IEEE 802.11e
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802.11e as of June 2003 is a draft 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.
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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 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 at it choses. 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 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
The 802.11e enhances the DCF and the PCF, through two new coordination functions: the Enhanced DCF (EDCF) and the Hybrid Coordination Function (HCF) (the HCF could have been called the Enhanced PCF). Both EDCF and HCF 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.
EDCF
With EDCF, 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. There are no real guarantees: it is a "best effort" QoS. Since it is quite simple to configure and implement, a lot of people seem to choose this coordination function.
HCF
The HCF 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, typically the AP) controls the access to the medium. During the CP, all stations function in EDCF. 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 Transmit Opportunity (TXOP): they may send multiple packets in a row, for a given time period selected by the HC. During the CP, the HC may chose to resume control of the access to the medium by sending CF-Poll packets to stations. In short, HCF is the most advanced (and complex) coordination function. With the HCF, QoS can be configured with great precision: things like bandwidth control, fairness between stations, classes of traffic, jitter, and much more can be configured within the HC.
Any 802.11e compatible Access Point must have both EDCF and HCF support. The difference between 802.11e APs will be in the configuration of the QoS for different TCs: some may actually only provide very simple bandwidth control configuration, other may go way further and offer jitter control, etc.
See Also
WMM is the Wi-Fi Alliance Wi-Fi Multimedia specification which is a subset of IEEE 802.11e.
External link
- For comparison of 802.15.3 and 802.11e standards (http://www.xtremespectrum.com/PDF/802.11eVs802.15.3.pdf)it:802.11e