In this post we will see the impact of enabling or disabling WMM (Wireless Multi Media) to QoS of wireless traffic in a given WLAN.
WMM setting is based on IEEE 802.11e standard to provide QoS to wireless network. An 802.11e (or WMM) client that obtain medium access must not utilize radio resources for a duration longer than specified limit (called transmission opportunity or TXOP). Also WMM client will classify their traffic in to four access classes (AC_VO, AC_VI, AC_BE & AC_BK). Below diagram show the difference with legacy system & WMM capable system.
Here is the topology for our post. Wireless Phone is connected to a WLAN configured with Platinum QoS profile. Traffic flow “D” & “E” will be looked at in detail for QoS analysis while WMM is enabled & disabled on this WLAN.
Here is the basic WLAN configuration
(4402-3) >config interface create vlan1 1 (4402-3) >config interface address dynamic-interface vlan1 192.168.2.249 255.255.255.0 192.168.2.250 (4402-3) >config interface dhcp dynamic-interface vlan1 primary 192.168.2.250 (4402-3) >config wlan create 1 ONE ONE (4402-3) >config wlan interface 1 vlan1 (4402-3) >config wlan radio 1 802.11a-only (4402-3) >config wlan security wpa disable 1 (4402-3) >config wlan qos 1 platinum (4402-3) >config wlan wmm require 1 (4402-3) >config wlan enable 1
Here is the QoS settings in GUI if you are familiar with it than CLI.
In this configuration we have restricted only WMM capable clients can join to this WLAN (by selecting WMM require option). WLAN is configured for Platinum profile where upto CoS =5 (or DSCP EF) QoS is allowed.
When 7921 phone make a call to 7965 phone here are the two different type of traffic (signalling – SCCP & media – RTP) wireless frame captures.
When AP receive these frames it will convert 802.11e priority value to CAPWAP header DSCP values. Since we have configured Platinum profile it will allow upto 802.11e priority value of 6. So in this case Voice traffic CAPWAP header DSCP would be EF & Signalling traffic CAPWAP header DSCP would be AF31 (as per the 802.11e to AVVID 802.1p mapping table). Below captures taken at AP connected switch port (Fa 1/0/4) prove this.
As long as you trust DSCP on AP connected switch port(Fa1/0/4) & CoS on WLC connected switchport (G1/0/1), this traffic will go to wired network with CoS=5 for voice & CoS=3 for signalling traffic.
Now let’s disable WMM on this WLAN & see what impact it would do on the traffic QoS. You can disable it via GUI or CLI & here is the CLI way of doing that.
(4402-3) >config wlan disable 1 (4402-3) >config wlan wmm disable 1 Warning: 802.11n requires WMM to be enabled (4402-3) >config wlan enable 1
As you see above this will impact the 802.11n functionality. If you disable WMM on a WLAN client won’t get 802.11n data rates.
Once WMM is disabled, AP is not negotiating any WMM settings and client wireless traffic comes without any WMM priority information. You can see both SCCP & RTP traffic wireless frames no WMM information available.
Since no priority values comes in wireless frames ALL traffic will get the QoS profile’s DSCP/CoS value in this time. So both traffic will get DSCP EF in CAPWAP header & translated into CoS of 5 prior to send to to wired network by WLC. So in this time all traffic coming from wireless client will mark as EF. Here is the capture of AP connected switch port.
Therefore it is important to understand this difference when you configuring QoS profile & WMM settings.
When enabling WMM there is another option called “WMM Allowed”. In this scenario it will allow both WMM capable client & non-WMM capable client to join the WLAN. But all non-WMM client traffic will mark as per the configured QoS profile.
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. Understanding Wireless QoS – Part 5
6. 3750/3560/2960 Wired QoS
7. Who do you trust ? (DSCP or CoS)
8. BYOD with QoS
9. QoS for H-REAP
10. VoIP Phone – Switchport Config
11. Autonomous AP – QoS
12. AAP QoS – A Closer Look
13. A Wireless Bridge with QoS