ey also had the disadvantage that it was very easy for people with scanners to listen in to conversations, a fact that several people, including some who are very famous, discovered to their cost. Analogue systems require voice and control channels in each direction for their operation as they operate full duplex. This gives a total of four channels as shown in Figure 10.6. The voice channels use narrow-band FM as this gives good immunity to general noise and signal strength variations. The capture effect exhibited when receiving FM signals also gives some immunity from signals from distant cells, thereby reducing the possibility of intercell interference. The data channels use frequency shift keying to carry the information to monitor the status of the phone while it is idle (switched on but not in use), to initiate a call and to allocate the voice channel. Problems may be encountered with the signalling or data functions. Here multipath propagation, where the signal may reach the receiver via Cellular telecommunications 263 Forward control channel Call set-up and management Reverse control channel Mobile station phone Forward voice channel Voice channels Reverse voice channel Figure 10.6 Control and voice channels required for an analogue system different paths and at slightly different times, means that errors are often present. Corruption becomes worse when signal strengths are low. To overcome this various forms of error detection and correction are employed. In order that many subscribers can be serviced within each cell there are a number of channels that can be used. These are split into two types: those for transmitting to the base station from the mobile phone, and those transmitting from the base station to the mobile station. These are split into two bands and arranged SO that there is a sufficient separation between the transmitter and receiver signal frequencies. This is required because the same mobile handsets and base stations have to handle signals in both directions at the same time. It is obvi