ring the control time slot period Figure 52.2 shows an example of the up-link and down-link time (which may be sub-divided into a number of signalling channels). frames for a system designed to operate at 19GHz (Ballance, 1984). If the signalling bursts from two or more OSs collide, no acknow- Each time slot includes a system overhead such as carrier recovery, ledgement is given in the down-link path. Requests are therefore symbol timing recovery, signalling, error checking and unique byte repeated after a random delay duration in order to decrease the identification. This is essential since all the OSs have sources probability of further collisions. operating at nominally the same frequency F2 Since the overhead bits for each time slot remains constant, the frame efficiency (i.e. 52.2.3 Ranging the ratio of total time for subscriber data to the frame time) can be improved by increasing the ratio of useful data to the overhead data. The burst transmissions from all the OSs, located at various dis- This can be achieved by increasing the frame period in the up-link tances, must arrive at the CS as non-overlapping bursts. A small direction. A very long time frame period can result in excessive guard time is generally allowed between adjacent bursts in order to transport delay which may be unacceptable to meet network re- cater for slight timing differences occurring in the OSs. When an os quirements for speech communication. The choice of time frame is first installed, it acquires a timing reference from the CS and period is a compromise between the frame efficiency, system capac- subsequently determines the time by which its transmission must be ity and ease of system implementation. adjusted. In one system (Ballance, 1984), when an os is first Urban multipoint systems 52/5 Central Outstation Sector station antenna Outdoor Outdoor unit (2 transceivers) Interface Interface Indoor Customer Telecom terminal Indoor network baseband TDMA access port card equipment System control computer (TAP) Figure 52.3 A simplified b