Call Admission Control

Planning to deploy Call Admission Control features in Lync Server 2013 is going to depend greatly on the network configuration discussed in the preceding section. CAC relies on determining an endpoint’s site and region through the network subnet. Call Admission Control in Lync Server 2013 applies to both audio and video traffic, but organizations can specify different limits for each type of traffic. Both a session limit (one-way traffic) and a total limit for all sessions can be specified.

The key to successful Call Admission Control deployment is to correctly define and associate the bandwidth policy profiles to sites and links by completing the following steps:

1. Identify the connection speed of each WAN link to sites that are bandwidth constrained.

2. Define the maximum audio and video session and total bandwidth limits to be used by Lync endpoints associated with site and policy. These limits vary based on the desired traffic type and the different audio codecs used.

3. Evaluate the site and make some estimates on the type of audio codecs used to create an appropriate limit. For example, if users make several Lync-to-Lync calls, RTAudio is predominantly used. If most calls are conferences, Siren or G.722 audio might be more prevalent.

It is also important to note that Call Admission Control applies only to Lync Server endpoints traversing a WAN link. Other applications transmitting data on the same WAN link are not affected by Lync Call Admission Control policies. Organizations can define a bandwidth limit for Lync traffic and still see that WAN link become saturated due to other applications. In this scenario, it makes sense to enforce QoS policies on the WAN link to ensure that Lync endpoints can always place calls. These QoS reservations should ideally match the CAC bandwidth limits.

The bandwidth override policy is enforced by the receiving endpoint and not the sender. When a call is placed, the receiving endpoint leverages its subnet information and checks whether the call will exceed the bandwidth policy limit.

The only clients that actually respect Call Admission Control policies are Lync 2010 endpoints. Earlier clients, such as Office Communicator 2007 R2, are not able to perform a bandwidth check when a Lync client calls. However, media calls from Office Communicator 2007 R2 to a Lync endpoint enforce Call Admission Control policies.