Editing IEEE 802.11e-2005 (section)

=== Enhanced distributed channel access (EDCA) ===
{{Disputed section|date=March 2014}}
EDCA is a supported [[Quality of service|QoS]] mechanism in 802.11e. With EDCA, high-priority traffic has a higher chance of being sent than low-priority traffic: a station with high priority traffic waits a little less before it sends its packet, on average, than a station with low priority traffic. This is accomplished through the TCMA protocol, which is a variation of CSMA/CA using a shorter [[arbitration inter-frame space]] (AIFS) for higher priority packets.<ref>M. Benveniste, "Tiered Contention Multiple Access' (TCMA), a QoS-Based Distributed MAC Protocol", Proceedings PIMRC 2002, Lisboa, Portugal, September 2002</ref> The exact values depend on the physical layer that is used to transmit the data. In addition, EDCA provides contention-free access to the channel for a period called a Transmit Opportunity (TXOP). A TXOP is a bounded time interval during which a station can send as many frames as possible (as long as the duration of the transmissions does not extend beyond the maximum duration of the TXOP). If a frame is too large to be transmitted in a single TXOP, it should be fragmented into smaller frames. The use of TXOPs reduces the problem of low rate stations gaining an inordinate amount of channel time in the legacy [[802.11]] [[Distributed Coordination Function|DCF]] MAC. A TXOP time interval of 0 means it is limited to a single MAC service data unit (MSDU) or MAC management protocol data unit (MMPDU).

The levels of priority in EDCA are called access categories (ACs). The contention window (CW) can be set according to the traffic expected in each access category, with a wider window needed for categories with heavier traffic.  The CWmin and CWmax values are calculated from aCWmin and aCWmax values, respectively, that are defined for each physical layer supported by 802.11e.

{| class="wikitable"
|+Calculation of contention window boundaries
|-
! AC
! CWmin
! CWmax
|-
| Background (AC_BK)
| aCWmin
| aCWmax
|-
| Best Effort (AC_BE)
| aCWmin
| aCWmax
|-
| Video (AC_VI)
| (aCWmin+1)/2-1
| aCWmin
|-
| Voice (AC_VO)
| (aCWmin+1)/4-1
| (aCWmin+1)/2-1
|}

For a typical of aCWmin=15 and aCWmax=1023, as used, for example, by [[OFDM]] (802.11a) and [[MIMO]] (802.11n), the resulting values are as following:

{| class="wikitable"
|+Default EDCA parameters for each AC
|-
! AC
! CWmin
! CWmax
! AIFSN
! Max TXOP
|-
| Background (AC_BK)
| 15
| 1023
| 7
| 0
|-
| Best Effort (AC_BE)
| 15
| 1023
| 3
| 0
|-
| Video (AC_VI)
| 7
| 15
| 2
| 3.008ms
|-
| Voice (AC_VO)
| 3
| 7
| 2
| 1.504ms
|-
| Legacy DCF
| 15
| 1023
| 2
| 0
|}

ACs map directly from Ethernet-level [[IEEE 802.1D|class of service (CoS)]] priority levels:
{| class="wikitable"
|-
! –
! colspan="3" | [[IEEE P802.1p|802.1p]]
! colspan="2" | 802.11e
|-
! Priority
! Priority code point (PCP)
! Abbreviation 
! Traffic type 
! Access category (AC) 
! Designation 
|-
| Lowest
| 1
| BK
| Background
| AC_BK
| Background
|-
|
| 2
| –
| Spare
| AC_BK
| Background
|-
|
| 0
| BE
| Best effort
| AC_BE
| Best effort
|-
|
| 3
| EE
| Excellent effort
| AC_BE
| Best effort
|-
|
| 4
| CL
| Controlled load
| AC_VI
| Video
|-
|
| 5
| VI
| Video
| AC_VI 
| Video
|-
|
| 6
| VO
| Voice
| AC_VO
| Voice
|-
| Highest
| 7
| NC
| Network control
| AC_VO
| Voice
|}

The primary purpose of QoS is to protect high priority data from low priority data. There are also scenarios in which the data needs to be protected from other data of the same class. Admission Control in EDCA address these type of problems. The AP publishes the available bandwidth in beacons. Clients can check the available bandwidth before adding more traffic.

[[Wi-Fi Multimedia]] (WMM) is the Wi-Fi Alliance specification which is a subset of IEEE 802.11e. Certified APs must be enabled for EDCA and TXOP.  All other enhancements of 802.11e are optional.