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Partial Recovery

Partial Recovery. Loss Recovery. Wireless medium is inherent lossy Large distance between sender and receiver Obstacles Collisions (e.g., hidden terminals) Current solutions to cope with loss?. Existing Solutions. Current solutions to cope with loss Automatic repeat request (ARQ) FEC

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Partial Recovery

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  1. Partial Recovery

  2. Loss Recovery • Wireless medium is inherent lossy • Large distance between sender and receiver • Obstacles • Collisions (e.g., hidden terminals) • Current solutions to cope with loss?

  3. Existing Solutions • Current solutions to cope with loss • Automatic repeat request (ARQ) • FEC • Rate adaptation • MIMO • Network coding • Pros & Cons?

  4. Bits in a packet don’t share fate (30 node testbed, CSMA on) Many bits from corrupted packets are correct, but status quo receivers don’t know which!

  5. Partial Recovery • Partial Recovery • Segment CRC • MRD • PPR • SOFT • … • Is partial recovery useful for wireline traffic?

  6. Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks Allen Miu, Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laboratory C. Emre Koksal Computer and Communication Sciences, EPFL

  7. Internet Today’s wireless LAN (e.g., 802.11) May use only one path • Uses only one communication path AP1

  8. MRDC Internet AP2 Multi-Radio Diversity (MRD) – Uplink Today’s wireless LAN (e.g., 802.11) May use only one path • Allow multiple APs to simultaneously receive transmissions from a single transmitter 10% AP1 20% Loss independence  simultaneous loss = 2%

  9. MRDC Internet Multi-Radio Diversity (MRD) –Downlink • Allow multiple client radios to simultaneously receive transmissions from a single transmitter AP1 AP2

  10. Are losses independent among receivers? • Broadcast 802.11 experiment at fixed bit-rate: 6 simultaneous receivers and 1 transmitter • Compute loss rates for the 15 receiver-pair (R1, R2) combinations • Frame loss rate FLR(R1), FLR(R2) vs. simultaneous frame loss rate FLR(R1 ∩ R2)

  11. Individual FLR > Simultaneous FLR y = x FLR R1 R2 R1*R2 FLR(R1 ∩ R2)

  12. Challenges in developing MRD • How to correct simultaneous frame errors? • Frame combining • How to handle retransmissions in MRD? • Request-for-acknowledgment protocol • How to adapt bit rates in MRD? • MRD-aware rate adaptation

  13. Patterns CRC Ok 1100 0000 1 R1 1100 0000 -- 1100 0010 X 1101 1010 R2 1 1100 1000 X 0001 1010 1100 1010 0 1. Locate bits with unmatched value 2. Select bit combination at unmatched bit locations, check CRC O Corrected frame Bit-by-bit frame combining Combine failure TX: 1100 1010 Problem: Exponential # of CRC checks in # of unmatched bits.

  14. Block-based frame combining • Observation: bit errors occur in bursts • Divide frame into NB blocks (e.g., NB = 6) • Attempt recombination with all possible block patterns until CRC passes • # of checks upper bounded by 2NB • Explore bursty bit-error • Failure rate increases with NB under uniform error

  15. Failure decreases with NB and burst size 1.0 Frame size = 1500B Probability of failure 0.8 0.6 NB = 2 0.4 NB = 4 0.2 NB = 6 … NB = 16 0 10 20 30 40 50 0 Burst error length parameter

  16. How to Perform Error-Control?

  17. Option 1: Directly use 802.11 retransmission scheme • Conventional link-layer ACKs do not work • Final status known only to MRD

  18. Option 2: Disable 802.11 retransmission scheme

  19. Option 2: Disable 802.11 retransmission scheme • Problems • Sending our own ACK is more expensive than sending 802.11 ACK (Why?) • Backoff and rate control both rely on MAC-layer ACKs

  20. Retransmission in MRD • Two levels of ACKs • Use 802.11 ACK for per-packet acknowledgement • 802.11 ACK can be directly used for CSMA • No need to content for the medium • Send MRD-ACK via ACK compression • Sender retransmits when MRD-ACK not received upon timeout

  21. DATA RFA DATA DATA MRD-ACK ACK Request-for-acknowledgment (RFA) for efficient feedback IP IP MRD MRD link link link MRDC

  22. MRD-aware rate adaptation • Standard rate adaptation does not work • Reacts only to link-layer losses from 1 receiver • Uses sub-optimal bit-rates • MRD-aware rate adaptation • Reacts to losses at the MRD-layer Implication: First use multiple paths, then adapt bit rates.

  23. Experimental setup ~20 m R2 R1 L • 802.11a/b/g implementation in Linux (MADWiFi) • L transmits 100,000 1,472B UDP packets w/ 7 retries • L is in motion at walking speed, > 1 minute per trial • Variants: R1, R2, MRD (5 trials each)

  24. 18.7 Mbps 2.3x Improvement 8.25 Mbps MRD improves throughput Throughput (Mbps) R1 R2 MRD Each color shows a different trial

  25. MRD maintains high bit-rate Fraction of transmitted frames Frame recovery data (% of total losses at R1) via R2 42.3% frame combining 7.3% Total 49.6% 0 6 9 12 18 24 36 48 56 Selected bit rate (Mbps)

  26. Delay Analysis Fraction of delivered packets User space implementation caused high delay 0 10-3 10-2 10-1 1 10-4 One way delay (10x s)

  27. Comments? • Partial recovery vs. spatial reuse • TCP Performance

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