7.2 CAC Overview

This topic describes the CAC options that are available in Cisco Unified Communications Manager.

CAC limits the number of calls between certain parts of the network to avoid bandwidth oversubscription:

  • QoS can be used to give priority to voice and video over data.
  • QoS cannot solve the problem of too much prioritized traffic (caused by too many voice and video calls).
  • Oversubscription results in delayed packets and packet drops:
    1. Any packets of any voice or video stream are affected (not just packets of the call that exceeds bandwidth limit).
    2. Results in quality degradation of all voice and video calls.
  • CAC avoids such problems by limiting the number of voice and video calls.

CAC limits the number of calls between certain parts of the network to avoid bandwidth oversubscription with too many voice and video calls. QoS is not able to achieve this result because QoS provides only the means to prioritize voice and video over data traffic. QoS does not avoid the situation in which too many (prioritized) voice and video streams are sent over the network.

If oversubscription occurs, any packets of any voice or video stream can be affected, not just packets of the particular call or calls that exceed the bandwidth limit. The result in this case is packet delays and packet drops of all voice and video calls, and hence oversubscription degrades the quality of all calls.

Therefore, to ensure good quality, you must use CAC to limit the number of voice and video calls.

CAC in Cisco Unified Communications Manager

Cisco Unified Communications Manager supports various CAC methods.

Cisco Unified Communications Manager supports the following CAC features:

  • CAC within a cluster:
    1. Locations
    2. RSVP-enabled locations
    3. If CAC denies call, AAR can be used to reroute call over PSTN.
  • CAC for calls leaving a cluster:
    1. Locations
    2. H.323 gatekeeper or SIP preconditions
    3. If CAC denies call and no other entries in route list are left to try, call fails.

In centralized call-processing deployments, you can use locations. Locations can be RSVP-enabled to provide RSVP-based CAC within a Cisco Unified Communications Manager cluster. If a call is not admitted by locations or RSVP-enabled locations due to bandwidth limitations, you can use AAR to reroute the call over the PSTN (off-net) instead of denying the call. AAR provides a service like PSTN backup, except that the reason for call backup is not that the call failed on the on-net path, but that there is no available bandwidth from a CAC point of view.

In distributed call-processing environments, you can use H.323 gatekeeper CAC with H.323 trunks (gatekeeper-controlled intercluster trunks and H.225 trunks). If you have SIP trunks, you can use SIP preconditions, which allows RSVP-based CAC, or locations.

If calls are not admitted by the H.323 gatekeeper, standard backup functionality of route lists and route groups is applied. For example, to route calls that have not been admitted by the gatekeeper over the trunk, you can configure one or more PSTN gateways in another (lower-priority) route group of the same route list. In this way, the gatekeeper-controlled trunk is preferred over the PSTN as long as calls are admitted; after admission is rejected, calls are sent over the PSTN. The same principle applies to calls that are placed through SIP trunks that are configured for SIP preconditions.

Comparison of CAC Methods

This section compares CAC methods that are available in a Cisco Unified Communications Manager deployment.

Call Admission Control Features Traditional Location CAC Gatekeeper RSVP Enhanced Location CAC
Audio versus video differentiation
Priority and preemption
Video versus immersive video differentiation
Multicluster support
Multihop WAN topology
Topology awareness (support of dynamic network changes, multiple links, load balancing)

Locations support in Unified CM changed in Cisco Unified Communications Manager Release 9. The table shows the traditional Locations CAC support that was available before Release 9 and the Enhanced Location CAC features that are available in Cisco Unified Communications Manager Release 9 and later releases.

As shown in the table, Enhanced Location CAC supports all listed features with the exception of topology awareness. Dynamic network changes, such as link failures, are not supported. Therefore, the applied bandwidth limits may not correctly represent the actual network conditions in this case.

Enhanced Location CAC cannot take multiple links into consideration. The network model of Enhanced Location CAC only supports one logical link between two locations. If load-shared links exist between two locations, the administrator could simply aggregate the bandwidth of the two paths into one logical link in the network model. However, again, Enhanced Location CAC would not be able to react to any changes. In case of a failure of one of the links, the Enhanced Location CAC bandwidth limit would be too high. This condition will very likely result in overloaded links and quality degradation or even call drops.

Enhanced Location CAC adds CAC features to the traditional location CAC method and does not have the disadvantages of the gatekeeper CAC. RSVP lacks support for a dedicated, immersive video bandwidth pool but provides topology awareness. Topology awareness provides support for any network topology including multiple links, load-balanced links, and the capability to manage dynamic network changes. To offer these features end-to-end, RSVP must be enabled on the entire IP network, which means that every router in the network needs to be RSVP-enabled.

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