Endto End Signaling

End-to-end signaling procedures establish, maintain, and release connections. They also enable signaling endpoints to exchange information using the Information Request (INR) and Information (INF) messages, which are detailed in the "ISUP Call Control Messages" section later in this chapter.

You exchange signaling capabilities between the originating and terminating endpoints using call indicators. The originating end office indicates its signaling capabilities in the forward call indicators field of the Initial Address Message (IAM). The terminating end office indicates its capabilities in the backward call indicators field of the Address Completion Message (ACM), ANswer Message (ANM), or Call Progress (CPG) message.

ISUP uses two methods for passing end-to-end signaling: the pass-along method and the SCCP method.

In the pass-along method, signaling information travels from the originating switch to each intermediate switch until it reaches the terminating end office. The initial setup information uses the same path used for all subsequent information relating to this circuit.

The SCCP method is an alternative to the pass-along method. In this case, ISUP uses the SCCP to route signaling information through the network. The signaling path does not have to be the same for messages related to a particular circuit. SCCP enables ISUP messages to be routed directly from the originating to the terminating end office.

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