n of the call, either another mobile or a control office. The control software and circuitry within the base station and the central control processing area for the network sets up the network SO that a channel is allocated for the audio (the traffic channel). It also sets up the switching in the network to route the call to the required destination. To enable the mobile station to receive a call, it is paged via the incoming control channel data stream to indicate that there is an incoming call. Channels are allocated and switching set up to provide the correct routing for the call. There is no method to 'handover' the mobile from one base station to the next if it moves out of range of the base station through which a call is being made. In this way the system is not a form of cellular telephone. It is therefore necessary for the mobile station to remain within the service area of the base station through which any calls are being made. The control channel signalling structure has to be defined SO that all mobiles know what to expect and what data is being sent. Signalling on the forward control channel is nominally continuous with each slot comprising 64-bit codewords. The first type is the control channel system codeword (CSCC). This identifies the system to the mobile radio units and also provides synchronization for the following address codeword. 242 Newnes Guide to Radio and Communications Technology TSC to mobile radio units 1 slot Address Address Address codeword codeword codeword Mobile radio Address unit response codeword Figure 9.1 The control channel signalling structures As mentioned the second type of word is the address codeword. It is the first codeword of any message and it defines the nature of the message. It is possible to send data over the control channel. When this occurs, both the CSCC and the address codewords are displaced with the data appended to the address codeword. The mobile radio unit data structure is somewhat simpler. It consists fundamentally of synchronism bits followed by t