Message Timers

Like other SS7 protocol levels, ISUP uses timers as a safeguard to ensure that anticipated events occur when they should. All of the timers are associated with ISUP messages and are generally set when a message is sent or received to ensure that the next intended action occurs. For example, when a REL message is sent, Timer T1 is set to ensure that a RLC is received within the T1 time period.

ITU Q.764 defines the ISUP timers and their value ranges. In Figure 8-3, part A includes the timers for the messages that are presented for a basic call. The "Continuity Test" section of this chapter discusses the timers associated with the optional COT message. Following are the definitions of each of the timers in the figure:

• T7 awaiting address complete timer— Also known as the network protection timer. T7 is started when an IAM is sent, and is canceled when an ACM is received. If T7 expires, the circuit is released.

• T8 awaiting continuity timer— Started when an IAM is received with the Continuity Indicator bit set. The timer is stopped when the Continuity Message is received. If T8 expires, a REL is sent to the originating node.

• T9 awaiting answer timer— Not used in ANSI networks. T9 is started when an ACM is received, and is canceled when an ANM is received. If T9 expires, the circuit is released. Although T9 is not specified for ANSI networks, answer timing is usually performed at the originating exchange to prevent circuits from being tied up for an excessive period of time when the destination does not answer.

• T1 release complete timer— T1 is started when a REL is sent and canceled when a RLC is received. If T1 expires, REL is retransmitted.

• T5 initial release complete timer— T5 is also started when a REL is sent, and is canceled when a RLC is received. T5 is a longer duration timer than T1 and is intended to provide a mechanism to recover a nonresponding circuit for which a release has been initiated. If T5 expires, a RSC is sent and REL is no longer sent for the nonresponding circuit. An indication of the problem is also given to the maintenance system.

We list the timers for the basic call in part A of Figure 8-3 to provide an understanding of how ISUP timers are used. There are several other ISUP timers; a complete list can be found in Appendix H, "ISUP Timers for ANSI/ETSI/ITU-T Applications."