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QoS advice

I have a Cisco AP1142N access point where I can enable QoS.

I have a few questions relating to QoS and how it actually works with devices:

  1. How does the Cisco access point      differentiate between traffic streams?

  2. What happens if the source of the      stream is not a Cisco device, e.g.: Netperf traffic generator?

  3. What is the access point (whether      it is Cisco or not) examining to determine what sort of traffic      stream it is?

  4. Is it the TOS field?

  5. Should all applications in the      network be QoS-aware?

Cisco Employee

Re: QoS advice

Hi Reg,

Qos is a large topic but I'll try to shortly answer your questions.

1. There is a 802.11e (specific to wireless QoS) field with the traffic priority class on each wireless frame. So the AP easily know what traffic is voice/video/best effort

2. QoS is nothing Cisco specific. Devices should support WMM (it's a standard) to do wireless QoS. When they associate, clients mention to the AP if they are WMM capable or not.

The AP will only trust traffic that is WMM and that is tagged with QoS. Un-tagged traffic will be considered best-effort.

To answer the question directly, I doubt that iperf sends packets marked with QoS on the wireless. I've only see softphones applications doing that.

3.See answer 1

4. No. One thing to note is that your application can send a packet with a DSCP value or whatsoever, it will not have any effect on the wireless. The application needs to instruct the driver to send the frame with a QoS tag over the air.

5.No. Non-qos aware applications send untagged traffic that is considered "best effort" class and non-wmm capable stations are also in that category by default.

I'm adding few precisions :

6. The AP, upon receiving a frame with a 802.11e field of 6 will convert it to COS 5 on the wired so that the wired network has a way of recognizing the QoS. When COS-tagged frames arrive to the AP for a wireless client, the AP is adding the 802.11e field for the wireless transmission.

7. One thing is that the QoS tag is there for the AP to trust the traffic in high priority and keep the tag for wired processing afterwards. but the most important part is when you are sending the QoS frame. QoS frame enjoy shorter time to access the medium on wireless, so they have a better chance to be sent before any other station send its data. This means that the client has to send the frame in the QoS fashion (the AP does it as well when it sends to client). The point I'm reaching with this is that the application needs to specifically instruct the wireless adapter to use shorter timers and to add this 802.11e field.

Most applications just add a DSCP field on the packet and hope the network will take care of it. This is why only few applications really work with wireless QoS.

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