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Mathilde Benveniste mbenven@avaya Avaya Labs

Power Save in 802.11e WLANs. Mathilde Benveniste mbenven@avaya.com Avaya Labs. UPDATE. Purpose of this presentation Describe the differences between the WME draft dated 3/9/04 (IEEE doc. 802.11-03-0504-07) and TGe draft D8.0

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Mathilde Benveniste mbenven@avaya Avaya Labs

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  1. Power Save in 802.11e WLANs Mathilde Benveniste mbenven@avaya.com Avaya Labs Mathilde Benveniste (Avaya Labs)

  2. UPDATE Purpose of this presentation • Describe the differences between the WME draft dated 3/9/04 (IEEE doc. 802.11-03-0504-07) and TGe draft D8.0 • Propose changes to the WME spec to make it consistent with TGe draft D8.0and with the changes to D8.0 recommended by the 802.11e ad hoc group on Power Save, May 13, 2004 References: docs IEEE 802.11-04/584r2 and 04/620r2 Recommended changes • Stations may continue to use Unscheduled APSD for all-traffic (periodic and non-periodic) • At the same time, stations have more information and control over the amount of buffered traffic received per service period • As a result of these changes, TSPECs are not needed for U-APSD Note: Text in purple indicates recent additions/changes, following the 802.11e ad hoc group’s recommendations of May 13, 2004 Mathilde Benveniste (Avaya Labs)

  3. SUMMARY • The power-save method inserted in the WME draft dated 3/9/04 (IEEE doc. 802.11-03-0504-07) is not consistent with the current 802.11e draft standard, D8.0, or earlier drafts • The key difference relates to the meaning of signaling – the TIM and More Data bits • As a consequence, the WME triggered power-save method has multiple limitations • Causes priority inversion (or causes stations to consume more power) • Cannot be used by many applications (i.e. nonperiodic traffic); these applications must use the less efficient legacy power save • The 802.11e power-save method Unscheduled APSD (U-APSD) does not have the above problems • The issues the WME method tried to address can be remedied with simple changes to the 802.11e draft Mathilde Benveniste (Avaya Labs)

  4. Ways WME differs from 802.11e • WME changes the definition of TIM and More Data bit • In the 802.11e draft, the TIM and More Data bit indicate whether traffic is buffered at the AP • In the WME spec, the TIM and More Data bit reflect only the traffic in the legacy buffer • WME changes the number of frames the AP must release in a service period • The 802.11e draft requires one buffered frame to be released; the AP may release additional frames • The WME spec requires the AP to empty the releasing triggered buffer • WME restrictsthe AC of the buffered frames released in a service period • The 802.11e draft poses no restriction on the AC of the released frames • The WME spec requires it to be the same as the AC of the trigger frame Mathilde Benveniste (Avaya Labs)

  5. Priority inversion or battery exhaustion with WME method Ending the Service Period WME power save requires the AP to transmit all buffered frames before ending the service period One of two problems arise: • Priority Inversion: If the AP transmits the contents of the triggered buffer before transmitting any other frames, lower-priority frames for the power-saving station would be transmitted before higher priority frames to other stations • Battery exhaustion: If the AP interrupts transmission to a power-saving station in order to transmit higher priority frames to other stations, it could keep the station awake too long. Knowing the backlog, the AP should be able to allow the station to go back to sleep, and thus postpone transmission of additional frames till another service period. The AP should be allowed to determine the end of service period as it deems appropriate, as in 802.11e Mathilde Benveniste (Avaya Labs)

  6. Delaying receipt of signaling or other frames in WME methodAnother reason for fixing the TIM and More Data bit If the service period may be ended before the triggered buffer of an AC is emptied, signaling, control, or maintenance frames that arrive in that AC may not reach the station in a timely manner, as the station will not know of their arrival without the TIM (or More Data bit) indication In order for a station to receive such frames, the TIM and More Data bit must indicate the presence of these frames at the AP To receive all frames, the TIM (and More Data bit) must reflect triggered buffers, in addition to legacy, as in 802.11e Mathilde Benveniste (Avaya Labs)

