1. Mathilde Benveniste mbenven@avaya.com Avaya Labs
April 2004
Power Save in e WLANs
Mathilde Benveniste
Avaya Labs
Mathilde Benveniste (Avaya Labs)

2. April 2004
SUMMARY
The power-save method inserted in the WME draft dated 3/9/04 (IEEE doc ) is not consistent with the current e 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 e 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 e draft
Mathilde Benveniste (Avaya Labs)

3. April 2004
Ways WME differs from e
WME changes the definition of TIM and More Data bit
In the e 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 e 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 restricts the AC of the buffered frames released in a service period
The e 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. April 2004
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 e
Mathilde Benveniste (Avaya Labs)

5. April 2004
Delaying receipt of signaling or other frames in WME method Another 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 e
Mathilde Benveniste (Avaya Labs)

6. Other changes needed in WME Relax AC of the releasing buffer
April 2004
Other changes needed in WME Relax AC of the releasing buffer
The WME method restricts the 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 e
Mathilde Benveniste (Avaya Labs)

7. April 2004
Added gains from adopting the e 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 e
Mathilde Benveniste (Avaya Labs)

8. April 2004
Otherwise, WME forces non-periodic traffic to use legacy power save The relative Inefficiency 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 e
Mathilde Benveniste (Avaya Labs)

9. Proposal: A station uses a single power save method
April 2004
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. Proposal: The EOSP bit must be in last transmitted frame
April 2004
Proposal: The EOSP bit must be in last transmitted frame
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. April 2004
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
AC=3
AC=1
AC=0
Arriving frames by AC
2 triggered buffers per
trigger-enabled station
Triggered Buffer 2
Triggered Buffer 1
Mathilde Benveniste (Avaya Labs)

12. April 2004
Dealing with possible confusion at the stations The 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 gets 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
April 2004
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 indicate whether there is data remaining in the buffer from which the DL frame was released. They could show the queue size, or one of these bits, RB, could indicate the buffer status.** I.e.
RB=1 if the are frames remaining in the buffer from which the current DL frame was retrieved
RB=0 if the the buffer from which the current DL frame was retrieved is empty
______________
* 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 must be fixed.
Mathilde Benveniste (Avaya Labs)

14. Interpretation of added signaling
April 2004
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
More Data bit
RB bit
Interpretation
No more frames remain buffered at the AP 1
Frames of lower priority than the received frame remain buffered at the AP
Frames of comparable priority to the received frame remain buffered at the AP
Not meaningful
Mathilde Benveniste (Avaya Labs)

15. Summary of proposed changes to the WME spec
April 2004
Summary of proposed changes to the WME spec
To make the WME power save method consistent with the e 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 e to address relevant issues:
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 buffer
Mathilde Benveniste (Avaya Labs)

16. April 2004
CONCLUSION
The WME has modified the power save method of the e 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 e method are proposed to remedy the problem that motivated the WME changes
Mathilde Benveniste (Avaya Labs)