802.11e, referred to in full as IEEE 802.11e, is an amendment to the IEEE 802.11 standard created by the Institute of Electrical and Electronics Engineers LAN/MAN Standards Committee (IEEE 802), a standard that allows devices to connect to a wireless local area network (WLAN).
It was created by a Task Group called the IEEE TGe. It features modifications to the Media Access Control (MAC) layer, such as a time division multiple access (TDMA) construct, in order to enhance Quality of Service (QoS) for WLAN applications. 802.11e is very important for delay-sensitive applications such as Voice over Wireless IP (Wireless VoIP) and streaming multimedia, as it offers error-correcting mechanisms for these applications.
802.11e networks operate at radio frequencies between the 802.11b range of 2.400 GHz to 2.4835 GHz, or the 802.11a range of 5.725 GHz to 5.850 GHz. The latter has advantages such as faster data transfer speed, more channels, and less susceptibility to interference from external sources.
A MAC technique known as the Distributed Coordination Function (DCF) is used in 802.11e to share the medium between multiple station. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and optional 802.11 RTS/CTS (shorthand for "Request to Send, Clear to Send") are employed as the medium access method. These methods come with a number of limitations: the system is prone to collisions if several stations communicate simultaneously; there is no notion of high or low priority traffic; there is no limit to how long a station can keep control of a medium once it has access to it, which is disadvantageous to other stations if the station has a low bit rate and takes a long time to send its data; and there are no Quality of Service guarantees.
In the "infrastructure" mode of an 802.11 MAC, where stations are connected to the network through an Access Point (AP), a coordination function known as the Point Coordination Function (PCF) is used. The APs send "beacon" frames at intervals about 0.1 second in length, and in between these beacon frames, there are two periods defined by the PCF, which are the Contention Free Period (CFP) and the Contention Period (CP). The DCF is used during the CP. In CFP, Contention Free-Poll (CF-Poll) packets are sent sequentially by the AP to each station, to enable the station to send a packet. A better management of the QoS is achieved because of this.
The Hybrid Coordination Function (HCF) was introduced in order to enhance the DCF and PCF. It has two methods of channel access, HCF-Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA), both of which were previously specified in the original 802.11 legacy standard. The two methods determine Traffic Classes (TC), assigning data to either low-priority class or high-priority class. Of the two, only EDCA is a requirement for Wi-Fi Multimedia (WMM) certified APs; HCCA and other enhancements are optional.
EDCA gives high-priority traffic a greater chance of being sent than low-priority traffic. Each priority level is given a Transmit Opportunity (TXOP), a bounded time interval when a station can send as many frames as possible for as long as the transmission does not take longer than the time length of the TXOP. A time interval of 0 means that the station is limited to a single MAC Service Data Unit (MSDC). One advantage provided by the usage of TXOP is that it avoids problems such as low-rate stations getting too much channel time.
Although similar in design to PCF, HCCA differs in that it allows for CFPs to be initiated at almost any time during a CP. The CFP in this case is called a Controlled Access Phase (CAP). It is initiated by the AP whenever it wants to send or receive a frame to or from a station without contention, and during a CAP, the Hybrid Coordinator (HC) gains control of medium access. All stations function in EDCA during the CP. The HC can provide a per-session service, since it defines both Traffic Class (TC) and Traffic Streams (TS). The streams may be coordinated in any fashion that the HC chooses. Like in EDCA, each station is also assigned a TXOP.
802.11e provides smooth interoperability among business, home, and public environments. It is ideal for use in networks that include multimedia capability, offering high-speed internet access with full motion video, high fidelity audio, and Voice over Internet Protocol (VoIP).