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308 ISDN USER PARTFigure 11.5-3. End-to-end signaling, SCCP method.Call References. In SCCP end-to-end signaling, exchanges A and C have toknow each other’s point codes and need a mechanism to identify the call towhich a INF or INR message pertains. This is done by including a Par.5 (call reference)in the message. The parameter consists of a point code (PC) and a callidentity (CID).When exchange A sends its IAM for the call, it assigns a call identity (CID ¼ x)and includes a Par.5 with PC ¼ a and the assigned CID (Fig. 11.5-4).When exchange C receives the IAM, it associates A’s call reference with the call.It also assigns a call identity (CID ¼ y) and, in its first backward message, includes aPar.5 with PC ¼ c and the assigned CID. On receipt of the message, exchange Aassociates C’s call reference with the call.Exchanges A and C now know each other’s call references. Then, if exchange Aneeds to send an INR message, it includes a Par.5 with C’s call reference and passesthe message to its SCCP with an N-unitdata request, identifying the message destinationby DPC ¼ c. Likewise, exchange C responds with an INF message thatincludes a Par.5 with A’s call reference and passes it to its SCCP, identifying themessage destination by DPC ¼ a.