The question about WMM 802.11e

[David Tian]

Hi Experts,

I would like to have a discussion/consult about some confuse about the wireless QoS WMM 802.11e, as I understand, it work on Layer2, between the clients and Access points, to achieve that let the critical traffic have a high priority during CSMA/CA.

My question is, how to identify which frames are voice, video or others just also like wired network, to let the clients to mark it when send it ?

Please correct me if I misunderstand , Thanks,

Br,

David 

[Mitch D · Accepted solution]

Hello David,

To simplify my answer as much as possible you can use AVC to identify and remark traffic after it enters the AP. For example if a video packet was sent unmarked with a WMM UP Value (Access Category - AC) of 0 and no DSCP markings you could at the AP level (in newer version of 8.0) identify the traffic as say Facetime traffic. The AP can set the outer DSCP markings of that packet to whatever you choose probably 34. It will be sent via the wired network encapsulated in CAPWAP to the controller. The controller can then mark the return traffic with 34 as it sends data to the AP. The AP will then use it's DSCP to UP Value lookup table to mark the traffic with an UP Value of 4 or 5. The effect of this is the AP will queue this frame in a higher priority queue and therefore will have a lower cw_min and cw_max value. 

A client would have to mark its own traffic it wants to transmit to the AP to ensure it gets the priority it needs. Some clients just don't mark traffic. Another issue is a client on Free Wifi can choose to set their WMM value to 6 or 7 and take priority away for your more important clients. Luckily you can change the markings on the return traffic to whatever you want.

Someone can correct me if I'm wrong. Cisco Wireless QoS is pretty complex.

[David Tian]

Hi Mitch, 

Yes, you are right, I make sense on that after checking some documents.

However, some more discussion.

for the Downstream traffic (from WLC to Client), the controller will use the DSCP of IP packet to map the outer DSCP of CAPWAP packet. and the AP will use the CAPWAP DSCP value to map 802.11e UP value to prioritize the packet.

for the Upstream traffic, yes, the client (MS application, Apple iOS) will send with the UP value and inner DSCP value together.