Now it may not be that the IP WAN is necessarily down. Let's go ahead and take a situation where we have limited bandwidth between our locations. And we know that we can comfortably have five phone calls for example going across that WAN link. If a sixth phone call came across, what would happen – it's kind of a little bit different than what you would think – all of the calls would suffer. So if we didn't have enough bandwidth and we started dropping packets and having excessive delay all calls would suffer not just that sixth phone call that tried to squeak across. So what can we do about this? We can set into motion what we call, Call Admission Control. Call Admission Control or CAC says okay five phone calls if the sixth one comes across re-route that, re-route it to the PSTN as that backup path. And in order to do this we can set up Automated Alternate Routing or AAR. AAR can say alright we're going to need to send this call out through the PSTN because only due to call admission control not if the WAN goes down that's a different strategy.
AAR only kicks in if call admission control says "oh we've reached our limit we now need to re-route that call across the public switch telephone network". AAR does this. It's very transparent to your end user. They don't have to dial any more digits or do anything crazy, but we can go ahead and behind the scenes make sure that we add on the appropriate digits in order to reach that destination.
With AAR – again this only kicks in due to Call Admission Control saying we've got too many calls going across that WAN we now need to re-route that call over the PSTN. And, another kind of "gotcha" that you need to understand is that it only works if we are using the Communications Manager to control locations. It's called locations based CAC, because we can go in and say via the Communications Manager there's X amount of bandwidth and that's it. So if the Communications Manager is in control of that bandwidth allocation now we can use AAR to re-route that call. Now, some of the scenarios that they bring up for you shows if a call originates from an IP phone within one location and terminates at an IP phone within another, AAR kicks in. If it's an incoming call through a gateway device within one location and terminates at an IP phone within another location, again AAR kicks in. Doesn't help going out to the PSTN, doesn't matter about the PSTN. It's the IP WAN in between our controlled locations that this can take place. AAR does not work with SRST. Again it's because it does not work if we have a WAN failure, so WAN failures are handled differently than if we've run out of bandwidth going across that WAN.