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 IP 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

DCF

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 communicate at the same time, many collisions will occur, which will lower the available bandwidth (just like in Ethernet, which uses CSMA/CD)
*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 all other stations will suffer from that.
*generally, there are no Quality of Service (QoS) guarantees.

PCF

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 CP, the DCF is simply used. In 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. This allows for a better management of the QoS. Unfortunately, the PCF has limited support and a number of limitations (for example, it does not define classes of traffic).

802.11e MAC Protocol Operation

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 Classes (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.

EDCA

With EDCA (Enhanced Distributed Channel Access), 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. In addition, each priority level is assigned 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 MSDU or MMPDU.

The purpose of QoS is to protect high priority data from low priority data but there can be scenarios in which the data which belongs to same priority needs to be protected from data of same priority. Example being suppose a network can accommodate only 10 data calls & an eleventh call is made. Admission Control in EDCA address this type of problems. The AP publishes the available bandwidth in beacons. The clients can check the available bandwidth before adding more traffic in the network that cannot be entertained.

Wi-Fi Multimedia (WMM) certified APs must be enabled for EDCA and TXOP. All other enhancements of the 802.11e amendment are optional.

HCCA

The HCCA (HCF (Hybrid Coordinator Function) Controlled Channel Access) works a lot like the 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. Nevertheless, implementing the HCCA does not require much overhead, as it basically uses the existing DCF mechanism for channel access (no change to DCF or EDCA operation is needed). In particular, the station side implementation is very simple as stations only need to be able to respond to poll messages. On the AP side, however, a scheduler and queuing mechanism is needed. Given that AP's are already equipped better than station transceivers, this should not be a problem either.

Other 802.11e Specifications

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

APSD

Automatic Power Save Delivery is a more efficient power management method than legacy 802.11 Power Save Polling. Most newer 802.11 STAs (station = device using the 802.11 protocol) 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 are sent to the Access Point, the Access Point is triggered to send the buffered Voice data in the other direction. After that the Voice over IP phone enters doze state until next Voice data have to be sent to the Access Point.

BA

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.

DLS

Direct Link Setup allows direct STA-to-STA frame transfer within a BSS (what is a BSS??). This is designed for consumer use, where STA-to-STA transfer is more commonly used.

Microsoft previously announced a 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 STA adapters to have multiple MAC addresses. The release date of this capability is unknown.

ee also

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

External references

* [http://standards.ieee.org/board/rev/905recomm.html Approval from the IEEE RevCom]
* [http://grouper.ieee.org/groups/802/11/Reports/tge_update.htm 802.11e TGe Status (finished)]
* [http://standards.ieee.org/getieee802/download/802.11e-2005.pdf 802.11e Amendment]

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