating par- shows a typical network arrangement. ameters as necessary. It also checks the location information being To other MSCs To PSTN Figure 47.1 Cellular network configuration 47/4 Cellular radio systems Mobile registers new location Location area 1 Location area 2 Path of mobile Location register Figure 47.2 Mobile location registration broadcast by the new cell and, if this differs from the previous cell, number. As part of the call set up procedure, the network will the mobile automatically informs the network of its new location by validate the mobile requesting the call to ensure that it is a legitimate means of an interchange on the signalling channel (Figure 47.2). By customer. Many networks incorporate specific security features to means of this location registration procedure, the network is able to carry out this validation. keep updated a database of the location area of all mobiles. This When the network receive