 Figure 51.11 Frame and burst format of the INTELSAT TDMA system. RB1 and RB2 are the reference bursts from reference stations 1 and 2 respectively. The drawing is not to scale Applications 51/15 On board switched TDMA has become feasible in multi-beam spectrum. When these two factors, cost and spectrum availability, satellites like INTELSAT VI, using switch matrices which can have been taken into account there are four main areas of applica- operate within the TDMA frame to route one burst to down-link tion where satellite communication is already established or is beam A and the next burst to another down-link beam, B. becoming established and is likely to remain in significant use: The functioning of TDMA systems which make efficient use of the time dimension demands precise timing and complex control of 1. Trunk telecommunications. The global trunk network, linking access. Such systems may be costly. Where the traffic flowing fixed points at national centres and provincial centres all over through the system is light, much simpler systems which use prin- the world, carrying massive amounts of telephone, data and ciples first explored within the ALOHA system may provide adequ- video channels and all the other kinds of telecommunication ate availability. In these, the transmission path is normally open and facilities that can be conveyed by such channels, is routed an earth station with information to send verifies that no down-link mainly by terrestrial and submarine cable (increasingly optical burst from another earth station is in progress; it then transmits its fibre) and terrestrial radio relay systems. Such systems are very burst. However, several hundreds of milliseconds elapse before the competitive in cost, relative to the satellite medium, for short start of a signal from an earth station, sent via a geostationary distances. However, the distance between link terminals is satellite, can be received at another earth station. Two earth stations sometimes large, and for these long links the satellite med