service (GoS) of the system. The GoS depends on the number of available timeslots or trunks and the offered traffic in Erlangs (E) The rural multipoint systems generally provide transparent 2-wire per subscriber. Traffic intensity is a dimensionless quantity ex- analogue telephony connections between telephone instruments and pressed in Erlangs. If a timeslot is occupied all the time its maxi- an exchange, although traffic is usually concentrated and de-con- mum capacity is 1E. The loss probability, p, of a system is given by centrated digitally over the radio path. The two wire connection the Erlang loss formula, Equation 52.2, where y is the offered traffic gives maximum flexibility but is expensive in terms of line card in Erlangs and n is the total number of circuits. costs. Therefore consideration has been given to provision of 1.5Mbit/s or 2Mbit/s interfaces from the CS to the exchange (Chas, (52.2) 1986). At the CS end 2Mbit/s links between radio and telephony sections have also been used to provide flexibility in siting the radio section remote from the telephony section (De Couesnongle, 1987). The system design generally permits extension of 2-wire analogue lines a few kilometres from an OS. Some of the systems have additional facilities such as intra-call and automatic ring back mode This is tabulated, for instance, in CCITT, 1989a. Further informa- (Beaupre, 1984). In systems with intra-call facility there is intel- tion on loss probability for network connections is given in CCITT ligence in an os to recognise when an outgoing and the correspond- (1989a). ing incoming call involve two subscribers who are served by the As an example, for a typical busy hour offered traffic per sub- same OS. scriber of 0.07E, the GoS for a system with 15 trunks and 116 Once the call is established via the exchange, the signall- subscribers from equation 1 is equal to 0.01. ing/metering connection to it via the CS is retained over the man- agement or control time slot, but the os connects the two 52.3.5 subscribers direc