• I'm studying for CWAP exam and need some clarification on QoS.  I understand the that the upper app layers give an 802.1D tag to a frame and these tags are mapped to a user priority (UP) and then EDCA defines these UPs as one of four access categories (ACs) for prioritized QoS.  However, I do not understand when paramaterized QoS kicks in (admission control).  Why/when is it required to do a ADDTS action frame to introduce a new traffic stream (TS) into the cell when a voice or video frame is already tagged as a high priority frame (AC_VO or AC_VI)?  Does the ADDTS action frame supercede the AC category or add an additional priority ranking over an AC_VO or AC_VI frame?  It does not seem to be real clear on when this "new" TS is needed, why it is needed or how it is added.  Any clarification or web links would be appreciated.

    Page 174 in the CWAP text states: "Managing QoS in wireless cells is complex."  Trying to understand it is complex, too.

  • -> Admission control is required when a STA desires a guarantee of the amount of time that it has available to access the channel.
    ->When AP receives ADDTS it will reserve Medium Time for that particular TS. 
    -> It is APs job to protect the admitted TS.

    For example lets consider STA sends ADDTS for Voice and AP accepts it. From wired side saturate the air with un-admitted traffic[Best Effort]. It is APs job to protect the admitted traffic. It’s up to the vendor how it handles the un-admitted traffic; one method is that the AP could drop some or all of the un-admitted traffic stream.

  • Sathwik, Thank you for the response. A few other questions: 1. I am able to set the ADM control bit at my AP (confirmed in beacon WMM Parameter Set field) to try to capture packets that use admission control (VI or VO). However, I am not sure what apps request admission control (since requests for higher priority ACs are made at the app layers and not at layer 2, and admission control is suggested to be used only by VI and VO, not BK or BE). VoIP calls probably use admission control, I am thinking. What VI apps use admission control. 2. I would like to capture action frames to analyze these (ADDTS request/ADDTS response/DELTS), but need to know what apps would indeed use admission control prior to running the capture tool (Wireshark). Thank you.

  • The main use of WMM-AC is to reserve medium and to protect admitted TS from un-admitted TS. If there is some delay in BE & BK, its acceptable. However VO & VI needs to be quick. Hence normally ACM is used for VO & VI. Even WMM_AC certification mainly concentrates on VO & VI.
    You dont want to see some delay in Video when you streaming. Hence normally ACM is enabled for VI.

    I have seen Spectralink phones and Cisco phones sending ADDTS, DELTS etc.

    I remember Blackberry being used in WMM-AC certification. Not sure which app on it uses AC and sends ADDTS.

    I have the capture of ADDTS req/res & DELTS. If you want I can send it to you.

  • Sathwik,
    It would be great if you could send a link or a direct contact with a ADDTS/DELTS capture so I could study these.
    Thank you.

  • By Sathwik - edited: April 29, 2013


    I have uploaded the packet capture in the following link

    {Link removed by administration}

    There are so many download buttons, click {Link removed by administration} to download the file.

    All the best for CWAP. Even I am preparing for the same :)

  • I'm sorry Sathwick, we do not allow links to download sites that require the installation of an executable to download. If you want to provide the download, please place it in a ZIP file (not an executable) and then provide a direct download link.


    Thanks for understanding,


  • Hi Tom,

    Sorry for that.

    Actually that was free file hosting server. Those exe were  advertisement/may be virus from the server. But the link "Download from sendspace" would download the .pkt file directly without installing any exes. Thats why I explicitly mentioned to click that. Anyway going forward I wont post those links.

    Hi gcate,

     you can send unicast mail to my mail id "". I can forward you the packet capture.


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