2.8 Call Admission Control

This topic describes the purpose of CAC and how it can reduce IP WAN bandwidth consumption.

CAC is used to limit the number of calls:

  • Enhanced location CAC: Bandwidth limits are applicable to components within a cluster and across clusters.
  • RSVP-enabled locations:
    1. Applicable to calls within a cluster
    2. Allow calls to flow through two routers (RSVP agents)
    3. Call leg between routers uses RSVP for CAC
  • SIP precondition: Like RSVP-enabled locations, but between SIP domains (for example, two clusters interconnected via a SIP trunk)
  • Gatekeepers: Bandwidth control for calls between H.323 gatekeeper zones

Cisco Unified Communications Manager allows the number of calls to be limited by these CAC mechanisms:

  • Locations: Cisco Unified Communications Manager Enhanced Location CAC is applicable to calls within a Cisco Unified Communications Manager cluster and to intercluster calls. It is based on a network model and all bandwidth limits that are configured along the path of the network model are checked.
  • RSVP-enabled locations: RSVP is a special way to configure locations. When RSVP is configured to be used between two locations, the audio streams flow through two routers, so-called RSVP agents. The call leg between the two RSVP agents is subject to Cisco IOS RSVP CAC. RSVP-enabled locations only apply to calls within a cluster.
  • SIP Preconditions: SIP Preconditions is a solution like RSVP-enabled locations, except that it is designed only for SIP trunks. With SIP Preconditions, calls through a SIP trunk flow through a local Cisco IOS router at each end of the SIP trunk and are split into three call legs, in the same way that they are split with RSVP-enabled locations. However, in this case, the call is not within a cluster but between clusters.
  • Gatekeepers: Gatekeepers are used with H.323 and provide address resolution and CAC functions. H.323 gatekeepers can be configured to limit the number of calls between H.323 zones.