1. Some Power-save changes in 802.11e Draft
May 2004
Some Power-save changes in e Draft
Mathilde Benveniste - Avaya Labs
Mark Bilstad - Cisco Systems
Steve Emeott, Floyd Simpson, Stephen Wang - Motorola
Jarkko Kneckt, Jari Jokela - Nokia
Keith Amman, SpectraLink
Andrei Kajukhov, TI
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

2. Comments resolved with this proposal
May 2004
Comments resolved with this proposal
Amann/7
Amann/11
Amann/5
Barr/4
Barr/5
Barr/8
Barr/9
Eklund/2
Eklund/3
Eklund/4
Eklund/5
Eklund/6
Eklund/7
Eklund/8
Hansen/10
Hansen/12
Kandala/9
Kandala/22
Benveniste/4
Benveniste/5
Benveniste/6
Benveniste/7
Benveniste/8
Benveniste/9
Benveniste/10
Benveniste/11
Benveniste/12
Benveniste/13
Benveniste/14
Benveniste/15
Barr/18
Barr/19
Barr/20
Barr/21
Barr/22
Barr/23
Barr/24
Barr/25
Barr/26
Barr/27
Barr/28
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

3. The power-save method was simplified and enhanced as follows:
May 2004
SCOPE
The power-save method was simplified and enhanced as follows:
Stations may continue to use Unscheduled APSD for all-traffic (periodic and non-periodic)
At the same time stations have information and control over the amount of buffered traffic received per service period
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

4. May 2004
Overview of Power Save
Unscheduled APSD is made into a generalization of legacy
PS-poll releases a single frame
No overlapping service periods
Scheduled and Unscheduled APSD my co-exist
TIM/MD reflects the status of all frames (NO CHANGE)
Any uplink QoS data or QoS null frame can act as a trigger for the service period (NO CHANGE)
QoS trigger specifies maximum downlink frames will be in the service period
The last DL frame transmitted in the service period must have the EOSP set to 1
Downlink frames may contain a queue size and AC of highest priority buffered frame
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

5. What we had before in the 802.11e draft
May 2004
What we had before in the e draft
The TIM and More Data bit indicate whether traffic is buffered at the AP
The AP is required to send at least one frame (if one is buffered), but may send more
Unscheduled APSD was enabled on a per AC basis
TSPECs were used by a station to enable use of U-APSD for each AC/TS
Legacy and APSD can be used by the same station
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

6. May 2004
Use of APSD
Unscheduled APSD has become a generalization of legacy power save
Scheduled and Unscheduled APSD may co-exist
The choice of Scheduled APSD for an AC/TS will be indicated by sending a TSPEC
A scheduled SP will end upon receipt of a frame with the EOSP bit set and TID corresponding to the TSPEC; all other TIDs on a frame with the EOSP bit set will end an unscheduled SP
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

7. No Overlapping Service Periods
May 2004
No Overlapping Service Periods
Only one service period of a given type (scheduled or unscheduled) can be active at a time per station
Any frames received by the AP from a station while an unscheduled service period is already active for the station will not be regarded as triggers
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

8. May 2004
A Requirement: The EOSP bit must be set in the final DL frame transmitted in a service period
Problem: 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
This problem is addressed by the above requirement
A way to meet this requirement:
Frames 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
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

9. May 2004
Possible organization of triggered buffers & frame release (Informative)
If there are as many triggered buffers as ACs, there is one triggered buffer per AC
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
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

10. Dealing with possible confusion at the stations
May 2004
Dealing with possible confusion at the stations
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
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

11. Proposal: Add signaling for priority information
May 2004
Proposal: Add signaling for priority information
An APSD station seeing the TIM indication will decide whether to generate a trigger frame, wait until its next trigger frame, or wait longer before responding to a TIM (or More Data bit) indication
To help with this decision, the AP conveys 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) are used to convey information about data remaining buffered when the DL frame is released.
They would show
TID of the highest-priority frame buffered in bits 8-11
the queue size – I.e. total buffered traffic (in units of 4096 octets), found in bits 12-15
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

12. Proposal: Limit the SP length
May 2004
Proposal: Limit the SP length
To provide more control to the station on the amount to buffered traffic sent per SP, the station may indicate the maximum number of frames in an SP in a trigger frame
When bit 7 is set to 1, the value in Bits 8-10 in the QoS control field of the trigger frame is the Max SP Length
A PS-Poll may be used as a trigger frame with maximum SP length of 1 frame
Max SP Length
(frames)
Interpretation
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
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

13. Summary of proposed changes
May 2004
Summary of proposed changes
Unscheduled APSD is made into a generalization of legacy
PS-poll releases a single frame
Scheduled and Unscheduled APSD my co-exist
The choice of Scheduled APSD will be indicated by sending a TSPEC
A scheduled SP will end upon receipt of a frame with the EOSP bit set and TID corresponding to the TSPEC; all other TIDs on a frame with the EOSP bit set will end an unscheduled SP
No overlapping service periods
TIM/MD reflects the status of all frames (NO CHANGE)
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI

14. Summary of proposed changes - 2
May 2004
Summary of proposed changes - 2
Any uplink QoS data or QoS null frame can act as a trigger for the service period (NO CHANGE)
QoS trigger specifies maximum downlink frames will be in the service period
0 => do not start service period
1-6 => corresponding number of downlink frames
7 => no limit on frames released
The last frame transmitted in the service period has the EOSP set to 1
AP may limit frames in SP to a single AC; most likely this will be the highest priority AC w/ buffered frames
Downlink frames may contain a queue size
AP provides queue size for all ACs and indicates the highest-priority AC with frames remaining buffered at the AP
Avaya, Cisco, Motorola, Nokia, SpectraLink, TI