PSTN Fallback Scalability

Examples 8-13 and 8-14 in the preceding section describe a simple network in which the remote site router acts as the terminating gateway for the voice conversation. The IP address of the remote site router and the network congestion values of the link between the remote and host router are held in the cache of the host site router. When the host site router receives the digits 12 followed by 2 additional digits, the host cache is consulted to determine whether the call can be successfully placed.

As the network becomes more complex, such as with the addition of IP telephony, it becomes necessary to designate a single terminating gateway to represent a number of IP destination devices. Consider the example illustrated in Figure 8-16. There are a large number of IP Phones at Site 6, each one having a unique IP address.

If Site 1 calls an IP Phone at Site 6, the cache at Site 1 does not need to contain an entry for each separate IP destination at Site 6. All IP call destinations at Site 6 can be mapped to the IP address of the WAN edge router at Site 6 so that a single probe from Site 1 to Site 6 can be used to probe CAC information for all calls destined to Site 6. The same principle applies if there were multiple terminating gateways at Site 6.

The probe traffic can therefore be reduced substantially by sending probes to IP destinations that represent the portion of the network that is most likely to be congested, such as the WAN backbone and WAN edge. This same scalability technique also provides a mechanism to support IP destinations that do not support SAA responder functionality.

Figure 8-16 PSTN Fallback Scalability

Figure 8-16 PSTN Fallback Scalability

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