ISDN exchange, which may be on a 2-wire user access line, adopts the HDLC protocol with References 43/11 opening and closing flag delimiters and error check by a CRC bit The network is informed by the calling user that the user-to-user field. signalling facility is a requirement. This requirement is included in A command and response technique is recommended, a command the call set-up message from the caller. signal message being discarded if checked as error. The command signal is transmitted continuously within a certain time period and ceased by receipt of an error free response. CCITT, 1988j gives the 43.4 References detail of the various signal messages. CCITT (1988a) Specifications of signalling system R2, Blue Book Vol. VI Fasc. VI.4, Recomms. Q.350 - Q.368, ITU, Geneva. 43.3.2.2 Primary rate access signalling CCITT (1988b) Specifications of signalling system No.6, Blue Book Vol.VI, Fasc. VI.3, Recomms. Q.251 - Q.300, ITU, Geneva. Here, the ISPEX (or LAN etc.) incorporates the NT function, the CCITT (1988c) Introduction to CCITT signalling system No. 7, Blue access being 4-wire to the N-ISDN exchange. The D signalling Book Vol. VI. Fasc. VI.7, Recomms. Q.700, ITU, Geneva. channel is 64kbit/s time slot 16 in 30-channel PCM (time slot 24 in CCITT (1988d) No.7 functions and codes of the signal unit fields, 24-channel PCM). Common channel signalling is adopted, the Recomms. Q.703, item 2.3. system serving the 30 B traffic channels (23 B in 24-channel PCM). CCITT (1988e) No.7 error control, Recomms. Q.703 items 4-6. The signal messages, CRC checked, are in sequence numbered CCITT (1988f) ISDN user-network interface - reference configu- HDLC frames delimited by opening and closing flags. Frames may rations, Blue Book, Vol.111 Fasc. 111.8, Recomms. I. 411, ITU, be retransmitted continuously until an error free acknowledgement Geneva. of correct reception is received from the other end of the access link. CCITT (1988g) ISDN user-network interfaces, - interface struc- CCITT, 1988k, gives the detail of the various s