What’s New in Communications Server’14 (Microsoft Lync Server 2010) – Call Admission Control

Today, I would like to talk about Microsoft Lync Server 2010 new feature – Call Admission Control (CAC). When I visited many organizations to talk about Microsoft Unified Communication, I always encounter a discussion about VoIP and Audio / Video Conferencing over the existing IP network.

The questions always like, What can we offer this functionality to our users? Can we do this with the desired quality? How will my existing infrastructure respond?
Is my network is bandwidth enough? what other options are available? etc.…

Actually it is quite simple and It is alright to worried. So now with the new feature from Microsoft Lync Server 2010, the network bandwidth problem is solve.
Microsoft Lync Server 2010 Call Admission Control provides supports highly adaptable audio and video codecs that can be 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, Microsoft Lync Server 2010 introduces support for call admission control. It is now possible to prevent users from establishing calls that would result in quality degradations for everyone.

Below a summary and a breakdown.

Part I – What is CAC?
Part II – How does CAC work?
Part III – How to design?
Part I – What is CAC?
Call Admission Control offers more flexible control for IT professionals to architect their network traffic. This helps to prevent unexpected spikes in calls from affecting the entire network, line of business applications, and impacting the quality of current calls. Call Admission Control protects the network and prevents Microsoft Lync Server traffic from consuming all the bandwidth available on the network. Unlike other call admission control solutions that are available from different vendors, Microsoft Lync Server 2010’s Call Admission Control does not require additional hardware; it is built into Microsoft Lync Server 2010 and Microsoft Lync 2010.
Part II – How does CAC work?
When the network link is about to become oversubscribed, Call Admission Control in Microsoft Lync Server 2010 will reroutes new calls through the PSTN or offloads the media portion of the call over the Internet. This offers more flexibility for handling call spikes instead of just letting the oversubscribed sessions fail. When rerouting calls, Call Admission Control attempts to redirect the audio through the Internet first before attempting to re-route it through the PSTN.
Call Admission Control provides reports on blocked calls and redirected calls. Similar to the PSTN, blocked calls is 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 call admission control policy for their particular network as they go. This helps reduce the anxiety of optimizing call admission control at first use.
Figure 1 describes the following scenario and the process in which Bobby (from Site B) to call Alice (to Site A) an intermediate WAN link.
CAC-1

  1. Bobby initiates call to Alice.
  2. Alice’s Communicator receives a call notification.
  3. Alice’s Communicator checks the call admission control policy to determine whether the call can be established.
  4. Alice’s Communicator accepts the call.
  5. Call is established and audio flows across the WAN link.
Figure 2 describes the following scenario and the process in which Bobby (from Site B) to call Alice (to Site A) an intermediate Internet link (via Edge)
CAC-2

  1. Bobby initiates call to Alice.
  2. Alice’s Communicator receives a call notification.
  3. Alice’s Communicator checks the call admission control policy to determine whether the call can be established.
  4. Call is not allowed to be established over the WAN.
  5. Call is accepted and directs audio path to be redirected to the Internet. (Via Edge Server)
Figure 3. Below describes the scenario and the process in which Paul (from Site B) to call Bob (to Site A) an intermediate PSTN link (not via CS Edge “14”).
CAC-3

  1. Bobby initiates call to Alice.
  2. Alice’s Communicator receives a call notification.
  3. Alice’s Communicator checks the call admission control policy to determine whether the call can be established.
  4. Call is not allowed to be established over the WAN.
  5. Call cannot be routed over the Internet.
  6. Call is re-routed to PSTN.
  7. Call audio flows across the PSTN.
Part III – How to design CAC?
Before you can start design CAC, it is important to be aware of varying terminologies, network regions, network sites and network links. All basic components for a properly functioning CAC within your organization.
Call admission control is managed by the Bandwidth Policy service in Microsoft 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. You can configure the Bandwidth Policy service by using PowerShell or Communications Server Control Panel.
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 that belongs to an organization such as a branch office or a regional office.

Network links refer to the WAN link that connects two network regions. Such WAN links have limited bandwidth capacity compared to LAN links; therefore, call admission control is enforced on such network links.

Configuring call admission control involves the following steps:

  1. Identifying network regions.
  2. Identifying network sites.
  3. Determining the IP subnets assigned to each network site.
  4. Identifying the network links that connect network regions. For each network link:
    • Determine the maximum bandwidth capacity.
    • Define the bandwidth capacity (meaning, the call admission control policy).
  5. Determining the network route between every pair of network regions.
It is most likely that the networking Team in your organization will 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. The hardest part will be chasing down this information.

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

Call admission control 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 (session)
  • Maximum bandwidth that can be allocated for a single video call (session)
To optimize per-session bandwidth use, you need to 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.

Note: Microsoft Lync Server 2010 does not enforce call admission control at layer-2 networking, which is managed by routers. To ensure that network traffic is routed in the network path that you want when multiple paths are possible, routing will need to be enforced at the router level.

Call admission control is enforced at the client level. It is enforced by the receiving client not the call initiating client as explained earlier. Because only Microsoft Lync 2010 is call admission control aware, clients such as Communicator 2007 R2 and Communicator 2007 won’t enforce call admission control when those clients are the recipients.
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About David Lim

David has over 15 years of experience in IT industry in designing and implementing Microsoft Solutions ranging from small to enterprise customer. He also has experience in designing and developing Microsoft Unified Communications, Collaboration and Office 365 solutions with focus on Exchange, Lync & SharePoint as well as strategic migration planning in complex business environments. He is specializing in architecture and design of Lync Voice deployments. He has been actively involved in various speaking engagements, the recent being the sessions on Lync On-Premise and Office Interoperability in September 2011 and Office 365 Introduction in November 2011. In recognition of his high-quality real-world technical excellence with the community and with Microsoft, David received the prestigious Microsoft Most Valuable Professional (MVP) Award in April 2011.
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