Inter-Gateway Alternate Routing (IGAR) provides alternative routing over the PSTN for inter gateway calls that would otherwise be precluded from traversing the IP network. Communication Manager offers the capability to use H.323 and SIP trunks in the alternative routes. The reasons for an intergateway call to be rerouted over the PSTN include:
The Call Admission Control limit for the link in question was already reached
VoIP RTP resources are unavailable
The parties on the call are members of incompatible (in the sense of codec) network regions
The call was forcibly redirected over the PSTN for testing or debugging purposes
Dial Plan Transparency is somewhat similar to IGAR in that calls whose primary routes are through IP networks are rerouted through the PSTN. However, IGAR applies only to intra-Communication Manager calls, and Dial Plan Transparency applies only to inter-Communication Manager calls. For example, consider a Communication Manager system in which endpoints in two distinct geographic sites can only talk to each other via a particular WAN or via the PSTN. Suppose that the WAN is lost because of a failure, and that the main server complex is coresident with one of the two sites. In that case, the other site must have a survivable core or remote server to keep the endpoints in that site active. In such a scenario, the call in question becomes an inter-Communication Manager call (that is, a call between an endpoint controlled by the main server and an endpoint controlled by a survivable server), and could be rerouted through the PSTN through the use of Dial Plan Transparency. IGAR would not apply to such a scenario.
When engineering a configuration supporting IGAR or Dial Plan Transparency, it is important to engineer the PSTN trunks to be able to support the traffic that would be rerouted if IGAR or Dial Plan Transparency was invoked. For example, if Dial Plan Transparency is being used to provide inter-site connectivity over the PSTN in the event of a WAN failure, the PSTN trunks in both sites should be engineered to an appropriate grade of service, assuming the PSTN call usage includes all of the traffic that would be rerouted pursuant to a WAN failure. For more information see Sizing of PSTN trunks.
