Editing IEEE 802.11e-2005 (section)

== 802.11e MAC protocol operation {{anchor|Hybrid coordination function}}==
[[Image:OSI-80211e.png|thumb|right|300px|A diagram of the seven-layer OSI model with the modifications made by the 802.11 standard and the 802.11e amendment<ref>{{Cite web|title=802.11n: Next-Generation Wireless LAN Technology|url=http://www.broadcom.com/collateral/wp/802_11n-WP100-R.pdf|date=2006-04-21|publisher=[[Broadcom Corporation]]}}</ref>]]
The 802.11e enhances the DCF and the PCF, through a new coordination function: the hybrid coordination function (HCF).  Within the HCF, there are two methods of channel accessed, similar to those defined in the legacy 802.11 MAC: HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA).  Both EDCA and HCCA define Traffic Categories (TC).  For example, [[email]]s could be assigned to a low priority class, and voice over wireless LAN (VoWLAN) could be assigned to a high priority class.

=== 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.

=== HCF controlled channel access (HCCA) ===
The HCF (hybrid coordination function) controlled channel access (HCCA) works a lot like PCF. However, in contrast to PCF, in which the interval between two beacon frames is divided into two periods of CFP and CP, the HCCA allows for CFPs being initiated at almost anytime during a CP. This kind of CFP is called a Controlled Access Phase (CAP) in 802.11e. A CAP is initiated by the AP whenever it wants to send a frame to a station or receive a frame from a station in a contention-free manner. In fact, the CFP is a CAP too. During a CAP, the Hybrid Coordinator (HC)—which is also the AP—controls the access to the medium.  During the CP, all stations function in EDCA.  The other difference with the PCF is that Traffic Class (TC) and Traffic Streams (TS) are defined. This means that the HC is not limited to per-station queuing and can provide a kind of per-session service.  Also, the HC can coordinate these streams or sessions in any fashion it chooses (not just round-robin).  Moreover, the stations give info about the lengths of their queues for each Traffic Class (TC).  The HC can use this info to give priority to one station over another, or better adjust its scheduling mechanism.  Another difference is that stations are given a TXOP: they may send multiple packets in a row, for a given time period selected by the HC.  During the CFP, the HC allows stations to send data by sending CF-Poll frames.

HCCA is generally considered the most advanced (and complex) coordination function.  With the HCCA, QoS can be configured with great precision.  QoS-enabled stations have the ability to request specific transmission parameters (data rate, jitter, etc.) which should allow advanced applications like VoIP and video streaming to work more effectively on a Wi-Fi network.

HCCA support is not mandatory for 802.11e APs.  In fact, few (if any) APs currently available are enabled for HCCA.{{Citation needed|date=July 2010}} Implementing the HCCA on end stations uses the existing DCF mechanism for channel access (no change to DCF or EDCA operation is needed). Stations only need to be able to respond to poll messages. On the AP side, a scheduler and queuing mechanism is needed.