IEEE 802.11e-2005

[1] The standard is considered of critical importance for delay-sensitive applications, such as voice over wireless LAN and streaming multimedia.

The basic 802.11 MAC layer uses the distributed coordination function (DCF) to share the medium between multiple stations.

[citation needed] APs send beacon frames at regular intervals (usually every 100 TU or 0.1024 second).

This is accomplished through the TCMA protocol, which is a variation of CSMA/CA using a 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).

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.

The HCF (hybrid coordination function) controlled channel access (HCCA) works a lot like PCF.

This means that the HC is not limited to per-station queuing and can provide a kind of per-session service.

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.

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.

With APSD, multiple frames may be transmitted together by the access point to a power-saving device during a service period.

With S-APSD, service periods start according to a predetermined schedule known to the power-saving device, thus allowing the Access Point to transmit its buffered traffic without the need for any signaling.

[1][4] APSD is a more efficient power management method than legacy 802.11 Power Save Polling, leading to lower power consumption, as it reduces both the signaling traffic that would otherwise be needed for delivery of buffered frames to power-saving devices by an AP and the collision rate among power-save polls, typically transmitted immediately after the beacon TIM.

S-APSD is more efficient than U-APSD because scheduled service periods reduce contention and because transmission between the access point and a power-saving device starts without the need for any signaling.

U-APSD is attractive for VoIP phones, as data rates are roughly the same in both directions, thus requiring no extra signaling—an uplink voice frame can trigger a service period for the transmission of a downlink voice frame.

In QoS mode, service class for frames to send can have two values: QosAck and QosNoAck.

For example, when streaming video to a television across the living room, or printing to a wireless printer in the same room, it can be more efficient to send Wi-Fi frames directly between the two communicating devices, instead of using the standard technique of always sending everything via the AP, which involves two radio hops instead of one.