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Call Admission Control in Lync Server 2010

Brass Contributor

May 20, 2019

First published on TECHNET on Sep 13, 2010

Join us for a brief walkthrough of how to protect your network from being overloaded by voice and video traffic using Call Admission Control in Lync Server 2010.

Author : Rui Maximo , Microsoft Principal Technical writer—with Subbu Chandrasekaran and Thomas Binder

Editor: Susan S. Bradley

Publication date : June 2010

Update date : March 25, 2013

Product version : Lync Server 2010

One of the big concerns that IT often raises when they consider deploying VoIP and video conferencing is whether their IP network is prepared to sustain the network load imposed by this media traffic. VoIP and video conferencing require high bandwidth and low latency to offer a good end user experience. The concern is legitimate. While upgrading existing IP networks may be the ideal solution, this option is likely to be cost prohibitive and often occurs over a long period of time. In the interim, what other options are available?

Call Admission Control (CAC) prevents degradation of quality by limiting the number of concurrent calls over limited bandwidth links, such as wide area networks (WAN).

Benefits

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 CAC. It is possible now to proactively prevent users from establishing calls that would result in quality degradations for everyone.

CAC offers more flexible control for IT to architect their network traffic. This helps prevent unexpected spikes in calls from affecting the entire network and line of business applications, and from impacting the quality of current calls. CAC protects the network and prevents Lync Server traffic from consuming all the bandwidth available on the network.

Unlike other CAC solutions available from different vendors, Lync Server CAC does not require additional hardware. CAC is built into Lync Server 2010 and Office Communicator 2010.

When the network link is about to become oversubscribed, CAC in Lync Server 2010 provides the ability to re-route new calls through the PSTN or offload the media portion of the call over the Internet. This offers more flexibility for handling call spikes, instead of simply failing the oversubscribing sessions. When re-routing calls, CAC first attempts to redirect the audio through the Internet before attempting to re-route it through the PSTN.

CAC provides reports on blocked calls and redirected calls. Similar to the PSTN, blocked calls are preferable to poor quality calls. Redirecting calls to alternate routes (Internet or PSTN) is even better than blocking calls. This data helps organizations fine-tune their CAC policy for their particular network as they go. This helps reduce the anxiety of optimizing CAC at first usage.

Figure 1 below illustrates a call being authorized through the WAN link. Figure 2 below illustrates the same call; however, instead of being blocked, the call is re-routed through the PSTN. Figure 3 below illustrates the same call, but in this case, the media is redirected over the Internet. Notice that the signaling path is still through the WAN link.

Figure 1. Call established over WAN link

Table 1 below describes the details of each step in the process of establishing a call between the calling party (Alice) and the called party (Bob) over the WAN link.

Table 1. Details of call established over WAN link

Step	Description
1	Alice initiates call to Bob
2	Bob’s Communicator 2010 receives call notification
3	Bob’s Communicator 2010 checks CAC policy whether call can be established
4	Bob’s Communicator 2010 accepts call
5a	Call is established and audio flows across the WAN link

Figure 2 below shows the call media being redirected over the Internet.

Figure 2. Call media redirected over the Internet

Table 2 below describes the steps of the call’s audio being redirected over the Internet, as shown in Figure 2.

Table 2. Steps of call audio redirected over the Internet

Step	Description
1	Alice initiates call to Bob
2	Bob’s Communicator 2010 receives call notification
3	Bob’s Communicator 2010 checks CAC policy whether call can be established
4	Call is not allowed to be established over the WAN
5b	Call is accepted and directs audio path to be redirected to the Internet

Figure 3 below illustrates CAC re-routing the call through the PSTN.

Figure 3. Call re-routed to PSTN

Table 3 below describes how the call is rejected over the WAN link, but is re-routed to the PSTN.

Table 3. Steps of call re-routed to PSTN

Step	Description
1	Alice initiates call to Bob
2	Bob’s Communicator 2010 receives call notification
3	Bob’s Communicator 2010 checks CAC policy whether call can be established
4	Call is not allowed to be established over the WAN
5b	Call cannot be routed over the Internet
5c	Call is re-routed to PSTN
6c	Call audio flows across the PSTN

Design

Let’s explore at a high level how CAC works in Lync Server 2010.

CAC is managed by the Bandwidth Policy Service in Lync Server 2010. This service is automatically installed as part of every pool, and benefits from the high availability design of the Enterprise pool. Bandwidth Policy Services across multiple pools within an organization automatically synchronize with each other to maintain integrity of bandwidth subscription. The Bandwidth Policy Service is configurable using PowerShell or Lync Server Control Panel (LSCP).

Before you set out to configure CAC, the terms, network regions, network sites, and network links used in CAC need to be understood.

Network Regions consist of multiple network sites. A network region represents a network hub or backbone. Network regions are interconnected through a wide area network (WAN) link

Network Sites identify locations within a network region. A network site represents a physical location belonging to an organization, such as a branch office or a regional office.

Network Links refers to the WAN link connecting two network regions. Such WAN links have limited bandwidth capacity compared to LAN links, and therefore CAC is enforced on such network links.

The process of configuring CAC involves 5 steps:

Identifying network regions.

Identifying network sites.

Determining the IP subnet(s) assigned to each network site.

Identifying the network links connecting network regions. For each network link:

Determine the maximum bandwidth capacity.

Define the bandwidth capacity (i.e. CAC policy).

Determining the network route between every pair of network regions.

More likely, the networking group in your IT organization has already defined your organization’s network regions and network sites, and identified the IP subnets for each network site and the bandwidth capacity of the WAN links between network regions (steps 1-4a). The hardest part is locating this information.

After you obtain this information, you can decide how much bandwidth capacity to allocate for audio and video across each network link, and determine the preferred route between network regions.

CAC policy defines the following information:

Maximum total bandwidth to allocate for audio.

Maximum total bandwidth to allocate for video.

Maximum bandwidth that can be allocated for a single audio call (i.e. session).

Maximum bandwidth that can be allocated for a single video call (i.e. session).

To optimize per session bandwidth utilization, you must consider the type of codecs being used to avoid under-allocating sufficient bandwidth needed for a particular codec or over-allocating bandwidth by setting the maximum bandwidth per session too high. In a subsequent NextHop article, we’ll provide you guidance on how to optimize your CAC policies based on the codecs allowed.

Considerations

Lync Server 2010 doesn’t enforce CAC at layer 2 networking, which is managed by routers. To ensure that network traffic is routed in the network path desired when multiple paths are possible, routing must be enforced at the router level.

CAC is enforced at the client level. It is enforced by the receiving client, not the call initiating client. How CAC works is explained in the Design section. Because only Lync 2010 is CAC aware, this implies that down-level clients, such as Communicator 2007 R2 and Communicator 2007 won’t enforce CAC when those clients are the recipients.

Summary

CAC ensures quality of experience of audio and video calls by enforcing that audio and video traffic managed by Lync Server 2010 doesn’t take more bandwidth than configured by the policy. The support of CAC in Lync Server 2010 makes Enterprise Voice a robust VoIP solution for the enterprise. CAC helps IT proactively prevent introducing additional high bandwidth, low-latency traffic into saturated network links, which would quickly degrade the IP network under spike conditions. When combined with layer 2 QoS at the routing level, CAC helps ensure high quality audio and video between endpoints across WAN links.

Leverage CAC to your advantage!