Call Admission Control and Mediation Server

Topic Last Modified: 2011-01-26

Call admission control (CAC) is a new Lync Server 2010 capability that manages real-time session establishment based on available bandwidth to help prevent poor Quality of Experience for users on congested networks. To support this capability, the Mediation Server, which provides signaling and media translation between the Lync Server VoIP infrastructure and a gateway or SIP trunking provider, is responsible for bandwidth management for its two interactions on the Lync Server side and on the gateway side. The philosophy for call admission control is that the terminating entity for a call handles the bandwidth reservation. The gateway peers (PSTN gateway, IP-PBX, SBC) that the Mediation Server interacts with on the gateway side do not support Lync Server 2010 call admission control. Thus, the Mediation Server has to handle bandwidth interactions on behalf of its gateway peer. Whenever possible, the Mediation Server will reserve bandwidth in advance. If that is not possible (for example, the locality of the ultimate media endpoint on the gateway side is unknown for an outgoing call to the gateway peer), bandwidth is reserved when the call is placed. This behavior can result in oversubscription of bandwidth, but it is the only way to prevent false rings.

Media bypass and bandwidth reservation are mutually exclusive. If a media bypass is employed for a call, call admission control is not performed for that call. The assumption here is that there are no links with constrained bandwidth involved in the call. If call admission control is used for a particular call that involves the Mediation Server, that call cannot employ media bypass.

For details about media bypass or call admission control, see Media Bypass or Call Admission Control in the Planning documentation.