ible to provide equipment for its pilot ISDN, work began in earnest within CCITT full duplex transmission over the single telephony pair of wires by to define a set of recommendations relating to ISDN. At quite an the adoption of suitable modulation techniques. early stage in the CCITT studies it became apparent that the emerg- Two main techniques are used for providing digital transmission ing standards for signalling in an ISDN would be significantly over the local network, the burst mode technique and a more different from BT's initial ISDN access arrangements. The notable complex echo cancelling technique. differences arose from the CCITT requirement to support two 64kbit/s B channels and a 16kbit/s D channels for signalling and for the interface to support a passive bus arrangement which enabled up 44.6.1.1 Burst-mode technique to eight terminals to compete for access to either B channel. BT were anxious to gain experience of the CCITT standard In its pilot service BT adopted the burst mode technique operating interface at the earliest opportunity. Consequently, following the at 80kbit/s because of its simplicity. In this technique illustrated in publication of the CCITT Red Book in 1984, BT began to specify Figures 44.10 and 44.11, the 80kbit/s input data is loaded into a an ISDN basic rate access arrangement using the I-Series interface buffer and then clocked out into the cable pair in bursts of 22 bits described by recommendation 1.420. BT placed a contract for the (20 information bits and 2 marker bits) at an increased rate of development of a multiplexer and a network termination which 256kbit/s. The interval between the bursts of data, 250us, is such supported the 1.420 interface. Although initially based on the 1984 that there is ample time to receive bursts of data from the remote Red Book recommendations the interface specification was kept in transmitter. Full duplex transmission is therefore achieved without alignment with changes made to the recommendations. The multi- having to identify a low l