Download

11.2 ISUP MESSAGES, FORMATS, AND PARAMETERS 283for overlap address signaling. The IAM then holds the initial digits of the calledaddress, and the remaining digits are transferred in one or more SAMs.Continuity Message (COT). This message is sent forward, during call setup, ifthe outgoing exchange has made a continuity test on an outgoing trunk. It indicateswhether the test has been successful.Address Complete Message (ACM). This is a backward message. When sentby the terminating local exchange, it indicates that the exchange is ringing the calledsubscriber or has received a Q.931 ALERT message from a terminal on the calleddigital subscriber line (DSL). When sent by an intermediate exchange, it indicatesthat the exchange has seized an outgoing trunk that does not have ISUP signaling.Call Progress Message (CPG). This is a backward message sent by a terminatingexchange that has already sent an ACM message, when it needs to report theoccurrence of an “event” in the call setup.Answer Message (ANM). This is a backward message sent when the called partyanswers.Connect Message (CON). This message is sent on ISDN calls on receipt of aconnect indication from the called party when ACM has not been sent. It is equivalentto ACM followed by ANM.Release Message (REL). This is a general message that requests the immediaterelease of a connection. It can be sent forward or backward and includes—like theQ.931 DISC (Disconnect) message—a Cause parameter that indicates the reason forand the originator of the release. In other interexchange signaling systems, therelease of a connection is normally initiated by the calling subscriber. In ISUP signaling,the calling and the called ISDN users can initiate a release.A REL message is also sent by an intermediate exchange, or by the terminatinglocal exchange, when the setup of a call in not possible.Release Complete Message (RLC). This is a forward or backward message,sent by an exchange in response to a received REL message for a trunk. It indicatesthat the exchange has completed the release of the trunk at its end. The RLC messagehas the same function as the Release-Guard message of TUP signaling.Suspend Message (SUS). This is a backward message that is similar tothe Clear-Back message of TUP. It requests to suspend the call, but to leave theconnection intact.Resume Message (RES). This message requests to resume a suspended call.