Call Admission Control

CAC limits the number of calls between certain parts of the network to avoid bandwidth oversubscription, with too many voice calls over WAN links.

Locations:
The configured bandwidth limit is independent of the call's destination location. Unlike region configuration, in which the maximum permitted codec is configured for each pair of regions, the bandwidth limit of a location applies to all interlocation calls, regardless of the other location.

CUCM CAC is based on a hub-and-spoke topology, in which all calls over the WAN are monitored for CAC as if they go through HQ. Because the configured bandwidth limit does not consider the destination location, the 24-kbps limit of BR1 allows any call to go out or in, regardless of where it goes to or comes from. The headquarters limit is unaffected by such a call. Only locations BR1 and BR2 subtract 24 kbps from their limits. Because locations-based CAC does not provide topology awareness, CUCM does not even know that the call physically flows through headquarters. 

1. Add locations and configure the CAC bandwidth limit.
2. Assign locations to devices.

RSVP-Enabled Locations
RSVP can be enabled selectively between pairs of locations.
RSVP makes this CAC mechanism WAN topology-aware, because RSVP will communicate over the WAN. Standard locations do not contain in their configurations details of the WAN topology. RSVP-enabled locations work well with all topologies (full-mesh, partial-mesh, and hub-and-spoke) and adapt to network changes by considering the actual topology.
An RSVP agent is a device called a Media Termination Point (MTP) through which the call has to flow. RSVP is used only between the two RSVP agents. The Real-Time Transport Protocol (RTP) stream from the IP Phone to the RSVP agent does not use RSVP.
There are three separate RTP streams: Phonel talks to RSVP Agentl, RSVP Agentl talks to RSVP Agent2, and RSVP Agent2 talks to Phone2.
Media resource group lists are used to identify the RSVP agent to be used by an IP Phone.
Confiiguration:
1. Configure RSVP service parameters.
2. Configure RSVP agents in Cisco IOS.
!
dspfarm profile 1 mtp
 codec pass-through
 rsvp
 maximum sessions software 20
asswciate application SCCP
!
interface Serial0/0
 ip rsvp bandwidth 40
!
3. Add RSVP agents to CUCM.
 Cisco IOS Enhanced Software Media Termination Point
4. Enable RSVP between location pairs.
5. Configure media resource groups.
6. Configure media resource group lists.
7. Assign media resource group lists to devices.

AAR
AAR does not work with Survivable Remote Site Telephony (SRST). AAR is activated only after a call is denied by CAC, not by WAN failures.
AAR works only for calls placed to internal directory numbers. It does not apply to calls placed to route patterns or feature patterns such as Meet-Me or Call Park. However, it does work for hunt pilots and computer telephony interface (CTI) ports.
The alternative number used for the PSTN call is composed of the dialed directory number, a prefix configured per AAR source and destination group, and the external phone number mask of the called device.

Configuration
1. Configure AAR service parameters (Cisco CallManager service).
 - Automated Alternate Routing Enable
 - Out-of-Bandwidth Text
 - AAR Network Congestion Rerouting Text
2. Configure partitions and CCS.
3. Configure AAR groups. Assign a dial prefixes.
4. Configure phones for AAR:
 Phone Configuration:
 - AAR Calling Search Space
 - AAR Group (or directory number Group is used)
 Directory Number Configuration:
 - AAR Settings: Voice Mail checkbox
 - AAR Destination Mask (CFNB)
 - AAR Group
 - External Phone Number Mask