em on 2Mbit/s links using CCITT G.703 interface provide DTMF receivers for interpreting the address signals from with the signalling information carried on a common 64Kbit/s DTMF extension telephones. When public exchange DTMF lines channel in timeslot 16 for the 30 voice or data channels. Synchroni- became available DTMF senders also had to be provided for the sation is provided by the one remaining channel in timeslot 0. regeneration of signals and the pulsing out of short code dialling DPNSS extends the facilities normally only available between stores. This meant that the PABX had all the elements within the extensions on a single PABX to all extensions on all PABXs that are system for fast call set up using DTMF signalling over the private connected together in the DPNSS private network. It specifies the network and it was merely a software task to reorganise the PABX signalling for a wide range of facilities for voice and circuit to make better use of these existing facilities. switched data calls. 34/10 PABX and key systems DPNSS is organised on the ISO 7 layer reference model with the existing call. Branching functions may occur at the originating, Physical Layer 1 and the Link Access Layer 2 virtually identical to terminating, transit, or gateways. As far as each call is concerned the the DASS2 equivalent layers. Layer 3 provides the basic messaging branching function is always accompanied by either an END or a for DPNSS between the PABXs and these messages are im- Transit function. plemented entirely in the software of the PABX. The DPNSS specification is divided into a number of sections Messages can be transmitted transparently through a PABX in the with 1 through to 6 covering the basic call set up and mandatory network if the destination address points to a PABX not linked interworking requirements. Sections 7 to 39 define supplementary directly to the PABX originating the call. When a call is set up on services and it is these that make the networking of PABXs a very DPNSS the originating PABX send