E-DOC-CTC-20060609-0001 v2.0
Chapter 4
Wi-Fi Multi Media (WMM)
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Extension to the DCF mechanism
WMM is an enhancement of the MAC sublayer to add QoS functionality to Wi-Fi networks. WMM is an
extension to the legacy CSMA/CA-based DCF mechanism that gives all devices equal access rights and that is
based on a best effort, “listen-before-talk” algorithm.
This collision avoidance method gives all the devices the opportunity to transmit, but, under high traffic
demand conditions, networks get overloaded and performance of all devices is equally affected.
WMM introduces traffic prioritization capabilities (based on the four access categories) that meet DCF’s
inadequacy to support multimedia applications.
The WMM access point by default categorizes packets not assigned to a specific access category (from legacy
wireless devices and devices that are not WMM-enabled) to the best effort priority.
WMM operation
WMM functions as follows:
1 Applications assign each data packet to a given access category.
2 Packets are then added to one of four independent transmit queues in the wireless station.
3 The wireless station has an internal collision resolution mechanism to cope with collision among
different queues, which selects the frames with the highest priority to transmit. The same mechanism
deals with external collision, to determine which wireless station should be given the Opportunity to
Transmit (TXOP).
Collision resolution algorithm
The collision resolution algorithm that is responsible for traffic prioritization is probabilistic and depends on
two timing parameters that vary for each access category:
> The minimum interframe space, or Arbitrary Inter-Frame Space Number (AIFSN);
> The Contention Window (CW), sometimes referred to as the Random Back off Wait.
Both values are smaller for high-priority traffic. For each access category, a back off value is calculated as the
sum of the AIFSN and a random value from zero to the CW. The value of the CW varies through time. Initially
the CW is set to a value that depends on the access category.
After each collision the CW is doubled until a maximum value (also dependent on the access category) is
reached. After successful transmission, the CW is reset to its initial, access category dependant value. The
access category with the lowest back off value gets the TXOP. As frames with the highest access category
tend to have the lowest back off values, they are more likely to get a TXOP.
Once a wireless station gains a TXOP, it is allowed to transmit for a given time that depends on the access
category and the PHY rate. For instance, the TXOP limit ranges from 0.2 ms (background priority) to 3 ms
(video priority) in an 802.11a/g network, and from 1.2 ms to 6 ms in an 802.11b network.
This bursting capability greatly enhances the efficiency for high data rate traffic, such as AV (Audio/Video)
streaming. In addition, devices operating at higher PHY rates are not penalized when devices that support
only lower PHY rates (for example because of distance) contend for medium access.
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