Editing IEEE 802.11e-2005 (section)

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

=== Automatic power save delivery<span class="anchor" id="APSD"></span> ===
In addition to the Power Save Polling mechanism, which was available pre-802.11e, new power save delivery and notification mechanisms have been introduced in 802.11e.  APSD (automatic power save delivery) provides two ways to start delivery: ‘scheduled APSD’ (S-APSD) and ‘unscheduled APSD’ (U-APSD).  With APSD, multiple frames may be transmitted together by the [[Wireless access point|access point]] to a power-saving device during a service period.  After the end of a service period, the device enters a doze state until next 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.  With U-APSD, whenever a frame is sent to the Access Point, a service period is triggered, which allows the access point to send buffered frames in the other direction.  U-APSD can take a ‘full’ U-APSD or ‘hybrid’ U-APSD form. With Full U-APSD, all types of frames use U-APSD independently of their priority. With Hybrid U-APSD, either  U-APSD or the legacy Power Save Polling mechanism is used, depending on the access category.  S-APSD is available for both channel access mechanisms, EDCA and HCCA, while U-APSD is available only for EDCA.<ref name="benven1" /><ref>{{cite journal | url=https://ieeexplore.ieee.org/document/1561942 | doi=10.1109/MWC.2005.1561942 | title=Analysis of the integration of IEEE 802.11E capabilities in battery limited mobile devices | date=2005 | last1=Perez-Costa | first1=X. | last2=Camps-Mur | first2=D. | last3=Sashihara | first3=T. | journal=IEEE Wireless Communications | volume=12 | issue=6 | pages=26–32 | url-access=subscription }}</ref>

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.  A power-saving device using U-APSD must generate signaling frames to retrieve buffered traffic in the absence of uplink traffic, as for instance in the case of audio, video, or best effort traffic applications found in today's smartphones.  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.<ref name="benven2">M. Benveniste, "Guidelines for Power Management", [https://mentor.ieee.org/802.11/dcn/04/11-04-0073-02-000e-guidelines-power-management.doc Doc IEEE 802.11-04/073], January 2004</ref>  Hybrid U-APSD is less efficient than Full U-APSD because the Power Save Polling mechanism it employs for some access categories is less efficient than APSD, as explained above. The relative advantages of the various power-save mechanisms have been confirmed independently by simulations.<ref name="perez2">{{cite magazine |first1=X. |last1=Pérez-Costa |first2=D. |last2=Camps-Mur |url=https://ieeexplore.ieee.org/document/5547926 |title=IEEE 802.11e QoS and Power Saving feature: Overview and Analysis of Combined Performance |magazine=IEEE Wireless Communications Magazine (WirComMag) |volume=17 |issue=4 |date=August 2010 }}</ref><ref>{{Cite journal |doi=10.1016/j.comnet.2007.01.026 |title=On distributed power saving mechanisms of wireless LANs 802.11e U-APSD vs 802.11 power save mode |date=2007 |last1=Pérez-Costa |first1=Xavier |last2=Camps-Mur |first2=Daniel |last3=Vidal |first3=Albert |journal=Computer Networks |volume=51 |issue=9 |pages=2326–2344 }}</ref>

=== Block acknowledgments ===
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.

=== Direct Link Setup ===
Direct Link Setup allows direct station-to-station frame transfer within a [[basic service set]].  This is designed primarily for consumer use, where station-to-station transfer is more commonly used.  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. Also, If the AP is far away in some distant part of the home, sending all the frames to the AP and back may require them to be sent at a lower transmission rate. However, DLS requires participation from the AP to facilitate the more efficient direct communication, and few, if any, APs have the necessary support for this. Tunnelled Direct Link Setup was published as 802.11z ([[TDLS]]), allowing devices to perform more efficient direct station-to-station frame transfers without support from the AP. Both DLS and TDLS require that stations be associated with the same AP. Both DLS and TDLS improve the speed and efficiency of communications between members of a [[basic service set]], but they do not facilitate communication between devices that are near to each other but not associated with the same AP.

Nearby communication between devices not associated with the same AP can be performed using technologies like [[Wi-Fi Direct]], but so far Wi-Fi Direct has not seen widespread adoption.

Microsoft's Virtual Wi-Fi initiative was designed to accomplish the same goal as DLS.  Virtual Wi-Fi allows gamers to connect wireless while accessing the Internet through an AP by allowing station adapters to have multiple MAC addresses.<ref>{{Cite web|url=http://www.istartedsomething.com/20090516/windows-7-native-virtual-wifi-technology-microsoft-research/ |title=Windows 7 adds native Virtual WiFi technology from Microsoft Research |date=16 May 2009 |access-date=2010-07-07}}</ref>