e the establishment of personal communications net- works (PCN) over the next few years in the 1.7GHz to 1.9GHz band may well pose a challenge to multipoint radio systems providing fixed services. The envisaged mass markets for these networks will inevitably lead to hardware price reductions and reflect in lower telecommunications costs to the subscribers. Manufacturers of multipoint systems may adapt the cheaper hardware, for instance, to Number of circuits provide complete exchange to subscriber premises radio solutions. Figure 52.9 Cost per circuit as a function of number of To date, most multipoint radio systems have aggregate rates of circuits less than 20Mbit/s. The synchronous digital hierarchy (SDH) multi- plexer offers flexible 1.5Mbit/s or 2Mbit/s drop and insert facility together with maintenance/management protocols at 155.22Mbit/s 52.4.1 Transport delay considerations (STM-1) bit stream (see Chapter 42). Therefore designs of multi- point radio systems with STM-1 aggregate data rate may well Point-to-multipoint systems introduce group delay due to the pac- become attractive in future (Richman, 1990). ketised nature of the system. Thus the round trip delay of a system The role of satellites in rural communications is mentioned in is the sum of the up-link and down-link frame periods plus addi- passing. Interest in this has increased with the recent announcement tional processing delay which could be 10% to 30% of the frame of plans for Iridium, a digital voice/data system in the 1GHz to delays. In addition, where synchronised repeaters are used to in- 2GHz range providing global coverage via 77 low orbiting, net- crease the system range, the propagation delay of about 3.3 micro- worked satellites (Satellite Communications, 1990). In such a sys- seconds per kilometre also becomes significant since the maximum tem a LOS path would be possible to every point on the earth's delay corresponds to that of an outstation located at a maximum surface. However, it will be late nineties before service commences di