tly, thereby freeing two trunks or time slots for Channel plans other traffic. This facility increases the traffic carrying capacity. It is also possible to have intra-call facility even when the link to Most of the rural multipoint systems above 1GHz are designed to the CS is temporarily unavailable. Information on local calls during operate in the frequency band 1427MHz to 1525MHz, and use an this period may be stored for a limited time and transferred to the inter-leaved frequency plan. The channel spacing between adjacent CS when the link is restored. In systems with automatic ring back channels on the same polarisation depends on the channel bit rate facility, a subscriber attempting to make a call when all the circuits and the modulation method used. CCIR, (1990) gives channel on the radio system are in use, is notified of this fact. Subsequently, arrangements for systems operating in 2.3GHz to 2.5GHz band. For the subscriber is offered the opportunity to make a call as soon as a rural multi-carrier FDMA/TDMA BEXR systems in the 150MHz free circuit is available, thereby reducing outgoing lost calls. and 450MHz bands, channel spacings of 30kHz and 25kHz respec- Some of the systems designed for rural and urban applications tively have been specified (Bellcore, 1989). also allow access to ISDN (Bonnerot, 1987; Le-Ngoc, 1989) at 144kbit/s (2B+D). These connections can be at U or S interface in 52.3.6 System details the CS and OSs. Within the radio system, the entire capacity of 144kbit/s per call is not necessarily allocated all the time to the Table 52.3 shows details of some of the multipoint systems currently subscribers. Instead the B and D channels may be processed separ- on the market or under development. These systems use either ately SO as to use the radio capacity more efficiently, providing B binary or four level modulation, and system aggregate rates vary Table 52.3 Examples of multipoint systems System 150MHz 450MHz 1.5GHz 2.5GHz 500MHz 1.5GHz 800MHz 1.5GHz 1.5GHz 1.5GHz 1.4GHz 2.3GHz 1.5GHz 450MHz