ISDN Call Flows

During a basic call flow, the calling party is the origination of the call and the called party is the destination of the call. The calling party sends a Setup message to the called party through the ISDN network. The calling party gets a Setup Acknowledgment and Call Proceeding from the local ISDN switch. The Setup Acknowledge is only used for overlap signaling. If en-bloc signaling is used, Call Proceeding is sent without a Setup Acknowledge. The remote ISDN switch sends a Setup message to the remote ISDN terminal and receives a Call Proceeding and Alerting message in response. The Alerting message travels through the network to the originating ISDN switch, and it sends an Alerting message to the calling party. Also in a backwards direction from the called party, a Connect statement is sent back to the calling party to indicate that the call has been connected and that a voice/data path now exists between them. Refer to Figure 8-14 for an illustration of a basic call setup procedure.

Figure 8-14 Q.931 Call Setup Procedure

Setup Setup

Setup Acknowledgement ->






Connect Connect

Connect Acknowledgement * <-

*Connect Acknowledgement is normally sent from the network back to the called party but can also be implemented to be sent from the calling party back to the network as well.

Figure 8-15 shows the call disconnect procedure. A typical call disconnect is rather simple. For instance, say that the calling party hangs up the connection first. In this case, the calling party sends a Disconnect message to the local ISDN switch. The ISDN switch enters a Disconnect Request state and begins clearing the B channels from the call to send them back into the pool of available B channels.

The local ISDN switch requests that the call be disconnected at the called party end and send a Release message to the calling party. The Release message is locally significant and does not mean that the called party has disconnected. Upon receiving the Release message, the calling party releases the B channels and sends a Release Complete to the local ISDN switch. The ISDN switch is then able to allocate the specified B channels for other calls.

The remote ISDN switch for the called party sends a Disconnect message to the called party's customer premises equipment (CPE). The remote call clearing happens in tandem with the local call clearing but is independent of it. The CPE disconnects and sends a Release message back to its local ISDN switch, which in turn, responds with a Release Complete. At this point, the call has been cleared on both sides, and the resources are available for use.

Figure 8-15 Q.931 Call Disconnect Procedure

Called Party Calling Party

Disconnect Disconnect <- <-

Release Release

Release Complete Release Complete






Call clearing happens in tandem on both sides of the network, and each side is independent from the other with the exception of the Disconnect message.

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