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Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

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. The problem. New wireless applications demand high performance

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Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

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  1. 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

  2. The problem • New wireless applications demand high performance • But: wireless channels are loss-prone • Interference • Noise • Attenuation • Multi-path • Mobility, etc…  Inconsistent and poor performance

  3. Current solutions are inefficient for recovering losses • Coding (e.g., FEC) • Hard on highly variable channels • Retransmission • Wasteful: outage durations can be long (tens to hundreds of milliseconds) • Bit-rate adaptation • Hard on variable channels • Slows down other clients

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

  5. 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%

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

  7. 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)

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

  9. 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

  10. 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 TX: 1100 1010 Combine failure Problem: Exponential # of CRC checks in # of unmatched bits.

  11. 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 • Failure rate increases with NB

  12. 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

  13. Flawed retransmission schemes • Conventional link-layer ACKs do not work • Final status known only to MRDC • Two levels of ACKs are redundant • Cannot disable link-layer ACKs

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

  15. 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.

  16. Experimental setup ~20 m R2 R1 L • 802.11a/b/g implementation in Linux (MADWiFi) • L transmits 100,000 1,500B UDP packets w/ 7 retries • 802.11a @ auto bit rate (6, 9, 12, 18, 24, 36, 48, 54) • L is in motion at walking speed, > 1 minute per trial • Variants: R1, R2, MRD (5 trials each)

  17. 18.7 Mbps 2.3x Improvement 8.25 Mbps MRD improves throughput Throughput (Mbps) R1 R2 MRD Each color shows a different trial

  18. 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)

  19. 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)

  20. Related work • Physical layer spatial diversity techniques • Antenna diversity • 802.16 MIMO/802.11n • Macro-diversity in CDMA networks • Retransmission with memory [Sindhu ’77] • Opportunistic forwarding [Biswas ‘05][Jain ’05] • Bit rate selection (AARF, RBAR, MiSer, OAR, Sample Rate)

  21. Summary • Design of Multi-Radio Diversity WLAN • Block-based frame combining • Request-for-acknowledgment protocol • MRD-aware rate adaptation • Analysis of block-based frame combining • Experimental evaluation • MRD reduces losses by 50% and improves throughput by up to 2.3x

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