[This is pre-release documentation and subject to change in future releases. This topic's current status is: Milestone-Ready]

Topic Last Modified: 2010-05-03

For Unified Communications (UC) applications that are IP based such as telephony, video and application sharing, the available bandwidth of enterprise networks is generally not considered a limiting factor within LAN environments. However, on WAN links that interconnect sites network bandwidth is limited. When an influx of network traffic oversubscribes a WAN link, current mechanisms such as queuing, buffering, and packet dropping are used to resolve the congestion. The extra traffic is typically delayed until the network is decongested or if necessary the traffic is dropped. For data traffic, the receiving client application can recover in such situations. In the case of real-time traffic as in the case of UC applications, such mechanisms are ineffective. Network congestion cannot be resolved in this manner because the UC traffic is sensitive to both latency and packet loss. This results in very poor quality of experience for end users. For real-time traffic, it is better to deny calls under congested conditions.

Call admission control (CAC) is a solution that determines whether or not a real-time session can be established based on the available bandwidth to help prevent poor quality of experience for users on congested networks. CAC controls real-time traffic only for audio and video, and does not affect data traffic. CAC may route the call through an Internet path when the default WAN path does not have the required bandwidth. CAC is only available in Communications Server 2010.

This section describes the call admission control functionality and it explains how to plan for it. For details about support for Enterprise Voice, see Enterprise Voice.

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