APSD vs Reverse Polling for EDCA

1. APSD vs Reverse Polling for EDCA Mathilde Benveniste benveniste@ieee.org Avaya Labs - Research Submission addresses D5.0 ballot comments: 441, 210, (209, 330, 331, 324, 325, 460, 466, 467, 526)

2. Goal Compare various power-save mechanisms available for EDCA in TGe Draft 5.0 • APSD • Reverse Polling M. Benveniste, Avaya Labs - Research

3. APSD General Description • Downlink and uplink transmissions occur at a scheduled time, which is negotiated with the AP • The station wakes up according to schedule to receive buffered downlink traffic • Uplink frames pending transmission are sent then • APSD can be used with either • EDCA or • HCF polling M. Benveniste, Avaya Labs - Research

4. Reverse Polling General Description • Downlink transmission occurs following the receipt of an uplink frame • The time is not known in advance • If there are no uplink frames pending, the station must send a (possibly ‘null’) frame in order to receive any buffered downlink frames • There are several variations of Reverse Polling M. Benveniste, Avaya Labs - Research

5. Sleep mode Scheduled wake-up Downlink delay Downlink Frames ------------- Uplink Frames Frame arrivals time Uplink delay How APSD works • Upon TSPEC admission, the AP staggers the scheduled wake-up times of all APSD stations • The goal is to prevent collisions among APSD stations • The AP transmits frames to APSD stations at scheduled wake-up times • The AP may refrain from accessing the channel immediately following such transmission • The station wakes up at the scheduled wake-up time • … receives downlink frames (if there are any) • … transmits uplink frames (if there are any) • … goes back to sleep M. Benveniste, Avaya Labs - Research

6. Variations of Reverse Polling* Variations possible with TGe Draft 5.0 • ‘Legacy Reverse Polling’ – legacy stations go to sleep and, as needed, wake up to get buffered frames by canceling power save (PS bit=0) • ‘PS Poll’ – legacy stations request the transmission of a single buffered frame by sending a special poll frame • ‘Legacy+11e Reverse Polling’ – It is similar to Legacy Reverse Polling, using 11e frames [ See 03/663r0 ]. • This is the most efficient of the existing variations in Draft D5.0 • We compare APSD to this variation ______________________ *A new reverse polling method is being proposed in 03/661 for inclusion in D5.0 M. Benveniste, Avaya Labs - Research

7. Sleep mode Scheduled wake-up Downlink delay Downlink Frames ------------- Uplink Frames Frame arrivals time How Reverse Polling works • The AP buffers all frames destined for an RP station • … transmits all pending frames using using PIFS access (i.e., transmits after a PIFS idle) • The station schedules to wake up upon arrival of an uplink frame • … waits for channel to be released by the AP • … transmits uplink frames (if there is none, a ‘null’ frame is sent) • … receives downlink frames (if there are any) • … goes back to sleep • The AP sends buffered frames to an RP station if an uplink frame is received from that station indicating that the station is awake M. Benveniste, Avaya Labs - Research

8. Similarities between APSD and Reverse Polling – 1 – • Stations must adhere to a wake-up schedule for both methods • For APSD, the schedule is negotiated with the AP • For reverse polling, the schedule is self-imposed by the station • An RP station not operating on a wake-up schedule would save less power, as a ‘deep’ sleep mode would not be possible otherwise • Both uplink and downlink frames are exchanged when the station wakes up • For APSD stations, downlink frames are received before sending uplink frames • For RP stations, uplink frames are sent before receiving downlink frames M. Benveniste, Avaya Labs - Research

9. Similarities between APSD and Reverse Polling – 2 – APSD Downlink delay Uplink+ Downlink Delay – APSD Downlink Frames ------------- Uplink Frames • Similar delay penalties are incurred as a result of the wake-up schedule • Reverse Polling minimizes uplink delay, but at the expense of increasing downlink delay • On the average, the two methods experience the same combined uplink-downlink delay time Uplink delay Reverse Polling Downlink delay Uplink+ Downlink Delay – Reverse Polling Downlink Frames ------------- Uplink Frames time M. Benveniste, Avaya Labs - Research

