its each of which the last 8 bits are error international specification is framed as a common vehicle, admit- check bits. ting variants for national application. The signalling bit rate is 4. SUs grouped into blocks of 12, the 12th and last SU of each 64kbit/s time slot 16 30-channel p.c.m. Figure 43.9 shows the No.7 block being an acknowledgement, i.e. the number of the block general arrangement. in the other direction being acknowledged and the error control acknowledgements, positive that the relevant SU has been received error free; negative indicating error detected and thus 43.3.1.1 User parts a request for a re-transmission of that SU. Blocks are numbered sequentially, but individual message SUs No.7 adopts a functional concept (Figure 43.9), allowing change to in the block are not, being identified by their position in the a function without significant impact on others (CCITT, 1988c). block. User parts (telephony TUP, data DUP, etc.) are service dependent parts of the signalling system controlling message requirements for Digital network signalling 43/9 N ISDN N ISDN exchange A exchange B ISPBX (typical) Primary rate access 30B + D (or 23B + D) DSS multi line signalling Basic access Basic access 2B + D 2B + D User-network User network signalling signalling DSS single line DSS single line Inter-exchange common channel signalling CCITT No.7 (or variants) SPC = Stored programme control ISPBX = Integrated services PBX User - user signalling DSS = Digital subscriber signalling system N-ISDN= Narrowband integrated services digital network Figure 43.8 N-ISDN signalling requirements particular services. This allows system evolution for additional returned from the receive end. The former clears the signal message services by additional User Parts, permitting the s.p.c. processor to from the retransmission store at the transmit end on correct receipt; be a common utility from the signalling aspect. Appropriate coding the latter causes the signal message and all other signal messages of a bit field in the signal m