  7. Other changes needed in WME Relax AC of the releasing buffer The WME method restrictsthe AC of the releasing triggered buffer to be the same as the AC of the trigger frame in a service period  If there is a single TIM, the station cannot tell the AC of the buffered frames, and hence it cannot provide a trigger frame of the same AC as the buffered frames A frame of any AC should serve as the trigger frame, as in 802.11e Mathilde Benveniste (Avaya Labs)

  8. Added gains from adopting the 802.11e definition of TIM and More Data bit Nonperiodic traffic will be able to use triggered power save (U-APSD) By changing the TIM and More Data bit in the WME method to reflect traffic in triggered buffers, one could use triggered power-save for non-periodic traffic as follows: • the AC of the traffic must be trigger enabled • trigger frames are generated when the TIM (or More Data bit) indicates that there is buffered traffic If the TIM (and More Data bit) reflect all buffered traffic, devices with nonperiodic traffic would be able to use U-APSD, as in 802.11e Mathilde Benveniste (Avaya Labs)

  9. Otherwise, WME forces non-periodic traffic to use legacy power save The relativeInefficiency of legacy power save The WME method does not allow nonperiodic traffic to use triggered power save; it must use legacy power save instead Legacy power save is less efficient than triggered power save A single trigger frame can retrieve multiple frames in triggered power save One cannot tell which is the right legacy option to use because TIMs do not indicate how much traffic is buffered, leading to greater inefficiency • PS polls are inefficient if long packet bursts are buffered, as a PS poll is needed for every buffered frame • PM bit (exiting and returning to power save mode) is inefficient for independent frames, as it involves two extra frames for each buffered frame If the wrong legacy option is chosen, the inefficiency of legacy power save is even greater By enabling all applications (periodic and nonperiodic) to use the new power save, battery life is extended, as in 802.11e Mathilde Benveniste (Avaya Labs)

  10. Proposal: A station uses the same power save method for all ACs Since there is only one TIM, and the TIM and the More Data bit reflect all buffered traffic, ambiguity could arise in PS signaling. To prevent this, Station requirement: • If U-APSD is supported by the AP, a station may use either delivery method (legacy or triggered) for all its traffic • If U-APSD is not supported at the AP, a station will use legacy delivery for all its traffic AP requirement: An AP that supports U-APSD will maintain • 1 PS buffer for each legacy station • at least 1 PS buffer per WME station (more PS buffers provide better prioritization)  Thus, there will be no ambiguity on how a station should retrieve its buffered data (triggered frame vs PS poll/PM bit) when the TIM is set. Mathilde Benveniste (Avaya Labs)

  11. Max SP Length (frames) Interpretation 0 Do not start service period 1-6 Maximum number of downlink frames in the SP 7 No limit imposed on the maximum number of frames in the SP Proposal: Limit the SP length In order to provide more control to the station on the amount to buffered traffic sent per SP, the station shall indicate in a trigger frame the maximum number of frames in an SP • When the Max SP length is zero, no frames may transmitted by the AP in response to receiving an uplink frame; this disqualifies a frame as a trigger • A non-zero value for Max SP Length of an uplink data frame marks it as a trigger Bits 13-15 in the QoS control field of the trigger frame is the Max SP Length As in 802.11e, a PS-Poll may be used to receive 1 buffered frame regardless of AC Mathilde Benveniste (Avaya Labs)

  12. TSPECs are no longer needed for U-APSD TSPECs provided two functions for U-APSD: • to indicate the ACs for a station whose frames would be used as triggers • to designate the stations that would use U-APSD TSPECs can be eliminated because • All ACs of a station may use the triggered delivery method (see slide 10) – no TSPEC needed per AC • Trigger frames are marked (with a non-zero Max SP Length field); so they are distinguishable from other frames (see slide 11) – no TSPEC needed per station Mathilde Benveniste (Avaya Labs)

