ng methods (Table 52.3). At gency service restoration, provision of SCADA (supervisory control the CS, the phase of the carrier from each OS burst has to be and data acquisition), and data services in rural areas. The more established rapidly. To facilitate this, the os burst has a preamble recently developed systems offer facilities for connecting sub- for carrier recovery. However, when differential demodulation is scribers to ISDN. With the higher capacity systems, the OSs are used, it is not necessary to recover the carrier. This consideration has capable of serving a large (> 100) number of subscribers. Therefore, led to the use in some systems of differential demodulation at the it is possible to consider the advantages of installing a multipoint CS, while coherent demodulation is generally used at the OSs. os rather than a small exchange (Blake, 1989). Combined with an Recent BEXR systems have used 16 level differential phase shift intra-call option, the os then behaves effectively as a small ex- keying modulation (Saunders, 1988; Mullen, 1989). Others de- change. signed for similar applications have also used 4-, 8-, and 32-level quadrature amplitude modulation in narrower channels of 25kHz (Hampton, 1989; McGuire, 1989). 52.4 Economics of urban and rural systems 52.3.8 Power Generally, urban systems can potentially be high revenue earning In a rural environment lack of primary source of power is quite a systems since they would be used for serving the needs of business dominant feature. When power is available it may be subject to subscribers. On the other hand, rural systems give a poor return on interruptions. Therefore subscriber radio systems are designed with investment, and the decision to employ them is often a political one. the aim of minimising power consumption SO that, for instance, The cost per circuit depends on the system configuration which, solar power operation is possible. The radio equipment generally in turn, depends on subscriber distribution and terrain features requires either a.c