IEEE 802.11-24 Preemption TXOP Proposal
Explore the proposal for preemption transmission opportunities (TXOP) in IEEE 802.11-24 standard, focusing on latency reduction and enhancing medium access. The document delves into concepts like preemptor vs. preemptee in BSS preemption scenarios and the identity of non-AP STA as preemptee. Various cases are discussed, such as DL/UL preemption and transmission window preemption.
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July 2024 doc.: IEEE 802.11-24/1074r0 Preemption TXOP Date: 2024-07-03 Authors: Name Yuxin Lu Affiliations Address Phone email eeluyx@gmail.com Pei Zhou TCL Industries Zhanjing Bao Submission Slide 1 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Introduction Latency reduction is one key objective as stated in UHR proposed PAR [1] Several contributions [2]-[17] have made progress to support low latency medium access by constructing and furnishing the preemption mechanism In this proposal, we focus on the TXOP level preemption branch, present concepts and essential actions covering the preemption transmission window for the in BSS preemption scenario Submission Slide 2 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Concept: Preemptor VS Preemptee In BSS preemption Preemptor: A STA who acquires the preempted transmission duration by initiating preemption The STA can be either AP or non-AP For traffic of AC2 that takes precedence over the current traffic of AC1, such as higher-priority traffic over lower-priority traffic Preemptee: A non-AP STA who grants or accepts preemption by suspending the current traffic transmission or reception of AC1 In the top view of traffic delivery, AP helps STAs forward traffic (either DL or UL), but does not generate traffic, only traffic intended for non-AP STA can be preempted The non-AP STA is either TXOP holder or TXOP responder Exceptional scenario: Preempt DL/UL TXOP to transmit UL/DL higher-priority traffic Preemptor: EDCAF(AC2) Preemptee: EDCAF(AC1) Submission Slide 3 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Preemptee and Preemptor Identity Non-AP STA1 is always the preemptee TXOP obtained by STA1 TXOP obtained by STA1 Note: Definition/descrip tion of LL data is under discussion Whether to support all cases is TBD For example, in case Ba, if AP would schedule LL data to STA2 together with data to STA1, we may or may not call this scenario preemption RTS RTS Data Data (Preemptee) STA1 (Preemptee) STA1 LL data for STA2 arrives at AP Ack / BA LL Data Ack / BA Ack / BA CTS CTS (Preemptor) AP (Third STA) AP LL data for AP arrives at STA2 Ack / BA LL Data (Preemptor) STA2 (Third STA) STA2 Case Aa: AP s DL LL preempts STA1 s UL TXOP Case Ab: STA2 s UL LL preempts STA1 s UL TXOP TXOP obtained by AP TXOP obtained by AP LL data for STA2 arrives at AP LL Data Ack / BA RTS RTS Data Data (Preemptor) AP (Third STA) AP Ack / BA Ack / BA CTS CTS (Preemptee) STA1 (Preemptee) STA1 LL data for AP arrives at STA2 Ack / BA LL Data (Third STA) STA2 (Preemptor) STA2 Case Ba: AP s DL LL preempts STA1 s transmission Case Bb: STA2 s UL LL preempts STA1 s transmission Submission Slide 4 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Hidden Node Preemption Issue and Solution Hidden node issue: STA1 and STA2 are hidden from each other, collision happens while STA2 initiates preemption Solution candidates: Assumption: STA1 or AP indicated preemption support in a signal that can be received by STA2 TXOP obtained by STA1 Data RTS Data either of the following two If non-AP STA1 is hidden from STA2, then STA2 shall not initiate preemption during the transmission between STA1 and AP If non-AP STA1 is hidden from STA2, then when STA2 initiates preemption and error occurs, STA2 shall not perform error recovery and shall stop initiating preemption STA1 Collision Ack / BA CTS AP LL data for AP arrives at STA2 Preemption request STA2 Hidden node issue example Submission Slide 5 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Tx/Rx Mismatch Due to Tx/Rx Switch and Solution Tx/Rx mismatch issue1: After Tx to STA1, AP needs to switch from Tx to Rx Solution: STA2 shall transmit the first preemption frame to AP at least a t0 time after the last PPDU that AP transmits to STA1, no sooner than that Tx/Rx mismatch issue2: After Rx from STA1, AP needs to switch from Rx to Tx Solution: STA2 shall transmit the first preemption frame to AP at most a t0 time after the last PPDU that AP receives from STA1, no later than that Frame Frame t0: Tx/Rx switch duration STA1 Original Frame Tx/Rx switch duration Rx duration/ preemptable duration Frame AP LL data for AP arrives at STA2 Preemption request Preemption request STA2 (preemptor) Submission Slide 6 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 How Long May Preemptor Preempt Preemption ends Preemption starts TXOP limit (AC1): EDCA parameters to protect fairness within a BSS The longest duration in which a TXOP holder can maintain uninterrupted control of the medium and transmit its data The TXOP holder shall, subject to several exceptions, ensure that the duration of a TXOP does not exceed the TXOP limit, when nonzero Preemption duration limit (AC2) For such longest duration, preemptor may preempt the current transmission For such longest duration, preemptee would suspend the current transmission Preemptor shall ensure preemption duration does not exceed such longest duration/limit Preemption duration: How Long May Preemptor Preempt? LL data (AC2) Preemption request Preemption ends (optional) Preemptor Preemptor LL data transmission Preemptee continues transmission Preemption grant Third STA Preemption grant Preemptee Preempt for transmission with third STA Preemption ends Preemption starts Preemption duration: How Long May Preemptor Preempt? LL data (AC2) Preemption request Preemption ends (optional) LL data Preemptor Preemption grant Data Ack/BA Preemptee Preempt DL/UL to transmit