ile stations continuously listen to a common channel of the net- work in order to receive a call. Figure 5.1 presents the basic elements of a simplified cellular network. BSs are radio transmitter/receivers by which the mobile stations (MSs, such as telephones) are connected to the wire-line network. The BSs are connected to the mobile switching center (MSC) by primary rate digital connections. The MSC acts as a local exchange in the fixed network. In addition to the switching and other functions of an ordinary telephone exchange, the MSC also keeps track of the subscribers' locations with the help of location regis- ters. We discuss this equipment in the following section. Note that all cellular networks are designed to act as access networks. Their main purpose is to make mobile subscribers accessible from the global (fixed) telecommunications network. The mobile cellular networks always rely on a fixed network. They have no switching hierarchy similar to that of a fixed network (see Chapter 2) and international calls are connected via a fixed network. Mobile Communications Public Land Mobile Network station station Mobile switching To public switched center station telephone network station Primary rate 2 or 1.5 Mbps digital connection station Figure 5.1 Basic structure of a cellular radio network. 5.2 Structure of a Cellular Network This section reviews the structure of a general cellular network. The detailed structure of a cellular radio network, the terminology of network elements, and their detailed functions are dependent on the network technology in question. 5.2.1 Cellular Structure Instead of covering an entire area with high-power fixed radio stations, the way older generation radio systems had to, the area of a cellular network is divided into small cells of only a few kilometers or less across as shown in Figure 5.2. Areas where subscriber density is high are covered by smaller cells than areas where subscriber density is low. The power BSs and MSs are auto- matically decreased with the decreased c