TAILIEUCHUNG - Signaling System No.7 Protocol Architecture And Sevices part 24

ISUP-ISUP Tandem Calls Previous scenarios have focused on line-ISUP and ISUP-line calls. ISUP processing at a tandem switch occurs in the same sequence as the line to ISUP calls we discussed previously. | ISUP-ISUP Tandem Calls Previous scenarios have focused on line-ISUP and ISUP-line calls. ISUP processing at a tandem switch occurs in the same sequence as the line to ISUP calls we discussed previously. However in the case of ISUP-ISUP calls the trigger for call processing events on the originating and terminating side are incoming ISUP messages. This section discusses the following three areas that are related to ISUP processing at a tandem node ISUP Message Processing Continuity Testing Transporting Parameters ISUP Message Processing at a Tandem In Figure 8-17 the call origination at SSP B is based on an incoming ISUP origination IAM from another exchange. The fields that are necessary for number translation such as CdPN are extracted from the IAM and used to process the call at the tandem node to determine the outgoing destination. The translation and routing process results in the selection of an outgoing ISUP trunk. An IAM is sent in the forward direction to SSP C updating fields in the message as necessary. For example a new CdPN might be inserted as a result of translations. The NOC field is updated based on information such as whether a satellite is being used for the voice circuit or whether a continuity check is being performed. Figure 8-17. ISUP-ISUP Tandem Calls View full size image When the ACM and ANM are received at SSP B they are propagated to SSP A updating fields such as the BCI as necessary. Each leg of the call cuts through the speech path in the same manner discussed in the Detailed Call Walk-Through section of this chapter. When SSP A sends an REL message SSP B responds with an RLC. It does not need to wait for the RLC to be sent from SSP C. Next SSP B sends an REL to SSP C and waits for RLC to complete the release of that leg of the call. Keep in mind that even though some messages in a multi-hop ISUP call are propagated the entire call actually consists of independent circuit segments. The release procedure is a reminder of this fact because

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