Early ACM
After outgoing side ISUP sends IAM, it expects ACM and start T7 timer. If no ACM receives in T7 duration, the call will be released.
Once incoming side ISUP receives IAM, it will start T11 timer. Once T11 expires, and ISUP can only determine the address is complete and it cannot determine the status of the called party (e.g. no paging response), it wil send an Early ACM message to previous exchange. In Early ACM message's Backward-call-indicators IE, called-line-status is set to "No indication". After sending Early ACM, once it can determine the called party's alerting, it will send a CPG message with "alerting" indication.
本文探讨了早期应答计费消息(ACM)在ISUP中的应用过程。当发送IAM消息后,若未在T7定时器周期内收到ACM,则会释放呼叫。接收方在T11定时器到期且无法确定被叫方状态时,会发送早期ACM消息,设置被叫线路状态为“无指示”。一旦确定被叫方处于振铃状态,将发送带有振铃指示的CPG消息。
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