Editing IEEE 802.11e-2005 (section)

=== 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>