• Hi all. I'm studying the CWDP (great cert) and wanted to clear up my own confusion around wireless and QoS.....

    1. The AP broadcasts (beacons) include the four WMM AC (Access Categories) and for each AC it states the CWmin, CWmax, AIFSN, etc. The first question is: Assuming that a client associates to the SSID in the beacon, does the client use those AC values (CWmin, etc)? For example, a client has a packet for AC Voice, does he have to use the AIFSN contained in the beacons?

    2. From what I understand, it is the clients responsibility to appropriately 'prioritise' the uplink traffic (traffic from client to AP). For example, if I have a voice application running on my PC, the application MUST set the UP (User Priority) correctly and as this goes down the clients own stack, at the MAC layer the client will map the UP to a WMM AC (in this case AC Voice). Is that correct?

    3. Assuming the answer to 2 is "yes" then surely this trust model is open to abuse (for uplink taffic). For example, I could (theoretically) write an application that marks the UP as 7 which would be mapped to WMM AC Voice. This means that all 'my' traffic would be preffered (statistically) over all other client traffic (assuming all other traffic was WMM AC_BE - Best Effort). Any comments?

    4. Next, is the UP-to-WMM AC mapping fixed on the client? I can see in the 802.11-2007 standard where the mapping is shown (UP 4 maps to WMM AC_VI for example) but I wasn't sure if this mapping was statically fixed on the clients or you configure it? Can anyone confirm?

    5. Finally, in Cisco (and probably other vendors) you can assign a WMM AC to a client using AAA attributes. For example, an SSID in Cisco speak is assigned to the "Gold" profile which is WMM AC_VI. However, when a client authenticates the RADIUS server passes a RADIUS atrribute to the wireless LAN controller which says this client is assigned to the "Silver" profile which is WMM AC_BE. My question is, this QoS policy only applies to downlink traffic (traffic from AP-to-client) right? I just want to confirm that this does not affect uplink traffic (traffic from client-to-AP) as this QoS mapping depends on the UP set by the clients AP (see point 2 above). 

    Thanks for any pointers or comments guys, it would be appreciated.


  • Hi Darren,

    1. Client has to follow the values which are negotiated,.i.e Assoc response from the AP and the client has to follow the same AIFSN. 

    2. Yes

    3. Yes, you can toggle the bits, if you write ur own software 

    4. I think it could be configurable somewhere for sure, but I am not aware of the config

    5. Not worked on Cisco AP's


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