Performance and Engineering of Common Channel Signaling Networks Supporting ISDN

A fundamental aspect of ISDN is the use of common channel signaling (CCS) for the passing of control, transaction and user-to-user information. On ISDN access facilities this signaling capability is provided on a D-channel employing the Q.921 (LAPD) protocol at layer 2 and Q.931 layer 3. Within the network, signaling information is exchanged using the CCITT Signaling System No. 7 (CSS7). There are two standard types of ISDN access defined: the basic rate access consisting of two 64 kb/s bearer channels (B-channels) and one 16 kb/s D-channel. The D-channel can carry 's' type signaling packets, consisting of call control and user-to-user information, and 'p' type X.25 data packets. The signaling information is given priority over data.