The interactions between the user and the network are all carried by messages in the D channel. These can be categorised into various groups

• Call Establishment
• Call Clearing
• Call Information
• Miscellaneous

The user at one or other end of the potential call negotiates with the network via his D Channel.

A successful call set-up between the two users will normally establish an end-to-end connection across the network on a B Channel.

Upon completion of the call, the disconnection procedure will also take place on the D Channel.

An understanding of the steps involved are essential for analysing problems encountered during call establishment.