n voice terminal, a route to another signalling system this over a data link between the two PABXs. Each PABX is or a conference bridge. The end function may be provided by an equipped with an asynchronous signalling circuit which converts originating PABX, a gateway to another signalling system, a gate- the PABX processor bus into a serial signalling output that can be way from another signalling system, a terminating PABX, or a connected to a modem. The modem is used to convert the digital conference PABX. A branching function may occur when some signals from the PABX into analogue signals to run on the analogue supplementary services result in a new (related) call sharing a tie line between the PABXs. It is selected to run at a low speed 1200, channel on part of its path from the originating PABX with the 2400 baud etc. depending upon the private circuit available between PBX A DPNSS Network PBX B Ringing tone Ringing Answer Conversation phase Busy tone Figure 34.9 DPNSS call set up messages Line circuits 34/11 the two PABXs and the expected message traffic for the number of 34.5.3 Digital line circuits speech channels between the PABXs. The voice traffic is carried on separate private circuit tie lines that Digital line circuits take many forms depending upon the trans- do not have any signalling system and are kept open at all times. mission system employed between the telephone and the line circuit Availability of circuits and feature transfer is controlled on the and within the PABX and key system environment many of the inter-PABX signalling channel via the modems with messages systems are proprietary, being designed for the most cost effective identical to those used in DPNSS. arrangement. If the system is 4-wire then one pair will be used for transmit and one for receive and the circuits at either end are connected with transformers. CCITT have now defined a system 34.5 Line circuits that works on a 4-wire basis between the terminal and the line circuit (in CCITT 1.420/430) which operates to the S refer