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Basic Rate Interface 139

NOTE

All fields are one byte except for the Address field, which has two bytes. Data is variable in
length.

ISDN Call Setup

The setup procedure for ISDN calls is very similar to that of other circuit switched technologies.
It begins with a request, which is acknowledged. The acknowledging switch then forwards the
setup request on to the next switch in the line, and so on. Once the called party is reached, a
connect message is sent, which also must be acknowledged. Figure 6-7 depicts the ISDN call
setup procedure.

Figure 6-7

ISDN Call Setup

Prior to the actual Connect and the Call Proceeding (CALL PROC) messages, there can be a
number of different progress messages indicating call progress. For instance, the calling party
sends setup messages to the switch. The switch responds with the Setup Acknowledgement
(SETUP ACK) and Call Proceeding messages. The remote switch then sends the setup message
to the called party, which acknowledges with a CALL PROC message. Alerting messages can
then be sent, although they are optional, depending on carrier implementation. Alerting messages
are normally associated with voice traffic and are not usually implemented in data calls.

Connect messages flow from the called party to the calling party when the connection is
established and can be followed by a Connect Acknowledgement (CONNECT ACK), which is
also optional. Once the calling party receives the CONNECT ACK, the call setup is complete.

Calling

party

Setup

acknowledge

Call

proceeding

Alerting

Connect

Connect
acknowledge

Called

party

Setup

Call proceeding

Alerting

Connect

Connect
acknowledge

Provider switch network