IP Destination Caching

Unlike AVBO, PSTN fallback does not require the static configuration of the IP destinations. The software keeps a cache of configurable size that tracks the most recently used IP destinations to which calls were attempted. If the IP destination of a new call attempt is found in the cache, the CAC decision for the call can be made immediately. If the entry does not appear in the cache, a new probe is started and the call setup is suspended until the probe response arrives. Therefore, an extra postdial delay is imposed only for the first call to a new IP destination. Figure 8-15 illustrates these possible scenarios.

Figure 8-15 PSTN Fallback Call Setup y y y

Call Setup

Cache entry for IP address found; CAC values are acceptable.

Cache entry for IP address found; CAC values are acceptable.

Call Setup

Call Setup

Cache entry for IP address found; CAC values not acceptable and no secondary dial-peer exists.

Call Reject / Hairpin

Call Setup

Cache entry for IP address not found. SAA Probe

SAA Response

Call Setup

Call Setup

Figure 8-15 demonstrates three possible scenarios. In all scenarios, a call setup message is send to router 1 (R1). R1 consults its cache to determine whether a path exists and, if so, that the ICPIF or delay/loss thresholds have not been exceeded. In scenario one, both conditions are true and the call setup message is forwarded to router 2 (R2) to connect the call. In scenario two, a path to the IP destination is found in cache; however, the ICPIF or loss/delay exceed the threshold for that path and the call is either rejected or hairpinned back to the origination PBX, depending on the interface type connecting the PBX with R1. In scenario three, a path to the IP destination is not found in cache. An SAA probe is sent to the IP destination to determine the ICPIF or loss/delay values. If the response shows that the thresholds have not been exceeded, the call setup message is forwarded on to router 2 (R2) to connect the call.

After an IP destination has been entered into the cache, a periodic probe with a configurable timeout value is sent to that destination to refresh the information in the cache. If no further calls are made to this IP destination, the entry ages out of the cache and probe traffic to that destination is discontinued. In this way, PSTN fallback dynamically adjusts the probe traffic to the IP destinations that are actively seeing call activity.

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