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  • Question about QoS and Admission Control


    We currently have a two site ShoreTel 9.1 system connected via T1s (MPLS). We also have a PRI coming into each site for external calls. My question is about Admission Control and QoS.

    Currently, our Admission Control bandwidth for both sites is set at 768 kbps. It's been set like that ever since the system was installed, and I don't believe anyone has ever been denied when trying to dial over to an internal extension at the other office.

    I was just speaking to our WAN provider, and they said that they have QoS set at 75% for VoIP at the highest service level, and 25% for Citrix at a level below. So when the line is at full utilization, 75% of that will be reserved for VoIP.

    Is this necessary? We've noticed that Citrix over the WAN slows when the line gets clogged, so we'd like to bump the QoS percentage up on that, but not at the expense of our phones!

    Given this situation, would a 50/50 split (theoretical 786 kbps each) work safely for our phone system? I can't think why it wouldn't but then again, I really don't want to be wrong either.

    Thanks in advance,

  • #2

    This all comes down to how many simultaneous calls between sites you want to be able to support. Find out which codec you are using for inter-site calls, multiply the bandwidth per call by how many calls you want to be able to support and use that as a number for your QOS %.

    Something doesn't add up though.

    if your have a 75% priority on VOIP and 25% priority on Citrix traffic, then what does all your other traffic get during contention? Nothing? If this is true, then this is a REALLY bad QOS setup.................

    It is very likely that your phone company only supports QOS on PART of the circuit. You may be setup running 75% and 25% on 768K only..., or 512k only. The other "half" of the circuit is set to "default/open". If this is the case you would have 576k prioritized for voice, 192 for ICA traffic, and the other half of the circuit for everything else. Make sure the circuit is actually having QOS set for the whole 1.54mbs, and not a portion of it.

    If they are tagging QOS for the entire 1.54mbps, and you have your Admission control set to 768k already, then you are already limiting VOIP related traffic between sites to half of the t1 anyway.... having QOS set for VOIP over 50% isn't doing anything for you.

    I would recommend trying something multi-layered like this:

    VOIP Traffic (TRUE VOICE, not PCM, etc)
    40% - True Priority Queue

    PCM and other Shoretel related NON VOICE traffic- call control, etc 5% - Bandwidth Queue

    Citrix Traffic 40% True Priority Queue

    Low Priority Queue - Bandwidth Queue - 2% - Put any Non latency
    dependant traffic here - SMTP - WINDOWS UPDATES, DFS File Replication, ETC ETC things that you dont care how long it takes to transfer.

    Default - unclassified remaining traffic - Bandwidth Queue 13%

    Any Bandwidth NOT being used is divided up according to your percent rules. For example, if there is no VOIP traffic, citrix traffic could use 100% of the line. If Citrix and some gigantic email were fighting for bandwidth, the SMTP low priority queue would get 2% or 30K and citrix would get its 40% guranteed + 98 percent of the remaining bandwidth, or 617k (guranteed) + 908k (98% of the remaining bandwidth).

    Adjust the values as you like to suit what you need.

    You have to be really careful at how they "classify" the groups. Are they including 1494 TCP and UDP and 2598 TCP AND UDP in the QOS group for Citrix traffic? Many people just think of 1494 TCP, and forget the other possible ports. If you are missing a port, then you may not even be prioritizing the traffic!

    I have found in QOS that restricting LOW priority traffic is often as important (or more so) than prioritizing high priority traffic. For example, if you are running DFS replication between the sites, it is very easy to limit it to 2% bandwidth. If you dont, it can starve out any "unclassified" data. Most people prioritize their voice traffic only, leaving the PCM traffic out to dry. You would surely want your PCM traffic to get priority over your DFS traffic, or an email with a 15MB attachment. Who cares if the email takes another minute to get where it is going right? If you can't hang up your phone call, that is bad.
    Last edited by eazeaz; 04-08-2010, 01:54 PM.


    • #3
      Thanks for the info. I don't control the firewall myself - that's hosted by our ISP. I just told them that I need VoIP and Citrix prioritized, as if nothing else those must get through. I don't know the exact rules that they set up.

      Lots of good stuff here to think about.

      Thanks again,