Cisco Cisco Aironet 3700i Access Point Livre blanc
© 2015 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
Page 9 of 10
The 802.11e amendment introduced prioritization through Enhanced Distributed Channel Access (EDCA). EDCA is
still obsessively polite, but introduces a controlled unfairness in order to enable an advantage in onto-the-air
access for some traffic types. Essentially, the mandatory minimum wait time for some traffic types is less than that
for other traffic types. Also, the subsequent mandatory randomly selected wait time is less for some traffic types
compared to other traffic types. That is, the set of random numbers is fewer for some traffic types.
At a high level, EDCA creates a set of access categories based on traffic type. Traffic classified as voice waits less
time than traffic classified as video, which waits less time than traffic classified as best effort, which waits less time
than traffic classified as background. Best-effort traffic in EDCA has similar, but not identical, access opportunity to
legacy Distributed Channel Access (DCA).
In Figure 7, WMM is the Wi-Fi Alliance certification for EDCA behavior.
Figure 7. WMM Prioritization
As can be seen, the net effect is that some traffic types get on the air sooner than other traffic types (waiting less
time equals a higher priority). Each traffic type contends equally for airtime within its corresponding access
category, but contention between traffic types, and access categories, is unequal.
However, this also means that the nature of the traffic itself determines access to the air. There is no way the
802.11 standard can be used alone to segment or partition airtime, and therefore, allow the network administrator
to determine or define fairness. This is the issue the Cisco Air Time Fairness solution addresses.