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Call Setup

In this section you will:

• See the step-by-step process for establishing and disconnecting an ISDN call

• See the exchange of messages between Terminal Equipment and the Network on

both sides of a call.

• Understand how several devices attempting to answer a call are managed

Network Services

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.

Making a call

The following sequence of diagrams shows the process when a user requests a call

through an ISDN network.

• The SETUP is a request to create a connection by the user on the left to a user on

the right. The SETUP message usually includes at least the following information:

o The called number (the destination number)

o The call type (Bearer Capabilities)

o It may also include other information, such as the calling number (the origin

of the call).

• The network chooses a B channel for this half of the B channel and uses the

SETUP_ACK to tell the user which B channel it is.

• The network sends the originating user a SETUP_ACK. This only acknowledges the

SETUP. It carries no information about the outcome of the call.

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Eicon Networks ISDN NT1 Specifiche Pagina 33