10. ACK ( Not drawn to scale) Time awake Data Contention Differences between APSD and Reverse Polling – 1 – APSD More Data field = 1 More Data field = 0 Station in power save mode Data Data • APSD stations spend less time (and power) than RP stations powered and waiting for the channel to clear (performing CCA) • An RP station must wait, powered and ready to transmit, for the AP to finish sending all its pending frames before it can send frames uplink • An APSD station need not wait for AP transmissions, as the latter refrains from accessing the channel per schedule • The result is lower power drain under APSD ACK ACK Reverse Polling (per 03/663r0) X More Data field = 1 More Data field = 0 ACK Data ACK ACK Data Data PM field=0 ACK Null+ACK PM field=1 Data PM field=0 Station wakes up Waiting for the AP to finish Tx M. Benveniste, Avaya Labs - Research

11. ACK ( Not drawn to scale) Time awake Data Contention Differences between APSD and Reverse Polling – 2 – APSD More Data field = 1 More Data field = 0 Station in power save mode Data Data • APSD stations spend less time than RP stations contending for the channel • Since the AP staggers their wake-up schedules, APSD eliminates contention among power-saving stations • Reverse polling power-saving stations contend among themselves, leading to more collisions • The result is lower power drain and better channel utilization under APSD ACK ACK Reverse Polling (per 03/663r0) X More Data field = 1 More Data field = 0 ACK Data ACK ACK Data Data PM field=0 ACK Null+ACK PM field=1 Data PM field=0 Station wakes up Collision with peer or contention M. Benveniste, Avaya Labs - Research

12. Problem of the Lost Ack to a Downlink frame • If the Ack to the last downlink frame is the last frame sent by a station before going back to sleep, the AP will keep retransmitting the same frame • APSD is not vulnerable to this problem because the station keeps awake to send uplink frames after receiving downlink frames • In reverse polling downlink frames are transmitted last; if no special measures are adopted (like those in 03/663), retransmission of the frame whose Ack was lost will keep repeating M. Benveniste, Avaya Labs - Research

13. Differences between APSD and Reverse Polling – 3 – • APSD stations are not subject to repeated retransmissions due to the loss of the last downlink Ack; reverse polling may be • Because uplink transmissions follow downlink transmissions, APSD stations are awake to receive a retransmitted downlink frame in case the Ack is lost • Downlink transmissions follow uplink transmissions in reverse polling. Unless special measures are adopted (as in 03/663), an RP station will go to sleep after sending the last Ack • The result is better channel utilization under ASPD • APSD generates less signaling traffic • APSD generates no redundant frames • During a station silence, reverse polling requires ‘null’ frames to be generated and sent uplink in order to fetch buffered downlink frames • The result is lower power drain and better channel utilization under APSD M. Benveniste, Avaya Labs - Research

14. Differences between APSD and Reverse Polling – 4 – • APSD requires that the station’s wake-up schedule be known to both the AP and station • Determining a schedule is easy if handling service intervals with a common denominator (see Appendix I) • The AP can reject requests with odd service intervals during admission control of the APSD TSPEC; those can be handled with the existing power save mechanisms M. Benveniste, Avaya Labs - Research

15. Discussion APSD uses less power, causes less jitter and delay, and uses channel more efficiently Reasons: • The APSD station spends less time awake waiting for the AP to transmit pending frames (Lower power consumption, less delay and jitter) • APSD eliminates contention among all power-saving stations (Uses channel more efficiently, lower power consumption, less delay and jitter) • APSD needs no extra ‘null’ frames to serve as uplink ‘polls’ (Uses channel more efficiently, lower power consumption, less delay and jitter) • APSD does not risk repeated retransmissions in case the ACK to the last downlink frame is lost; reverse polling needs special prevention measures These benefits outweigh any additional computation necessary to establish the wake-up schedule M. Benveniste, Avaya Labs - Research

16. Appendix I APSD Scheduling

17. Slot 1  10 ms Slot 2  10 ms Slot 3  20 ms , 20 ms Slot 4  10 ms Slot 5  20 ms , 20 ms Slot 6  10 ms Slot 7  10 ms APSD Scheduling SP=20ms SP=20ms Scheduling Example • AP accepts APSD TSPECs with service periods that are multiples of 10 ms • Odd service periods do not use APSD; they can use Reverse Polling • 10 ms cycle is slotted to fit the frames exchanged sequences during wake-up • Unused APSD slot time is used by other EDCA traffic • Streams with service period greater than 10 ms can be interleaved into a single 10 ms slot time 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 SP=10ms SP=20ms M. Benveniste, Avaya Labs - Research