res positive and negative acknowledgement signals to be link carries the total signalling load for 4000 traffic circuits. 43/10 Signalling systems Traffic circuit group Exchange A Exchange B Switchblock Switchblock Error Error control control Switch Signalling Signalling software Switch software Data link 64Kbit/s Input/ signalling Signalling input/ output terminal terminal output modules hardware hardware modules Signalling link Others Others parts Message Transfer Part parts Figure 43.9 CCITT No. 7 C.C.S. system generalised arrangement The polynomial for the error check is given by expression 43.1. 43.3.2 User-network signalling The CRC (sometimes called FCS, frame check sequence) is a 16 bit field. Digital Subscriber Signalling No.1 (DSS 1 is the CCITT name for the N-ISDN access protocols. (43.1) In N-ISDN, the network is extended down to the user's location, terminating in a Network Termination (NT). The NT forms the interface between the user's Terminal Equipment (TE) signalling Exchange Exchange and the network signalling and performs the signalling conversion. The CCITT regulates for a concept of S and T reference points at the interface (CCITT, 1988f). The main types of user-network capabilities recommended by the Digital traffic CCITT are (CCITT, 1988g): circuit group 1. Basic access 2B + D. 2. Primary rate access 30B + D (30-channel PCM networks) or C.C.S. link 1 23B + D (24-channel PCM networks). B is a 64kbit/s traffic channel; D the signalling channel (16kbit/s in basic access, 64kbit/s in primary rate access). C.C.S. link 4 43.3.2.1 Basic access signalling C.C.S. 4 link Information TE-NT and NT-TE is transferred in 48 bit 250 micro- signalling second frames in each direction (CCITT, 1988h; CCITT, 1988i). module Each frame contains four traffic octets (two B1 and 2 B2), four outslot D signalling bits giving the 16kbit/s signalling channel, plus Figure 43.10 Common channel signalling module other information. 4-wire operation applies on a passive bus. principle The signalling in the two directions NT-