s in service or has been taken out of service. The software will also be expected to log all 1. Support for high user densities. the problems encountered and generate trouble reports e.g. that a 2. Good subjective speech performance. particular circuit has been seized numerous times but released 3. High reliability and grade of service. immediately without call completion, indicating a faulty circuit 4. Data capability. which may be, for instance, the private tie line. This indication can 5. High security. be used to pass messages to the outgoing call handling software to 6. Support for user mobility. remove the circuit from service and raise an alarm. Self adjusting frequency allocation and re-use. In environments such as city business centres with adjacent multi- Data transmission storey buildings, fully equipped with cordless business systems, traffic densities of the order of 10000 Erlang per square kilometre The PABX is not the ideal mechanism for data transmission as it is per floor are required. Good subjective speech quality comparable designed around the speech bandwidth of 4kHz and in digital to that of wired terminals is required at the cordless terminals, and switching uses circuit switched digital signals at 64kbit/s, which is the performance of the trunk network is often used as the yardstick. not the ideal medium for LANs running at 10Mbit/s or FDDI at Cordless business systems must be designed SO as to provide 100Mbit/s. However, for the occasional data user, where small acceptable grade of service (GoS) with respect to both call set up amounts of data are transferred from one point to the other, it can and forced call curtailment. Call set up GoS is a blocking phe- make an economical medium. PABXs have therefore concentrated nomena as in conventional switches, whereas forced call curtail- in this area of the data switching market, providing such facilities ment GOS is a phenomena of cordless systems only due to such as modem pools on the PABX to which users can have dial up interference of adjace