Call admission control (CAC) is a solution that determines whether a real-time session can be established based on the available bandwidth
to help prevent poor audio/video quality 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
https://technet.microsoft.com/en-us/library/gg398870.aspx
http://blogs.technet.com/b/jenstr/archive/2010/10/07/bandwidth-management.aspx
http://voipnorm.blogspot.com/2010/11/lync-2010-media-bypass-and-cac-part-3.html
I hope this helps...