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328 SIGNALING IN CELLULAR MOBILE TELECOMMUNICATIONS12.1.1 DefinitionsA cellular mobile network (CMN) provides communication services for mobilestations that are operating in its service area. A service area typically covers ametropolis and its surrounding suburbs, or a number of medium-sized cities. Thesize of a service area is typically in the range of 100–4000 square miles.The CMN service area is divided into a number of MSC areas, each of which containsan exchange known as a mobile switching center (MSC)—see Fig. 12.1-1.A MSC provides service to all MSs in its area. Some rural CMNs consist of justone MSC area. A MSC area is divided into a number of location areas, and eachof these areas is divided into a number of cells. The cells are approximately circular,with radii that range from about 2 to 15 miles. Each cell has a base station (BS)—also known as land station and cell site—which houses radiofrequency (RF) transmittersand receivers.A MSC has trunk groups (TGs) to nearby exchanges in the public switched telecommunicationnetwork (PSTN) (also known as the “fixed” or “wireline” network)and a base station trunk group (BSTG) to each base station—Fig. 12.1-2(a). When aCMN has several MSCs, there are also trunk groups (MSCTGs) between theseMSCs—Fig. 12.1-2(b).A MS operating in a cell communicates on a RF channel with the BS of the cell.There are two channel types: voice channels and control channels.Voice Channels. In a base station, each BS trunk is permanently wired to thetransmitter and receiver of a RF voice channel. The combination of a BS trunkand its associated voice channel is the functional counterpart of a trunk in thePSTN. A BS trunk and voice channel is assigned to a mobile at the start of a calland released when the call ends. Figure 12.1-3 shows the connection for a callFigure 12.1-1. Cellular mobile network. (From EIA/TIA 553. Reproduced with permissionof TIA.)