r facility programming. In order to 64kbit/s capability of the ISDN and these terminals are now coming facilitate interworking to V-series terminals and to provide a tele- onto the market. They feature high definition pictures with various phony capability, a range of terminal adapters were made available options and are able to transmit an A4 size document in 4 to 5 to connect onto the X.21 port of the network terminal. The new seconds over a 64kbit/s channel. national ISDN service based upon the CCITT recommendations required a new network terminal (NT1) which has been developed 44.7.5 Terminal adapters to support the 1.420 interface directly. A range of customer premises terminal equipment is now being developed on the open market Terminals with non-ISDN standard interfaces will continue to exist which includes Terminal Adapters (TA), PC plug in cards, Group 4 for some time. Terminal adapters which support the 1.420 interface facsimile machines etc. Before these can be connected to the net- on the network side and a range of non-ISDN standard interfaces on work however they have to go through a formal approval procedure the terminal side are therefore becoming widely available. They and be approved against a European Commission standard Net 3. support such interfaces as the standard analogue telephony inter- face, the V.24 asynchronous interface, various versions of the X.21 interface and the V.35 leased line interface. ISDN applications The extension of the 64kbit/s digital capability of the network 44.7.6 PC Terminals together with the provision of a much more powerful signalling capability to the customer opens up the opportunity for new services Personal Computers (PC) are currently being enhanced to support and applications together with an improvement in the performance access to the ISDN via 1.420 interface cards. These are seen as one of some existing services. of the main sources of ISDN terminals because of their ability to support a wide range of different software applications. Cards are therefore 