Topic Last Modified: 2011-01-28

For unified communications (UC) applications that are IP-based, such as telephony, video, and application sharing, the available bandwidth of enterprise networks is not generally considered to be a limiting factor within LAN environments; however, on WAN links that interconnect sites, network bandwidth can be 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 congestion eases or, if necessary, the traffic is dropped. For conventional data traffic in such situations, the receiving client can recover. For real-time traffic such as unified communications, network congestion cannot be resolved in this manner, because the unified communications traffic is sensitive to both latency and packet loss. Congestion on the WAN can result in a poor Quality of Experience (QoE) for end users. For real-time traffic in congested conditions, it is better to deny calls than to allow connections with poor quality.

Call admission control (CAC) determines whether there is sufficient network bandwidth to establish a real-time session of acceptable quality. In Lync Server 2010, CAC controls real-time traffic only for audio and video, but it does not affect data traffic. If the default WAN path does not have the required bandwidth, CAC can attempt to route the call through an Internet path or the PSTN. CAC is available only in Lync Server 2010.

This section describes the call admission control functionality, and it explains how to plan for CAC.

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