UL/DL Submission Slide 7 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Preemption Duration Limit Option 1: Preemption duration is counted into the current TXOP Preemption duration longest: t2 = TXOP limit(AC1) - t1 Continuing duration longest: t3 = TXOP limit(AC1) - t1 - t2 Option 2: Preemption duration is counted independently t2 = TXOP limit(AC2) t3 = TXOP limit(AC1) - t1 Option 3: Indicate new parameters termed such as Preemption TXOP limits t2 = Preemption TXOP limit(AC2) t3 = TXOP limit(AC1) - t1 TXOP holder indicates Preemption TXOP Limit for the current TXOP, or advertised in the EDCA Parameter Set element in Beacon and Probe Response frames transmitted by the AP (AC_VI) TXOP limit 3ms (AC_VO) TXOP limit 1.5ms TXOP holder gains TXOP Preemption starts Preemption ends Preemption transmission of AC2 Duration longest = t2 Continue current transmission of AC1 Duration longest = t3 Current transmission of AC1 Duration long = t1 t2 (AC2) = Preemption transmission duration limit 0 t1 + t3 (AC1) = Current transmission duration limit Submission Slide 8 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Preemption Duration Protection: Issue Preemption starts Assumption: TXOP holder protected initial TXOP till time T1 via multiple protection Non-AP STA1 is TXOP holder in the figure Issue: If preemption protected medium till a time < T1, the non-AP TXOP holder may fail to get remaining duration back LL data (AC2) Preemption request STA2 Preemptor LL data transmission Preemption grant Preemption ends (optional) AP Collision Preemption grant Frame STA1 AP protected till here < T1 STA1 is hidden from STA3 STA3 Frame AP has no knowledge of how long STA1 s next frame would be, but AP knows T1 Submission Slide 9 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Preemption Duration Protection: Solution To ensure fairness for preemptee/TXOP holder after preemption ends, at the best effort, preemptor or AP shall Transmit at least one frame to indicate duration protection till time T1, or longer than T1 Preferably till time T1+t2 , such that total transmission duration for AC1 remains T1, not shorted After preemption ends (indication is TBD), preemptee/TXOP holder can continue transmission w/o contention/backoff Related STAs may use a new timer to record the preemption duration causing medium busy, or re-use the existing NAV timer To be discussed TXOP holder gains TXOP Preemption starts Preemption ends Preemption transmission of AC2 Duration long = t2' Current transmission of AC1 Continue current transmission of AC1 Continue current transmission of AC1 0 TXOP holder protected TXOP duration (multiple protection) till T1 time 0 time T1 time T1+t2' Submission Slide 10 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Summary Concept and identity of preemptor and preemptee in several cases Non-AP STA is always the preemptee How to initiate preemption successfully Hidden node preemption issue and solution Tx/Rx mismatch issue and solution How long may preemptor preempt Preemption duration limit Preemption duration protection To ensure fairness for preemptee/TXOP holder after preemption ends Submission Slide 11 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Straw Poll 1 Do you agree that in 11bn the concept of preemptor and preemptee should be defined to facilitate the preemption mechanism? NOTE This can be defined along with the concept of preemption Submission Slide 12 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Straw Poll 2 Do you agree that in 11bn the preemption duration limit/parameter should be defined to facilitate the preemption mechanism? NOTE Preemptor or third STA shall ensure preemption duration does not exceed such limit NOTE How to defined it is TBD Submission Slide 13 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Straw Poll 3 Do you agree that in 11bn the non-AP preemption initiator (preemptor) Shall transmit the first preemption frame to AP at least a some time after the last PPDU that AP transmits, no sooner than that Shall transmit the first preemption frame to AP at most a some time after the last PPDU that AP receives, no later than that NOTE The some time is TBD Submission Slide 14 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Straw Poll 4 Do you agree that in 11bn the preemption duration protection should at least cover the initial ending time that the TXOP holder protected? NOTE Whether the preemption duration protection can extend the initial TXOP ending time is TBD Submission Slide 15 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 Straw Poll 5 Do you agree that in 11bn the preemption duration protection can extend the initial ending time that the TXOP holder protected? NOTE For how longer it can extend is TBD Submission Slide 16 Yuxin Lu, TCL Industries
July 2024 doc.: IEEE 802.11-24/1074r0 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 11-23/480r3, UHR Proposed PAR 11-23-0018-01-0uhr-low-latency-support-in-uhr 11-23-0092-00-0uhr-preemption 11-23-1886-03-00bn-preemption-techniques-to-meet-low-latency-ll-targets 11-23-2076-03-00bn-multiple-channel-access-in-preemption-sequence 11-24-0091-01-00bn-enhanced-scheduling-method-for-low-latency-traffic-follow-up 11-24-0103-01-00bn-txop-level-preemption-for-low-latency-application-in-802-11bn 11-24-0131-00-00bn-signaling-of-preemption 11-24-0168-00-00bn-txop-preemption-in-11bn 11-24-0247-00-00bn-considerations-on-ll-preemption 11-24-0390-00-00bn-a-uniform-procedure-for-preemption 11-24-0391-00-00bn-legacy-sta-and-obss-issues-for-preemption 11-24-0416-00-00bn-target-sta-prioritization-in-edca-based-preemption-mechanisms-during-a-dl-txop 11-24-0431-01-00bn-signal-for-preemption-request 11-24-0442-03-00bn-latency-reduction-for-immediate-real-time-application-traffic-transmission 11-24-0470-00-00bn-rethinking-preemption 11-24-0804-00-00bn-the-transmission-of-preemption-request-frame Submission Slide 17 Yuxin Lu, TCL Industries
