6). Applications include low bit rate data transmission and facsimile. Other applications include access to ISDN and videocon- 52.3.1 System operation ferencing at various bit rates. The use of high capacity radio systems has been studied for emergency use in situations where, for instance, an exchange serving several thousand subscribers is destroyed by The operation of a typical rural TDM/TDMA system is illustrated fire or other disasters (Nakayama, 1985). in Figure 52.6. Its operation is similar to that of an urban multipoint system. The system comprises a central station (CS) with a wide beamwidth antenna (e.g. omni-directional antenna), and a number Rural multipoint systems of subscriber outstations (OSs) which have directional horn or yagi antennas. For rural applications, telephony is the main requirement and sys- The CS is generally located at or near a telephone exchange. The tems are generally required to operate over long distances, often OSs, with line of sight (LOS) paths to the CS, are usually pole over hostile, mountainous or marshy terrain. Consequently several mounted and serve a number of subscribers over a distance of a few systems have been designed to operate in the 1.5GHz to 2.5GHz kilometres via overhead or buried cable pairs. Outstation Repeater Central station (local exchange) Outstation Outstation Figure 52.6 Rural multipoint system operation (James, 1990) 52/8 Point to multipoint urban and rural radio For telephony, the system effectively provides a transparent path channels as and when needed. Naturally the whole process is trans- between the telephone exchange and a telephone instrument at a parent to the subscriber. subscriber's premises. Typically the telephony signals from the exchange are encoded in PCM format (A or u law) by the CS, and 52.3.3 Channel access and ranging these are then time division multiplexed (TDM) before being trans- mitted in the down-link direction as a modulated signal at one In urban multipoint systems the path distances are relatively short frequency F in 