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

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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 the RLC can be sent immediately after receiving a REL.

Continuity Testing

When a call is set up across multiple exchanges, continuity testing is performed independently on each leg of the call. If a call traverses three trunks across four different exchanges and continuity is done on a statistical basis, it will likely only be performed on some of the trunks involved in the call. While the actual continuity test is performed independently on each call leg, the end-to-end call setup is dependent on each leg passing the test. If a continuity test is successfully performed on the second leg of the call (SSP B to SSP C), the results are not reported until the COT results have been received from the previous leg of the call (SSP A to SSP B). If a previous leg of the call connection cannot be set up successfully, there is no need to continue. For example, if SSP A reports a COT failure, it would attempt to establish a new connection in the forward direction by selecting another circuit to set up the call. There is no need to continue the previous connection from SSP B to SSP C because the new call attempt from SSP A will come in as a new origination to SSP B.

Transporting Parameters

A tandem node can receive ISUP parameters that are only of interest to the destination exchange. This is particularly true of many optional parameters, which are passed transparently in the outgoing messages across tandem nodes. However, the tandem might update some fields during call processing, based on new information encountered while processing. For example, a tandem node that selects an outgoing ISUP facility over a satellite connection would update the NOC Satellite Indicator field in the outgoing IAM. This distinction is made because the tandem node might be required to have knowledge of how to process some parameters, but not others. When parameters are passed across a tandem node without processing the information, it is sometimes referred to as "ISUP transparency." Since the parameters do not need to be interpreted by the tandem, they are considered transparent and are simply relayed between the two trunks.