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802.11 Arbitration Part 2: Getting your marbles in the right bucket

Part Two: EDCA

How is it different than DCF?

Wireless is a shared medium. The fundamental method for sharing this medium is DCF, Distributed Coordination Function. This is the method used when QoS is not present. When QoS is present EDCA (Enhanced Distributed Coordination Function) is used to access the channel.

EDCA observes many of the same steps as DCF. Carrier sense is performed exactly as it is in DCF.

The way a STA observes an IFS and the parameters of the contention window are different.

The IFS used in a QoS BSS (Basic Service Set) is called an AIFS (arbitration interframe space). The length of the AIFS is unique for each QoS access category.

In DCF the CW has one range, 0-1023. In EDCA, the CW size is defined for each access category.

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