IEEE 802.11e-2005

IEEE 802.11e-2005 or 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of Quality of Service enhancements for wireless LAN applications through modifications to the Media Access Control (MAC) layer. The standard is considered of critical importance for delay-sensitive applications, such as Voice over Wireless LAN and streaming multimedia. The amendment has been incorporated into the published IEEE 802.11-2007 standard.

802.11 is an IEEE standard that allows devices such as laptop computers or cellular phones to join a wireless LAN widely used in the home, office and some commercial establishments.

Original 802.11 MAC

Distributed Coordination Function (DCF)

Main article: Distributed coordination function

The basic 802.11 MAC layer uses the distributed coordination function (DCF) to share the medium between multiple stations. DCF relies on CSMA/CA and optional 802.11 RTS/CTS to share the medium between stations. This has several limitations:

• if many stations attempt to communicate at the same time, many collisions will occur which will lower the available bandwidth and possibly lead to congestive collapse.
• there are no Quality of Service (QoS) guarantees. In particular, there is no notion of high or low priority traffic.
• once a station "wins" access to the medium, it may keep the medium for as long as it chooses. If a station has a low bit rate (1 Mbit/s, for example), then it will take a long time to send its packet, and transmission from all other stations will be held off.

Point Coordination Function (PCF)

Main article: Point coordination function

The original 802.11 MAC defines another coordination function called the point coordination function (PCF). This is available only in "infrastructure" mode, where stations are connected to the network through an Access Point (AP). This mode is optional, and only very few APs or Wi-Fi adapters actually implement it. APs send beacon frames at regular intervals (usually every 0.1 second). Between these beacon frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP). In the CP, DCF is used. In the CFP, the AP sends Contention-Free-Poll (CF-Poll) packets to each station, one at a time, to give them the right to send a packet. The AP is the coordinator. Although this allows for a better management of QoS, PCF does not define classes of traffic as is common with other QoS systems (e.g. 802.1p and DiffServ).

802.11e MAC protocol operation

A diagram of the 7-layer OSI model with the modifications made by the 802.11 standard and the 802.11e amendment.

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 access, 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, emails 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)

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 by using a shorter contention window (CW) and shorter arbitration inter-frame space (AIFS) for higher priority packets. 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 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).

Default EDCA Parameters for each AC

AC	CWmin	CWmax	AIFSN	Max TXOP
Background (AC_BK)	31	1023	7	0
Best Effort (AC_BE)	31	1023	3	0
Video (AC_VI)	15	31	2	3.008ms
Voice (AC_VO)	7	15	2	1.504ms
Legacy DCF	15	1023	2	0

ACs map directly from Ethernet-level class of service (CoS) priority levels:

Priority	802.1p Priority	802.1p Designation	Access Category
Lowest	1	BK	AC_BK
	0	BE	AC_BE
	2	EE	AC_BE
	3	CA	Video (AC_VI)
	4	VI	Video (AC_VI)
	5	VO	Voice (AC_VO)
	6	IC	Voice (AC_VO)
Highest	7	NC	Voice (AC_VO)

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) 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 CP, 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. 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.

Other 802.11e specifications

In addition to HCCA, EDCA and TXOP, 802.11e specifies additional optional protocols for enhanced 802.11 MAC layer QoS:

Automatic power save delivery

Automatic power save delivery is a more efficient power management method than legacy 802.11 Power Save Polling. The literature includes an 802.11 Power Save Mode overview, an analysis of unscheduled and scheduled automatic power save delivery (APSD) and a comparison of APSD versus 802.11 Power Save Mode performance. Most newer 802.11 stations already support a power management mechanism similar to APSD. APSD is very useful for a VoIP phone, as data rates are roughly the same in both directions. Whenever voice data is sent to the access point, the access point is triggered to send the buffered voice data in the other direction. After that the VoIP phone enters a doze state until next voice data has to be sent to the access point.

Block acknowledgments

Block acknowledgments allow an entire TXOP to be acknowledged in a single frame. This will provide less protocol overhead when longer TXOPs are specified.

NoAck

In QoS mode, service class for frames to send can have two values: QosAck and QosNoAck. Frames with QosNoAck are not acknowledged. This avoids retransmission of highly time-critical data.

Direct Link Setup

Direct link setup allows direct station-to-station frame transfer within a basic service set. This is designed for consumer use, where station-to-station transfer is more commonly used.

Microsoft's Virtual Wi-Fi initiative designed to accomplish the same goal. Virtual Wi-Fi allows gamers to connect wireless while accessing the Internet through an AP by allowing station adapters to have multiple MAC addresses.

See also

Wi-Fi Multimedia (WMM), the Wi-Fi Alliance specification which is a subset of IEEE 802.11e.

External references

• Approval from the IEEE RevCom
• 802.11e TGe Status (finished)
• 802.11e Amendment

This protocol will be implemented on the international network, NFN.^{This project is still under construction. EST Completion Date November 2007}

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