) subscriber 44.6.1.2 Echo cancelling technique 2, although based upon the HDLC structure the use of the address and control fields was different. Similarly the structure of the layer The echo cancelling technique is now recognised as the standard 3 messages and their defined sequences was different. technique by most PTOs and is used by BT in its National ISDN The introduction of DASS for multi-line access to PABXs, the Service launched in 1991. Using this technique it is possible to development of digital PABXs and the use of digital leased circuits transmit and receive simultaneously. Figure 44.12 illustrates the to form digital private networks, led to a need for a digital inter- principles of the more complex echo cancellation system. A hybrid PABX signalling system. BT and a number of U.K. PABX manu- with a compromise balance provides some initial reduction of the facturers therefore collaborated on the definition of a signalling transmit signal passing into the receiver, but inevitably the transmit system based upon DASS but which has been enhanced to meet the signal will still be much larger than the receive signal on the longer inter-PABX signalling requirement. This has been called the Digital lines. An adaptive network therefore models the echo path response Private Network Signalling System (DPNSS). between the transmitter and receiver which results from the imper- fect hybrid and the reflections on the line. The signal generated is During the definition of DPNSS it became apparent that it was then subtracted from the receiver input to leave the true receive line desirable to align more closely certain of the messages at level 3 of signal. The adaptive network derives its signal by minimising the DASS with those of DPNSS. At the same time proposals were being residual echo components on the wanted receive signal, hence the made to enhance the repertoire of messages at level 3 in order to system's name. provide more facilities to the user. An enhanced version of DASS No. 1 was therefore defined, called