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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. 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)

  3. 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)

  4. 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)

  5. 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)

  6. 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)

  7. 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)

  8. 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)

  9. Proposal: A station uses a single power save method 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: • A station will use either U-APSD, or legacy power save, for all its traffic; never both • If U-APSD is not supported at the AP, a station will use legacy power save for all its traffic AP requirement: An AP that supports U-APSD will maintain • 1 legacy buffer for each non-APSD-enabled station • at least 1 triggered buffer, but no legacy buffer, per APSD-enabled station (more triggered 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)

  10. 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 • 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)

  11. AC=3 Triggered Buffer 1 Triggered Buffer 2 AC=1 AC=0 2 triggered buffers per trigger-enabled station Arriving frames by AC Possible organization of triggered buffers & frame release (Informative) If there are as many triggered buffers as ACs, there is one AC per triggered buffer If the AP employs fewer triggered buffers than ACs, ACs are (dynamically) mapped into buffers monotonically. E.g. • All higher priority triggered buffers, except for the lowest, receive frames corresponding to a single AC • The lowest-priority triggered buffer receives frames corresponding to the remaining ACs Frames released in a service period are transmitted from the same access/transmit buffer; this ensures that the EOSP frame arrives last. E.g. • In a service period, frames are released from a single triggered buffer and have the same AC Example: 3 active ACs 2 triggered buffers Mathilde Benveniste (Avaya Labs)

  12. 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 8) Mathilde Benveniste (Avaya Labs)

  13. Proposal: Add signaling for priority information To convey information on the priority of buffered frame, we propose the addition of signaling in a field that is currently transmitted, but reserved.* • Bits 8-15 in the QoS control field of the DL frame (which correspond to Queue Size in an uplink frame) could be used for signaling • These bits would convey information about data remaining buffered when the DL frame is released. They could show the queue size, or one or more of these bits, RB, could indicate the buffer status.** I.e. RB=1 if there are frames remaining in the buffer from which the current DL frame was retrieved or in buffers holding higher priority frames RB=0 otherwise ______________ * One could add priority-specific TIMs in the beacon, and extend the QoS control field to make room for priority-specific More Data bits in order to provide the priority of buffered traffic. This, however, is deemed impractical when using fewer buffers than ACs. Also, • More TIMs would increase channel overhead • A longer QoS control field would increase the frame size ** Note: Using RB this way gives more information than by fixing the AC tracked by RB to be the top priority AC. This is because it is assumed that the AP will transmit the highest priority frames first. If this assumption is not made, then the AC tracked by the RB bit must be fixed. Alternatively, one could use two bits, HPB and CRB: HPB indicates the status of buffers with higher priority frames; CRB indicates the status of the current releasing buffer. HPB=0 if the current releasing buffer is the highest-priority buffer. Mathilde Benveniste (Avaya Labs)

  14. More Data bit RB bit Interpretation 0 0 No more frames remain buffered at the AP 1 0 Frames of lower priority than the received frame remain buffered at the AP 1 1 Frames of comparable or higher priority than the received frame remain buffered at the AP 0 1 Not meaningful Interpretation of added signaling A station seeing the TIM indication will either wait until its next trigger (e.g. UL voice) frame before responding to a TIM indication, or generate a trigger frame The AP responds with the highest priority frame buffered for the station With the RB and the More Data indications, the station has more information to do what is needed Mathilde Benveniste (Avaya Labs)

  15. 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) • Additional changes proposed for both WME and 802.11e: • A station may use either U-APSD or legacy power save, never both • 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 bit in the QoS field of a downlink frame indicates the status (empty, or not) of the releasing and higher-priority buffers Mathilde Benveniste (Avaya Labs)

  16. CONCLUSION • The WME has modified the power save method of the 802.11e draft standard as follows: • The TIM (and more Data bit) reflect only legacy-buffered frames • The entire content of a triggered must be emptied before ending a service period • The trigger frame and releasing buffer must have the same AC • As a result, the WME method has these limitations: • Causes priority inversion (or causes stations to consume more power) • Cannot be used by most applications (i.e. nonperiodic traffic) • Is less efficient in preserving battery life • Simple additions to the 802.11e method are proposed to remedy the problem that had motivated the WME changes Mathilde Benveniste (Avaya Labs)