Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
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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:
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."
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