This would be the last post of the 5 part series “Understanding wireless QoS posts”. In this post we will look at How QoS value change under two specific conditions.

1.Cient is WMM capable, WMM is disabled on WLC
2.Client is not WMM capable, WMM is configured on WLC (Allowed option)

Network topology is same as the one used for previous posts (see below)Wireless QoS-3-1

Following shows the values I observed when Cisco7921 & IPcommunicator in Dell Laptop(XP sp2 operationg system & 3495ABG Intel wireless driver) used as wireless client. In this scenario I have disabled the WMM on WLC.Wireless QoS-5-2

This is how we can interpret this. When wireless device put its traffic into wireless media it will set up WMM user priority in wireless frames. When it comes to access point, this time it ignores this “UP” value comes from client’s wireless frame as WMM disable on the wireless network.Therefore AP will set outer DSCP value purely based on 802.1p configured for QoS profile. In my case it is configured with Platinum  QoS profile & therefore set to priority value of 6 which is equivalent to EF in DSCP terms according to 802.11e. Note that all traffic will be marked as EF in the outer DSCP field.

When this goes to WLC, WLC will set to 802.1p value to 5 before it put into wired media towards PC. If you configured trust CoS of the WLC connected port, switch will trust this CoS value & rewrite packet’s DSCP value prior to go out of Fa 1/0/4. Since every packets comes with CoS of 5 , all of them will re-write as DSCP EF.

Instead of trusting CoS, if you trust DSCP at the WLC connected switch port, original packet DSCP will go into wired media.

In the 2nd scenario I have to use non-WMM capable client. Tried to disable WMM on my 3495ABG wireless card to do this. But still I can see QoS control field in wireless frame & different UP values for RPT & Signalling traffic. As per  the Cisco document 7920 phone is non-WMM capable, which I could not find to test.(I will do more digging to find a wireless client to do this test & will post the result)

As per my readings & current understanding this is how it should work. When AP received wireless frames from client there is no WMM_UP value on it. When AP sends it to WLC it map inner DSCP to outer DSCP as long as QoS profile configured 802.1p value equivalent DSCP is higher than the incoming packet DSCP. I found the below blog post discussed about this and hence posted here as it gives useful insight on this topic.(http://onlinestudylist.com/archives/ccie_wireless/2011-March/001985.html)

If a packet received from wired site to WLC without 802.1p tagging (i.e native vlan traffic of WLC connected trunk port), WLC will not set any outer DSCP & traffic will handle as Best Effort.

Update 5th Jan 2014:
In the above 2nd scenario, if the client is non-WMM capable then outer DSCP value is based on the QoS profile configured on the WLAN. If WLAN is configured for platinum profile all non-WMM client traffic will get EF as outer DSCP. In the above my understanding was wrong.

To avoid such situation , your WLAN QoS to be configured as WMM required & in that way non-WMM clients are unable to connect to your wireless network.


Related Posts

1. Understanding Wireless QoS – Part 1
2. Understanding Wireless QoS – Part 2
3. Understanding Wireless QoS – Part 3
4. Understanding Wireless QoS – Part 4
5. 3750/3560/2960 Wired QoS
6. Who do you trust ? (DSCP or CoS)