| An ISDN call can be placed in numerous
ways. In an ISDN call, the calling party requests a call setup, as
shown in the Figure. Before the actual connect and call proceeding,
you might see several different messages, such as progress, which
indicate how your call is proceeding. This progress message is
optional.
The same is true for the alerting
message, which is typical of telephone messages, but is not
required. Alerting messages are not typical with data transmissions.
Most of the time, you see connection messages with data calls. You
may not see the connect acknowledge, but it is valid. The messages
depend on how successful the call and completion acknowledgments are
implemented.
Different ISDN switches use different
call setup and teardown procedures. Depending on the switch type,
you may or may not get all the steps shown in the Figure. At a
minimum, call proceeding, alerting, and connect should be exchanged.
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