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
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.