IGAR requests PSTN to provide bearer connections in any of the following conditions:
The number of calls allocated or bandwidth allocated through Call Admission Control- Bandwidth Limits (CAC-BL) is reached.
VoIP RTP resource exhaustion in a port network or media gateway is encountered.
The codec set between a pair of network regions is set to pstn.
Forced redirection is configured between a pair of network regions.
IGAR provides enhanced Quality of Service (QoS) to large distributed single-server configurations. You can use IGAR for configurations where the IP network is not reliable enough to carry bearer traffic. If you have more than one IP network available, you can use H.323 or SIP trunks for IGAR instead of the PSTN.
When Communication Manager needs an inter gateway connection and adequate IP bandwidth is unavailable, Communication Manager attempts to substitute a trunk connection for the IP connection. For example, Communication Manager can substitute a trunk connection in any of the following situations:
A user in one Network Region (NR) calls a user in another NR.
A station in one NR bridges on to a call appearance of a station in another NR.
An incoming trunk in one NR routes to a hunt group with agents in another NR.
An announcement or music source from one NR must be played to a party in another NR.
Communication Manager attempts to use a trunk for inter-region voice bearer connection when the following five conditions are met:
An inter gateway connection is needed.
IGAR requests PSTN to provide bearer connections.
IGAR is enabled for the NRs associated with each end of the call.
The Enable Inter-Gateway Alternate Routing system parameter is set to y.
The number of trunks used by IGAR in each NR has not reached the limit administered for that NR.
The SRC PORT TO DEST PORT TALKPATH page of the status station screen shows the IGAR trunk connectivity for an interNR call.
A Trunk Inter-Gateway Connection (IGC) is established using ARS to route a trunk call from one NR to IGAR Listed Directory Number (LDN) extension administered for the other NR. The Trunk IGC is independent of the call being placed. Therefore, Communication Manager can originate the IGC from the NR of the calling party to the NR of the called party, or vice versa. However, for users who use Facility Restriction Levels or Toll Restriction to determine who gets access to IGAR resources during a WAN outage, the calling user is considered the originator of the Trunk IGC for authorization and routing. However, if the outgoing trunk group is administered to send the Calling Number, the IGAR Extension in the originating NR is used to create this number using the appropriate administration.
The following are examples of failure scenarios and how Communication Manager handles the scenarios:
On a direct call, the call continues to the first coverage point of the unreachable called endpoint, or if no coverage path is assigned, busy tone is played to the calling party.
If the unreachable endpoint is being accessed through a coverage path, the coverage point is skipped.
If the unreachable endpoint is the next available agent in a hunt group, that agent is considered unavailable, and the system tries to route the call to another agent using the administered group type, such as Circular distribution and Percent Allocation Distribution.
