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How can QoS be maintained on 802.11?

Since 802.11 uses Time Division Duplex (TDD), only one device uses the medium at any given time and is either in transmit or receive mode. How can QoS be maintained on 802.11?
As you note, every 802.11 station has equal access to the shared medium (channel). Prioritization and the ability to managed Quality of Service (QoS) requires an extension to the 802.11 standard that is now underway.

The Wi-Fi Alliance's WMM (Wi-Fi Multimedia) is a subset of the IEEE 802.11e QoS draft standard. WMM defines four access categories: voice, video, best-effort, and background. Each packet is tagged according to category. Packets in higher priority categories remain in queue for shorter periods, while packets in lower priority categories stay in-queue longer. This is done using extensions to the CSMA/CD Distributed Coordination Function (DCF) which adjust a station's minimum interframe spacing and random backoff wait parameters that apply to each category. To learn more about how 802.11 QoS and WMM work, read this WMM white paper.

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