evel receive signal in the presence of a high plexer had to interface to the exchange via an existing 2048khit/s level transmit signal. interface. The multiplexer was therefore required to convert the When the transmit bursts of a number of systems in the same cable protocols of the 1.420 interface to those developed for the pilot are synchronised the problems of near end crosstalk are also ISDN as well as provide a transmission system capable of suppor- avoided. However, if systems on pairs with a long reach are mixed ting 144kbit/s over the local loop. Further contracts were placed for in cables with a short reach this advantage is to a certain extent the multiplexer SO that by the middle of 1991 BT had 90,000 lines nullified. A WAL, line code is used which ensures there is sufficient available throughout the country for service. timing information present and that no equalisation of the charac- In order to introduce these new services a number of new devel- teristic distortion of the line is required at the receiver. The system opments were required and these included: is therefore relatively simple and therefore cheap to implement. The disadvantage of the system is that because of the higher frequency 1. Digital transmission system for the local network. at which it operates it suffers high attenuation. This burst-mode 2. Message based digital customer-network signalling. system has a maximum loss of 34dB and will give a range of 3. New types of customer's Network Terminating Equipment with approximately 2.5km over 0.4mm copper pairs enabling 78% of standardised interfaces for the connection of data terminals. BT's customers to be connected directly to their local exchange. Exchange Subscriber 80 kbit/s 256 kbit/s 80 kbit/s Tx buffer Rx buffer 80 kbit/s 80 kbit/s 256 kbit/s Rx buffer Tx buffer Figure 44.10 Two wire duplex: burst mode transmission 44/12 The Integrated Services Digital Network Tx burst Rx burst Tx burst Rx burst Burst cycle period Figure 44.11 Cycle for burst mode transmission: (a) exchange; (b