 DASS No. 2, which would 44.6.2 Signalling enable PABXs to have a common type of signalling system for both inter-PABX and PABX to network signalling. The common features of DPNSS and DASS enable them to be interleaved on a common The introduction of a pilot ISDN in the U.K. required the specifica- tion of a new digital signalling system since no CCITT recommen- signalling link such as time-slot 16 in a 2048kbit/s multiplex struc- dations on digital access signalling protocols were available at the ture. time. A new signalling system DASS (Digital Access Signalling The use of DASS 2 has now been superseded by the new 1.440/1 System) was therefore defined. As with the current CCITT recom- and 1.450/1 recommendations (Q921 and Q931) on the basic rate mendations the structure of DASS is based upon the levels of the ISDN service. However because of the maturity of the DASS and ISO (International Standards Organization) model for OSI (Open DPNSS specifications they continue to be used on the primary rate Systems Interconnection). The main differences being that at layer service in the U.K. for a number of years. It is expected that a new Exchange Subscriber Hybrid Adaptive Hybrid Replica network signal Error signal Replica signal Adaptive network Error signal Figure 44.12 Echo cancellation ISDN applications 44/13 primary rate service based on the CCITT recommendations will be at 2Mbit/s they are now in use at 384kbit/s and 128kbit/s and introduced by the end of 1993. therefore can be supported on a standard ISDN basic access inter- face. 44.6.3 Network terminal equipment 44.7.4 Facsimile terminals For BT's pilot service a network terminal was developed with two Current analogue facsimile terminals conforming to the Group 3 X.21 ports supporting both circuit switched and leased circuit vari- standard can take between 1 and 3 minutes to transmit an A4 size ants for synchronous data rates up to 64kbit/s. They also incorpor- page. A new Group 4 standard has been agreed that can exploit the ated a V.24 port for use only fo