IEEE 802.11e
IEEE 802.11e as of late
2005 has been approved as a
standard that defines a set of
Quality of Service enhancements for
LAN applications, in particular the
802.11 WiFi standard. The standard is considered of critical importance for delay-sensitive applications, such as Voice over Wireless IP and
Streaming Multimedia. The protocol enhances the
IEEE 802.11 Media Access Control (MAC) layer.
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 bitrate (1 Mbit/s, for example), then it will take a long time to send its packet, and all other stations will suffer from that.
* more 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).
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 DCF 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 IP (VoWIP) could be assigned to a high priority class.
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. 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.
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 works a lot like the PCF: the interval between two beacon frames is divided into two periods, the CFP and the CP. During the CFP, the Hybrid Coordinator (HC) controls the access to the medium. During the CP, all stations function in EDCA. The main difference with the PCF is that Traffic Classes (TC) are defined. Also, the HC can coordinate the traffic 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. 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. The Wi-Fi Alliance has a forthcoming certification (WMM Scheduled Access) that will allow network integrators to easily distinguish APs that allow HCCA.
In addition to HCCA, EDCA and TXOP, 802.11e specificies 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 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. 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.
See also
WMM is the Wi-Fi Alliance Wi-Fi Multimedia specification which is a subset of IEEE 802.11e.
*
Approval from the IEEE RevCom*
802.11e TGe Status (finished)
