5.2 ISDN Protocol Layers
5.2.4 ISDN call setup
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.