ring of the signalling arrangement by means of which a high level of accessi- B channels can also indicate problems on the D channel, for bility and good transmission quality are achieved. example erratic traffic on the B channel may point to problems on Common to all systems is the use of a cellular structure for the the D channel signalling protocol. Performance testing can uncover optimal re-use of the available frequency channels. There are how- day to day operating problems that might otherwise pass inter-oper- ever significant differences in the signalling techniques and mess- ability testing. age formats used. With this approach, each area served by a base station is called a cell, and these cells are arranged to cover the ground in a regular 61.5.7.4 Protocol trouble-shooting pattern. The service area of each station is roughly circular, but is normally drawn as a hexagon. If a problem is discovered protocol trouble-shooting often finds All the base stations are linked into a wired network which the cause of the problem. ISDN defines new ways of performing switches calls to a mobile as it moves about; the cellular arrange- routine tasks such as making a phone call. In order to trace the flow ment has effectively made the radio link part of the network as short of protocol events, for a call being placed, a protocol analyser is as possible. Figure 61.20 shows a typical 7 cell cluster. often needed. Protocol analysers interpret protocol events by dis- For testing purposes cellular radio systems are divided into two playing to the user an 'English' breakdown of the protocol events major groups. Those systems which employ digital signalling and that have occurred on the line (see Figure 61.19). The events analogue speech transmission are generally referred to as analogue displayed on the screen are usually marked with some form of cellular systems, and systems which use both digitised signalling identification indicating which device the message was received and digitised speech transmission are referred to a