okes a service by keying in the appro- System X exchanges located in Birmingham, Manchester and Lon- priate sequence of digits delimited by and "#". Only the generic don, which were fully interconnected via their respective digital procedure has been specified and the sequence and digits for any main network switching centres with digital transmission links given supplementary service is network dependent. using CCITT No. 7 (BT) signalling. This was further extended to The feature key management protocol requires the network to more than 200 additional exchange areas in the major centres of hold a terminal or service profile for a given terminal. A given population. ISDN developments in the U.K. 44/11 This pilot service offered two new types of customer access, a 44.6.1 Local network transmission single-line access and a multi-line access which were structured as follows: A cable network has already been established in the local area to provide telephony to customers and this represents a large propor- 1. Single-line IDA: a 64kbit/s channel for speech or data; an tion of any PTO's capital assets. Although this cable has been 8kbit/s channel for data only; an 8kbit/s channel for signalling installed to carry telephony with a bandwidth of less than 4kHz, it only. Total 80kbit/s plus additional framing information. can be exploited to carry the wider bandwidth digital signals of and 2. Multi-line IDA: 30 X 64kbit/s channels for speech or data; 1 X ISDN basic access link. The bandwidths for the single line access 64kbit/s channel for signalling; 1 x64 kbit/s channel for framing will be of the order of 100kHz to 200kHz depending upon the digital and alarms. Total 2048kbit/s as for the PCM primary multiplex information rate and the modulation technique employed and as a structure. consequence will be subjected to much greater attenuation (up to 60dB) than the telephony signal (a maximum of 10dB or 15dB). During the period in which BT was specifying and developing Notwithstanding these high attenuations it is still feas