 on 16kHz signals. a LAN connected to the PABX. 34.3.4 2Mbit/s digital access Private networking 2Mbit/s digital access is presented to the PABX as a coaxial circuit carrying 30 channels for voice or data, a synchronisation channel on Many PABXs are interconnected by private networks carrying both timeslot 0 and a signalling channel on timeslot 16. In the UK voice and data traffic. Access to these networks is provided by tie DASS2 is the signalling system employed on the primary rate ISDN line circuits in the PABX which provide the signalling and intercon- service. The signalling system is based on the ISO 7 layer reference nection between the PABX and the transmission network of the model with Layer 1 using a CCITT G.703 2.048Mbit/s interface private system. Some private networks are not point to point and the utilising HDB3 line coding. The PABX must be designated as the PABX must then act as a tandem exchange interconnecting the tie slave as far as synchronising to the network is concerned and this is line circuits and providing a digit translation facility, to enable the achieved via timeslot 0. user to dial the least number of digits to get from one point to the The messaging between the PABX and the public exchange is via other. timeslot 16 using Link Access Protocol D (LAPD) which is de- With the deregulation of telecommunication services the PABX signed for end-to-end signalling over ISDN and is an extension of operator is able to offer 'break out' facilities over the private the ISO High Level Data Link (HDLC) control procedure. network. When the user dials a PSTN number, the PABX will LAPD is a frame orientated protocol with the information it interpret this number. If it is more economical to send it over the carries consisting of messages that are delimited by flags. Error private network, the call will be routed to the nearest geographical correction of information frames is by re-transmission until correct point in the private network to the required PSTN number, normally or until a timeout expires.