Inter-Gateway Alternate Routing (IGAR) enables systems with distributed gateways and distributed Call Centers an alternative means of providing bearer connection between port networks and branch gateways when the IP-WAN is incapable of carrying the bearer traffic. IGAR may request that bearer connections be provided by the PSTN under the following conditions:
VoIP RTP resource exhaustion in a MG/PN is encountered.
A codec set is not specified between a network region pair.
Forced redirection between a pair of network regions is configured.
The number of calls allocated or bandwidth allocated via Call Admission Control–Bandwidth Limits (CAC-BL) are reached.
IGAR takes advantage of existing public and private-network facilities provisioned in a network region.
Most trunks in use today are used for IGAR. Examples of the better trunk facilities for use by IGAR would be:
IGAR is the next logical step in providing Quality of Service (QoS) to large distributed single-server configurations.
IGAR relies on Call Admission Control. When all VoIP RTP resources have been used, the next attempt to get a VoIP RTP resource results in denial of the VoIP connection. Communication Manager attempts to use existing applications and features to redirect the call accordingly. Each IP audio stream requires a VoIP RTP resource from either a G4xx media gateway or a branch gateway. Exactly how many audio streams can be supported by these resources depends on the codec selection. Upon hitting the VoIP RTP resource limit, IGAR immediately attempts to use an alternative path for a bearer connection to the network region of the called party using PSTN facilities allocated for use by the IGAR feature.
