Cisco Cisco Aironet 3700i Access Point Weißbuch

 

 

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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.