Call Admission Control (CAC)
For IP-based real-time applications on wide-area network links between sites, bandwidth is a finite resource, which makes the appropriate provisioning of such links crucial for a Microsoft Lync Server 2010 administrator. To serve this need, the ability to manage bandwidth in a wide-area network (WAN) has been added in Microsoft Unified Communications Managed API (UCMA) 3.0. This ability addresses bandwidth utilization in unified communications scenarios, and provides an infrastructure that allows policy decisions (whether or not sessions can be established) to be made when setting up real-time sessions.
Bandwidth management provides two important benefits, by enabling the following:
Lync Server 2010 supports highly adaptable audio and video codecs that can adjust to varying network capacity. The RTAudio and RTVideo codecs make it possible to maintain good media quality in degraded network conditions. However, to prevent degradation of audio and video quality that users can perceive, Lync Server 2010 introduces support for call admission control. It is now possible to prevent users from establishing calls that would result in quality degradations for everyone.
Call admission control offers more flexible control for IT professionals to architect their network traffic. This can help to prevent unexpected spikes in calls from affecting the entire network and line-of-business applications, and impacting the quality of existing calls. Call admission control protects the network and prevents Lync Server 2010 traffic from consuming all the bandwidth available on the network. Unlike other call admission control solutions that are available from different vendors, Communications Server's call admission control does not require additional hardware; it is built into Lync Server 2010 and Microsoft Lync 2010.
Behavior Changes from Previous Versions
Microsoft Office Communications Server 2007 R2 always allowed calls between clients to complete regardless of the bandwidth resources they might consume. With the introduction of bandwidth controls in Lync Server 2010, a call that would have completed in the earlier version might now have two additional outcomes.
The call or session request is rejected because of insufficient bandwidth.
An audio call is redirected by Lync Server 2010 to a PSTN call, provided that the user policy is set to allow PSTN redirects.
These new outcomes might cause the user to perceive that the system is acting erratically. For this reason, special care must be taken to inform the user as to why a media session could not be established as intended, or at all, as in the following situations.
A user calls an application and the call fails, or the user is sent a busy tone. When the user calls again a short while later, the call succeeds, which can happen if bandwidth utilization for Lync Server 2010 drops due to other users ending their sessions.
An application is always able to perform video and application sharing with users in the same building, but is sometimes unable to add application sharing to a user in another building or site.
An application never has trouble adding local users to a conference, but occasionally is unable to add users from other sites.
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