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Real-time communications are sensitive to the latency and packet loss that can occur on congested networks. Call admission control (CAC) determines, based on available network bandwidth, whether to allow real-time communications sessions such as voice or video calls to be established. The Call Admission Control (CAC) uses the Policy Decision Point (PDP) to distribute policies defining some of the CAC rules. CAC controls real-time traffic for voice and video only. It does not control data traffic.

Administrators define CAC policies, which are enforced by the Bandwidth Policy Service that is installed with every Front End pool. CAC settings are automatically propagated to all Lync Server Front End Servers in your network.

This test helps you figure out how many client authentications failed and how many client connections were authenticated. Using this test, administrators can easily figure out the number of active audio and video call requests for which bandwidth is checked. This way, administrators can figure out the client connection that is consuming too much of bandwidth.

Measurement	Description	Measurement Unit	Interpretation
auth_failed	Indicates the rate at which client authentication failed due to timeout.	Failures/Sec	A low value is desired for this measure. If there are too many authentication failures, then check for certification issues. The value of this measure may be high due to an attack by the client on the machine through the creation of connections and consumption of resources from the Bandwidth Policy Service. If the rise in the value is due to an attack, then trace the machine and block the machine through firewall policies.
Frst_pac_timeout	Indicates the rate at which the sessions timed out before the arrival of the first packet.	Timeouts/Sec
server_auth_timeout	Indicates the rate at which the server connections timed out before an authentication message was received.	Failures/Sec	A low value is desired for this measure.
ActiveClientCon	Indicates the number of active client connections that are authenticated.	Number
videoBWReq	Indicates the number of video call requests for which bandwidth is checked.	Number
videoBWResrv	Indicates the number of active bandwidth reservations for video calls.	Number
audioBWReq	Indicates the number of audio call requests for which bandwidth is checked.	Number
audioBWResrv	Indicates the number of active bandwidth reservations for audio calls.	Number
coreBWReq	Indicates the overall number of core (both audio and video) call requests for which bandwidth is checked.	Number
coreBWResrv	Indicates the total number of active reservations for audio and video calls.	Number
segRecRate	Indicates the rate at which TCP segments were received by this service.	Number