  13. A Requirement: The EOSP bit must be in the last frame transmitted in a service period If the frame with the EOSP bit set is transmitted by the AP before other frames released in the same service period, the station will go to sleep before it receives all transmitted frames A way to avoid this: Frames must go through the same access/transmit buffer in a service period; e.g. • In a service period, frames are released from a single triggered buffer and all have the same AC (see slide 15) • The AP will chose the buffer for frame release • it will release frames from the highest-priority non-empty buffer Thus, there will be no frames arriving after the station has gone back to sleep Mathilde Benveniste (Avaya Labs)

  14. AC=3 PS Buffer 1 PS Buffer 2 AC=1 AC=0 2 PS buffers per WME station Arriving frames by AC Example: Organization of PS buffers (Informative) Organization of buffers at the QAP is independent of the QAP’s or the stations’ ability to support U-APSD A QAP that supports power save could keep multiple PS buffers, which would hold frames of different priorities, in order to enhance prioritization A QAP that does not support U-APSD would release frames in response to • PS polls A QAP that supports U-APSD would release frames in response to • PS polls or • Trigger frames (if the station supports U-APSD) • Example: 3 active ACs, 2 PS buffers • If the QAP employs fewer PS buffers than ACs, ACs are (dynamically) mapped into buffers monotonically. E.g. • All higher priority PS buffers, except for the lowest, receive frames corresponding to a single AC • The lowest-priority PS buffer receives frames corresponding to the remaining ACs Mathilde Benveniste (Avaya Labs)

  15. AC=3 PS Buffer 1 PS Buffer 2 AC=1 AC=0 2 PS buffers per WME station Arriving frames by AC Example: Final frame release in a SP (Informative) To ensure that the EOSP frame arrives last, frames released in a service period are transmitted from the same access/transmit buffer;. E.g. • In a service period, frames are released from a single PS buffer and have the same AC The buffered frames in this example would be emptied in 4 service periods. AC present in each SP: AC3, AC1, AC0, and AC1 Mathilde Benveniste (Avaya Labs)

  16. Dealing with possible confusion at the stationsThe WME method offers too costly a remedy Using a single TIM (and a single More Data bit) gives rise to ambiguity concerning the priority of the buffered frames; this may cause a problem for devices needing to perform other urgent tasks • With knowledge of the priority of buffered traffic, a station could retrieve higher-priority frames immediately; lower priority frames could be left to wait WME got around this problem by separating top priority from other traffic and by forcing the latter to use legacy power save WME imposes too costly a remedy, as legacy power save is inefficient (slide 9) Mathilde Benveniste (Avaya Labs)

  17. Proposal: Add signaling for priority information To help with this decision, the addition of optional priority/load signaling in a field that is currently transmitted, but reserved, is proposed • Bits 9-15 in the QoS control field of the DL frame (which correspond to Queue Size in an uplink frame) are used to convey information about data remaining buffered when the DL frame is released • This information consists of • flag indicating whether the AP will indicate the state of buffered traffic, found in bit 9 • AC of the highest-priority frame buffered, found in bits 10-11 • the AP buffer load – i.e. total buffered traffic (in units of 4096 octets), found in bits 12-15. When this value of this field is 14, the queue is greater than 57344 octets. Mathilde Benveniste (Avaya Labs)

  18. Proposal: There are no overlapping service periods To simplify implementation, no overlapping service periods are permitted for a station A frame received at the AP while the sending PS station is in a service period is not be treated as a trigger Mathilde Benveniste (Avaya Labs)

  19. Summary of proposed changes to the WME spec • To make the WME power save method consistent with the 802.11e draft, the following changes are proposed: • The TIM (and more Data bit) must reflect all buffered frames • At least one frame (not the entire buffer) must be released in a service period • There is no AC matching between the trigger frame and released frame(s) • A PS poll causes the release of a single frame • No overlapping SPs for a station • Additional changes proposed for both WME and 802.11e: • A station using U-APSD power save may use it for all its traffic • During a station’s service period, the frame with the EOSP bit set is the last frame transmitted by the AP to that station • A signaling field in the QoS field of a downlink frame indicates optionally the status of PS buffers • The station can limit the frames transmitted by the AP per SP • No TSPECs are needed for U-APSD Mathilde Benveniste (Avaya Labs)

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