The EDCA (Enhanced Distributed Channel Access) mechanism proposed different contention and back-off times for the different access categories. Voice access category is configured to wait the least before transmission, followed by Video, Best Effort and Back-ground traffic class. Hence a degree of un-fairness was introduced giving the higher traffic class more priority for transmission. The manner in which it was achieved is detailed below
- Each Access category queue is given an Arbitration Inter-Frame Spacing Number (AIFSN)
- The AIFSN value determines the Arbitration Inter-Frame Spacing for the different access categories. This is different from the fixed DIFS time-period in the DCF mechanism
- The Arbitration Inter-Frame Spacing is given by the formula
- AIFS = AIFSN(AC) * Slot Time + SIFS time – where AIFSN(AC) is the number assigned for the particular access category as explained in 1
- By providing a different AIFSN number to each of the access category, the different access categories in EDCA are now contending for the medium on an un-equal footing (AIFS) as is not the case in the DCF mechanism (DIFS = SIFS time + 2 * Slot Time)
- In addition to providing a different Inter-Frame Space wait time, the contention window values for Best Effort/Background/Voice and Video was also kept different. Hence, the back-off window for each of the traffic categories was also different
- Each Access Category was also provided a fixed Transmit Opportunity time
- The different AIFSN numbers/contention window values and the back-off mechanism is provided below in tabular column
| Access Category | AIFSN | CWmin | CWmax | TXOP time |
| Voice | 1 | 3 | 7 | 1.5ms |
| Video | 1 | 7 | 15 | 3.0ms |
| Best Effort | 3 | 15 | 63 | 0 |
| Back ground | 7 | 15 | 1023 | 0 |
NOTE: for 802.11b – the above table and values are different. Refer hostapd.conf file (Linux open source) to know the values for 802.11b
An example of how different access categories were given different weights and how it affects traffic prioritization is provided below
- the slot time in 802.11ac is 9 uS. Hence, the AIFS time is given by the formula above would be
- For Voice – 1 * 9 uS + 16 uS = 25 uS + exponential backoff (given by the minimum and maximum Contention window values)
- For Best Effort – 3* 9 uS + 16 uS = 43 uS + exponential backoff
- If exponential back-off for voice were to be 7uS and for Best effort as 63uS (from the table above – max window value)
- A Voice packet will wait 32 uS before it attempts transmission
- A Best Effort packet will have to wait 106 uS before it attempts transmission
The below picture depicts the EDCA mechanism
As can be seen from the picture above – the back-off counter of the access categories that do not win contention are frozen at that value till medium is again sensed to be idle. Hence, the Access categories with lesser AIFS value and back-off win the medium more often leading to better utilization of the medium by the higher priority Access categories.