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Cancellation of aggregate Multicast feedback – measurement results

Cancellation of aggregate Multicast feedback – measurement results. Date: 2010-07-09. Authors:. Abstract.

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Cancellation of aggregate Multicast feedback – measurement results

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  1. Cancellation of aggregate Multicast feedback – measurement results Date: 2010-07-09 Authors: Jochen Miroll

  2. Abstract • This presentation is an update on the Leader-based aggregate feedback Protocol (LBP) proposal previously made to TGaa by the authors and provides measurement results obtained on a consumer 802.111 hardware test bed • The feedback cancellation probability in the worst case of LBP is measured and compared to previous theoretical / simulation results • These results have also been published and presented at the IEEE ISCE 2010 conference in June 2010 Jochen Miroll

  3. Motivation • 11aa is standardizing Multicast ARQ: MRG • Gathering per-receiver feedback, the overhead due to the positive ACKs grows linearly with the number n of receivers • How does 11aa MRG compensate for this increased overhead? • Aggregation of multiple frames: single-TID, uncompressed Block-ACK (802.11n) for MRG • Per-frame ACK becomes multi-frame Block-ACK bitmap for the last k frames • Still: overhead increases linearly with receivers n • How to get rid of the dependency on n? • We have previously proposed a leader-based Multicast retransmission scheme to 11aa Jochen Miroll

  4. NACK NACK ACK Feedback aggregation in the same time slot • All receivers provide feedback, butit is aggregated in the same time slot - then(n = number of receivers) • overhead(n) = overhead(1) • Idea: Introduce NACK • Transmit a data frame • Then, ask for ACK/NACK • If STA i has received the frame:it responds with an ACK • If STA j did not:it responds with NACKat the same time ? AP1 STA 1 STA 2 STA 3 STA 4 Jochen Miroll

  5. Feedback cancellation premise • If ACK and NACK are approx. equally strong • Is it possible to cancel an ACK by a simultaneous NACK and thus enforce a retransmission? • The „capture effect“: • Describes the phenomenon that a frame (e.g. ACK) may be received correctly in the presence of another, similarly strong (e.g. NACK) • Main reasons for „imperfect collision“ • Locking the preamble and then Viterbi decoding the locked-onto frame is a very robust mechanism. • E.g.: ACK is BPSK, rate ½ and only 14 Bytes in length.It is the most robust 802.11 frame(OFDM: few dB difference between ACK and NACK may suffice to „capture“) Jochen Miroll

  6. Earlier comments from TGaa (resolved) • Will feedbackcancellationactuallywork? • answer: Yes, collisions are happening all of the time • answer: No, due to the capture effect • We have consequently provided Matlab and ns-2 results for feedback cancellation to Tgaa • cf. doc.: IEEE 802.11-09/1150r2 • Provided in this document: measurement results using real and cheap 802.11 hardware Jochen Miroll

  7. Leader-based feedback cancellation • Idea: Multicast is essentially handled as a unicast connection to a „leader receiver“ • „Non-leaders“ transmit a NACK if a frame is lost • Target: Larger Multicast groups (large n) • If ACK survives the somewhat weaker NACK, does it survive many? Does it survive many equally strong, many somewhat stronger? • Intuitive leader selection: choose the „weakest“ receiver (as seen by the AP, no power control, just due to path loss) • If no loss: Leader’s ACKs can be received (ACKs are most robust) • Else: Expect a good chance that whenever several somewhat stronger NACKs are transmitted at the same time, the Leader’s ACK will be cancelled Jochen Miroll

  8. Aggregation through Leader-Based feedback cancellation Protocol (LBP)cf. doc.: IEEE 802.11-09/0290r1 optional SEQ# indicator and NAV updater to synchronize aggregate feedback Jochen Miroll

  9. Feedback cancellation constraint • Failure of feedback cancellation results in uncorrectable packet loss at non-leaders • (i.e. capture of ACK happens, no collision) • Question that arises: • What is the error floor in the worst case? • What is the worst case for the leader-based feedback cancellation approach? • Intuitively: the „weakest“ receiver can not be distinguished • All receivers on average experience the same SNR • We assume that all are sending approx. equally strong feedback Jochen Miroll

  10. Feedback cancellation measurements • Examine two different cases of how feedback aggregation may be implemented • In the WLAN card‘s real-time OS • In the WLAN card‘s host OS (e.g. Linux) • Implications • Cards allow for strict timing constraints (similar to 802.11 ACK, ±900ns), so we can examine short feedback • Host OS is less accurate in timing, thus we examine feedback cancellation with frames of several tens of Bytes Jochen Miroll

  11. Feedback cancellation test setup (1) • We have used real consumer 802.11 hardware • Limited freedom in implementing MAC algorithms • But: We can fix some parameters in cancellation experiments • Here: Non-leaders transmit different frames • Examine different frame sizes and timings with what is possible… …out of the box: Let positive feedback be a 6 Mbps ACK and the negative feedback be a 12 Mbps ACK …own implementation: Driver level ACK/NACK implementation Jochen Miroll

  12. Feedback cancellation test setup (2) SEQ frametriggersfeedback, assumethisisthequestion „didyougetthedataframe“ Jochen Miroll

  13. Non-leader 1 Non-leader n-1 AP Leader Feedback cancellation test setup (3) • To obtain independence from the (fading) environment: • Move receivers slowly around the AP, changing their positions in the environment • Periodically change the roles (leader, non-leader) of the receivers(always have exactly 1 leader) Jochen Miroll

  14. CDF of SNR at receivers is very steep ~identical channel conditions for all receivers on average Error free reception rates of different frames at the end of measurement run yield valid results SEQ (trigger) loss?loss rate < 0.1% Validation of test setup Jochen Miroll

  15. Test results (representative example) • 1 leader, 3 non-leaders • Why? An example, assume • But: Assume large n • Virtuallyno SEQ loss • ~89% feedbackcancellationsuccessprobability • Resultseemsindependentofframelengthandtiming • Worstcaseresults (whereleader-selectionwould not work) Jochen Miroll

  16. Theoretical / Simulation results • Compare with ns-2 results • Scenario: Rayleigh fading channel, equal AP-STAs distance • feedback cancellation rate is about 76% for 2, • more than 90% for more than 2, and • already 99% for 5 receivers • Again: worst case whereleader selection fails Jochen Miroll

  17. Conclusion • Scalable Multicast error correction can be achieved by aggregation through cancellation • Real test bed results are backed up by simulations • Channel will not be arbitrarily reliable but limited by an error floor • Combined MAC-layer and “Application Layer” error correction feasible • Assume overlay packet erasure FEC • Audio/Visual streams typically can tolerate errors • Residual error requirement can be dealt with on layers above MAC Jochen Miroll

  18. Questions? (a further presentation will propose how this scheme should be incorporated into 11a) Jochen Miroll

  19. Recap: Hybrid LBP (HLBP)*cf. doc.: IEEE 802.11-09/0290r1 Phase I Transmit a block of frames, as in MRG BA. Here: systematic FEC part Phase II Parity phase. Instead of BAR/BA, do AggregateAckRequest/AggregateAck * Assume e.g. DVB-IPDC or Raptor code on upper layer, MAC somehow knows which packets are systematic (DATA) or parity Jochen Miroll

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