[ LiB ]

Examining ISDN Call Setup and Teardown

ISDN call setup and teardown reflect the activity of the Layer 3 Q.931 protocol. While an ISDN call is being set up, a number of messages are exchanged between the called and calling parties that identify the progress of a call setup.

Call Setup Process

As you can see in Figure 6-2, the called party requests a call setup. Some steps and messages that are displayed might not necessarily be a part of your particular call setup. It depends on the type of switches used in the exchange and their requirements.

Call Teardown Process

The teardown of a call may be initiated by either party. However, the switch handles the proceedings.

First, the Disconnect message is transmitted on the D channel. After the switch receives the Disconnect message, it starts the release of the B channel circuit and sends a Release message to the downstream switch. The involved switches eventually transmit the Release message to the final switch.

To make sure the call is being disconnected properly, each foregoing switch starts a T12 timer. It expects to receive a Released message from the neighbor switch, upon which it issues a Release Complete message back to the neighbor. If the Release Complete isn't received within the timer period, the Release message is reissued.

Keep in mind as you are consulting Figure 6-3 that call teardown is handled very rapidly throughout the network.

[ LiB ]