What is 802.11e WMM?

Aruba Employee
Aruba Employee

Product and Software: This article applies to all Aruba controllers and ArubaOS versions.


As users experience the convenience of wireless connectivity, they are beginning to demand support for the same applications they run over today's wired networks. Quality of service is increasingly important in 802.11 networks because wireless bandwidth availability is restricted.


To support bandwidth-sensitive applications like voice and video, IEEE proposed a new standard 802.11e. The original 802.11 media access control protocol was designed with two modes of communication for wireless stations.


·       Distributed Coordination Function (DCF) is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), and sometimes is referred to as "listen before talk." A station waits for a quiet period on the network and begins to transmit data and detect collisions.


·       Point Coordination Function (PCF) supports time-sensitive traffic flows. Wireless access points periodically send beacon frames to communicate network identification and management parameters specific to the wireless network. Between the sending of beacon frames, PCF splits the time into a contention-free period and a contention period. With PCF enabled, a station can transmit data during contention-free polling periods.


 However, DCF and PCF do not support any type of priority access of the wireless medium.


 802.11e uses Enhanced Distribution Coordination Function (EDCF) to categorize traffic. Each station has eight traffic categories or priority levels. Using EDCF, stations try to send data after detecting that the medium is idle and after waiting a period of time defined by the corresponding traffic category called the Arbitration Interframe Space (AIFS). A higher-priority traffic category will have a shorter AIFS than a lower-priority traffic category. Thus stations with lower-priority traffic must wait longer than those with high-priority traffic before trying to access the medium. 


The Wi-Fi Multimedia (WMM) specification is a subset of 802.11e. Makers of wireless access points as well as wireless client devices such as laptops, phones, and consumer electronics products have started to incorporate support for WMM into their products. WMM stipulates different fixed and random wait times for the four prioritization categories (voice, video, best effort, and background) to provide more favorable network access for applications that are less tolerant of packet delays. Devices that have less time to wait have a better chance of being able to transmit than those that have a longer wait.


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‎07-09-2014 04:02 PM
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