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No audio when video is added to a Lync 2010 conference in a CAC-enabled Lync Server 2010 environment

Consider the following scenario:
  • You enable the Call Admission Control (CAC) feature in a Microsoft Lync Server 2010 environment.
  • You create a bandwidth policy profile that has low audio and video session limits. For example, you set the audio session limit to 60 kilobits per second (Kbps) and the video session limit to 250Kbps. 
  • The two Lync 2010 users (user A and user B) who are affected by the bandwidth policy join an audio conference and can successfully talk to one another. 
  • User A adds video to the conference.
In this scenario, user B does not receive the conference audio temporarily.
This issue occurs because the G.722 codec is removed from the Session Description Protocol (SDP) in the INVITE request that is sent when video is introduced to the conference. However, the Audio Video Multipoint Conferencing Unit (AVMCU) does not start to use the renegotiated codec immediately. The AVMCU may take some time to switch from the G.772 codec to the SIREN codec. 
To resolve this issue, install the following cumulative update:
2740406 Description of the cumulative update for Lync Server 2010, Unified Communications Managed API 3.0 Runtime: October 2012
More information
For more information about how to configure the Call Admission Control feature, go to the following Microsoft website:For more information about how to plan for the Call Admission Control feature, go to the following Microsoft website:

Article ID: 2741296 - Last Review: 10/12/2012 06:49:00 - Revision: 3.0

Microsoft Lync Server 2010 Enterprise Edition, Microsoft Lync Server 2010 Standard Edition

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