9. Access protocols

9.1 Objective

ISDN protocols extend the concept of signaling by semaphore channel to the terminal equipment of subscriber installations. The D channel of the user-network access carries signaling related to the B channels of the same access.

The signalling implemented between the user and the network enables the connections necessary for the transfer of information between users to be established, and service complements to be implemented. Among these service complements, user-to-user signalling offers an additional means of transferring information between users.