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Cisco IOS Software Configuration Guide for Cisco Aironet Access Points
OL-30644-01
Chapter15 Configuring QoS
Understanding QoS for Wireless LANs
2. QoS Element for Wireless Phones setting—If you enable the QoS Element for Wireless Phones
setting, dynamic voice classifiers are created for are created for RTP-based traffic, which allows the
wireless phone traffic to be a higher priority than other clients’ traffic. Additionally, the QoS Basic
Service Set (QBSS) is enabled to advertise channel load information in the beacon and probe
response frames. Some IP phones use QBSS elements to determine which access point to associate
to, based on the traffic load.
You can use the Cisco IOS command dot11 phone dot11e command to enable 802.11e/WMM QBSS
Load IE. The 7920 phones with 1.05 firmware, and older, do not support the 802.11e QBSS IE.
If your network wireless clients are primarily 7920 phones with firmware 1.05 or older, enable dot11
phone.
If your network wireless clients are primarily 7920 with firmware 1.09 or later, or WMM compatible
VoWLAN phones, enable the IEEE 802.11e compatible QBSS IE with the command dot11 phone
dot11e.
This example shows how to enable IEEE 802.11 phone support with the legacy QBSS Load
element:
AP(config)# dot11 phone
This example shows how to enable IEEE 802.11 phone support with the standard IEEE 802.11e
QBSS Load element:
AP(config)# dot11 phone dot11e
This example shows how to stop or disable the IEEE 802.11 phone support:
AP(config)# no dot11 phone
3. Policies you create on the access point—QoS Policies that you create and apply to VLANs or to the
access point interfaces are third in precedence after previously classified packets and the QoS
Element for Wireless Phones setting.
4. Default classification for all packets on VLAN—If you set a default classification for all packets on
a VLAN, that policy is fourth in the precedence list.
Using Wi-Fi Multimedia Mode
When you enable QoS, the access point uses Wi-Fi Multimedia (WMM) mode by default. WMM
provides these enhancements over basic QoS mode:
•The access point adds each packet’s class of service to the packet’s 802.11 header to be passed to
the receiving station.
•Each access class has its own 802.11 sequence number. The sequence number allows a high-priority
packet to interrupt the retries of a lower-priority packet without overflowing the duplicate checking
buffer on the receiving side.
•WPA/WPA2 replay detection is done per access class on the receiver. Like 802.11 sequence
numbering, WPA/WPA2 replay detection allows high-priority packets to interrupt lower priority
retries without signaling a replay on the receiving station.
•For access classes that are configured to allow it, transmitters that are qualified to transmit through
the normal backoff procedure are allowed to send a set of pending packets during the configured
transmit opportunity (a specific number of microseconds). Sending a set of pending packets
improves throughput because each packet does not have to wait for a backoff to gain access; instead,
the packets can be transmitted immediately one after the other.
•U-APSD Power Save is enabled.