Skip Ribbon Commands
Skip to main content

Xadean's Empirical Musing


Quick Launch

Xadean's Empirical Musing > Posts > Call Admission Control for Lync
April 04
Call Admission Control for Lync
Although you can use CAC bandwidth policies to complement QoS settings, the point there is that it is highly recommended (basically required) to implement QoS lthough you can use CAC bandwidth policies to complement QoS settings, the point there is that it is highly recommended (basically required) to implement QoS if you plan on configuring CAC.  Not the other way around (if you plan on implementing QoS, CAC is optional).  There are possible quality implications if CAC or QoS is implemented solely without the other, implemented together, or neither implemented at all.


The reason for potential quality issues when CAC is implemented without QoS is because it operates for Lync at the application layer.  CAC is not aware of what is actually happening on the network link.  Therefore, other data could be consuming the bandwidth and there would be no mechanism to prioritize, police, or reserve bandwidth for audio/video traffic.  CAC would just see that there is not enough bandwidth to connect a stream and you may not be able to make any calls at all.

The reason for potential quality issues when QoS is implemented without CAC is because of the dynamic compression nature of the RTAudio/RTVideo codecs.  It will cumulatively degrade all streams to make room for new connections.  Therefore, existing audio/video streams could be affected by newly established ones if the bandwidth is not adequate.
When both QoS and CAC are implemented together, traffic is prioritized, policed, and reserved by QoS so the calculation you use to set the max number of allowable audio/video streams over the WAN link with CAC will be honored (for instance, you will be able to make 10 calls if you set that as the max).   The main consideration here is that you will have to create 2 separate CAC solutions for Lync and Cisco respectively that will have to fit in the QoS reserved/prioritized bucket for audio/video.  Depending on the calculation, the bandwidth requirement could be large, so it would seem the philosophy of increasing the bandwidth on the WAN link, if possible, is more appropriate.
Here are some additional considerations for the CAC topic:


  • CAC cannot be enforced on clients prior to Lync. So if you have Communicator 2007 R2 and lower registering to Lync Server during a migration period CAC will not work for those clients until they are migrated Lync.
  • CAC is enforced at the client level. It is enforced by the receiving client, not the call initiating client.  So if Alice initiates call to Bob, Bob’s Lync client checks CAC policy whether call can be established.
  • Lync doesn’t enforce CAC at layer 2 networking, which is managed by router.  Lync does CAC at layer 7 (application).
  • CAC does not affect data, which includes web conferencing and desktop sharing.
  • When deploy CAC anywhere in your network, all subnets need to be entered into Lync. This is critical as the Bandwidth Policy Server coordinates bandwidth limitations based on your current location and not where you are homed which it cannot do unless you map your entire network.
  •  The Mediation Server cannot enforce CAC when Media Bypass is in use.
  • For calls that exceed the predefined max, they can be redirected to an alternate PSTN gateway or the internet if edge servers are in place.
  • Key questions to ask when considering CAC:
    • How much BW do you have to each location?
    • How much available BW do you have, or can they set aside (using QOS)
    • Based on answers from the two questions above; How much BW do you want to restrict for Audio and Video traffic?
  • CAC Reroute Priority:
    • Internet (via Edge)
    • PSTN
    • Voicemail deposit
  • Lync CAC looks at two files: The Bandwidth Policy Service generates two types of log files in comma separated values (CSV) format. The check failures log file captures information when bandwidth requests are denied. The link utilization log file captures a snapshot of the network topology and the WAN link bandwidth utilization. Both of these log files can assist you in fine-tuning your CAC policies based on utilization. When you specify the file directories in the tool, Server logs is typically in <fileserver>\<choice of FE>\AppServerFiles\PDP where Bandwidth policy server logs are stored. Please ensure that sufficient file access to the server logs and the temporary file store folder is provided to the tool user. More info, you can refer to  the specification of Microsoft Lync Server 2010 Resource Kit Tools.
    • Must enable logging with the following command: Set-CsBandwidthPolicyServiceConfiguration -Identity "Global" -EnableLogging 1



Re: Call Admission Control for Lync

 on 9/28/2019 6:12 AM

 on 11/27/2020 10:29 AM

auto tuners

 on 12/24/2022 10:18 AM

Add Comment


Body *