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Unclear on how CAC works exactly RRS feed

  • Question


  • CAC is a great feature of Lync (when combined with QoS) but I am confused about a few aspects of how it works. I have actually looked in a couple of books and done numerous Google searches and nobody seems to understand or explain it clearly. 

    I think it is important to understand these things to properly configure Lync.

    To simplify my questions I give a simple example just considering audio (voice calls) through a single connection. CAC audio is configured to use up to 40% of the bandwidth of the connection; QoS is used to reserve that much bandwidth (40%) for Lync audio.

    Here are my questions:

    1. If the connection is saturated (including 40% being used for Lync audio calls), but the RTAudio codec for the existing calls is currently providing high quality audio, will Lync throttle back the quality on existing calls to open up bandwidth for a new call to be placed? It seems like this is a good idea as long as the audio quality is not reduced too much.

    2. If the audio is using 40% of the bandwidth but "other uses" on the connection are only using say 10% (leaving 50% unused) will Lync allow a new call to be placed to take advantage of the unused bandwidth? I have seen this implied but have not seen it confirmed anywhere else.

    3. If the answer to (2) is yes then what happens if "other uses" jumps to 60%? Does Lync terminate the extra call that is using the bandwidth above 40%?

    4. Also, if the answer to (2) is yes then how does Lync determine how much bandwidth is being used on the connection? I don't believe it has access to this low-level statistic.

    I hope these questions make sense and someone can answer them. Any other clarification would be greatly appreciated if my misunderstanding is obvious.

    Andrew.

    Tuesday, December 8, 2015 10:06 AM

Answers

  • 1. If the connection is saturated (including 40% being used for Lync audio calls), but the RTAudio codec for the existing calls is currently providing high quality audio, will Lync throttle back the quality on existing calls to open up bandwidth for a new call to be placed? It seems like this is a good idea as long as the audio quality is not reduced too much.

    There are two settings in CAC; traffic shaping for the connection similar to the 40% you have mentioned, second one session limit. if you limit the session to let say 100 Kbps, new call cannot establish if 100 Kbps not available within the 40% you have mentioned. So exiting call will continue without impact, new call can be establish only after ending existing call.

    2. If the audio is using 40% of the bandwidth but "other uses" on the connection are only using say 10% (leaving 50% unused) will Lync allow a new call to be placed to take advantage of the unused bandwidth? I have seen this implied but have not seen it confirmed anywhere else.

    It doesn't exceed BW limit set for Lync Audio and Video not for other streaming media or any other modality.

    4. Also, if the answer to (2) is yes then how does Lync determine how much bandwidth is being used on the connection? I don't believe it has access to this low-level statistic.

    No, it doesn't have access to low level statistic, it is not implemented at layer 2 or 3 network level.

      


    Malinda Perer | MCSM - Chatter Communication | MCM - Lync 2010 and OCS 2007 | http://malinblog.com/ | https://sg.linkedin.com/in/malindap

    Tuesday, December 8, 2015 1:12 PM