Class of Service: Automated Alternate Routing CSS
An AAR calling search space (CSS) is a special application of calling search spaces that can be applied at the line (directory number) level of Cisco IP phone configuration. This blog will discuss automated alternate routing (AAR) and the application of the AAR CSS. AAR technology allows Cisco IP phone to Cisco IP phone calls over the WAN to be re-routed to the public switched telephone network (PSTN) when the configured location-based bandwidth of the location is in use. AAR assumes the following configurations and technologies: • Centralized Call Processing • Direct Inward Dialing (DiD) • Location based Call Admission Control (CAC) • External Phone Number Mask configuration • AAR Group configuration • AAR Calling search space configuration The purpose of this blog is to talk to the AAR CSS application and not all of the other components involved in AAR. I will gladly write subsequent blogs on AAR if it is of interest… let me know. A new CSS for the application of AAR may be required when Cisco IP phone calls are re-routed over the PSTN. Normal inter-office dialing is achieved with an abbreviated dial plan, but the PSTN requires 7, 10, or 11 digit dialing. The AAR CSS is a requirement when the CSS of the calling device is restricting the number to be dialed for a call re-routing attempt. It is possible that a restricted phone should be able to route a call over the PSTN, but only when location-based CAC has rejected the call attempt. AAR CSS is not required in a distributed multi-cluster call processing model because inter-site call routing will use the route patter -> route list -> route group(s) -> trunk/gateway call routing logic. The route list is pointed to multiple route groups where different digital manipulation can be applied to the WAN_RG and PSTN_RG. If the WAN is not available, the PSTN route group normally prefixes the digits required to extend the call to the destination CUCM cluster. Centralized call processing models do not have this call routing logic. Calls in a centralized model are routed to destination IP addresses which may route calls to the next cubicle or across the world over a VPN tunnel. In the next blog, we’ll take a look at subscribe calling search spaces.
