his topic describes how to use the PSTN as a backup path when calls over the IP WAN are not admitted.
AAR allows rerouting of calls over the PSTN if there is not enough bandwidth for VoIP calls:
- An alternate destination is derived from the external phone number mask and a prefix that is configured per AAR group.
- Individual destinations can be configured per phone.
- AAR is applicable to calls within a cluster.
The figure illustrates how AAR improves availability in multisite environments.
If a call over the IP WAN is not admitted by CAC, the call can be rerouted over the PSTN using AAR. The AAR feature includes an option, which allows the alternate number to be set per IP phone. In the example, because the remote site does not have PSTN access, the call is not rerouted to the IP phone over the PSTN (instead of over the IP WAN). It is alternately rerouted to the cell phone of the affected user. AAR and CFNB improve availability in multisite environments by making it possible to reroute on-net calls that failed CAC over the PSTN.