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Understanding Lossy Radio Links in an 802.11b Mesh Network

This talk explores the reasons for packet loss in an 802.11b Mesh Network and investigates the causes of intermediate delivery rates, including marginal signal-to-noise ratios, interference (long and short bursts), and multi-path interference. Results are relevant for sensors and community meshes.

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Understanding Lossy Radio Links in an 802.11b Mesh Network

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  1. Link-level Measurements from an 802.11b Mesh Network Dan Aguayo John Bicket, Sanjit Biswas, Robert Morris MIT Glenn Judd CMU

  2. What this talk is about • Roofnet is a multi-hop, wireless mesh net • Packet loss makes protocol design hard • This talk explores the reasons for loss • Results relevant for sensors and community meshes • Focus on long outdoor links

  3. Roofnet provides Internet access 1 kilometer

  4. Omni-directional antennas + Easy to deploy + Provide high connectivity - Don’t allow engineered link quality

  5. Lossy radio links are common Broadcast packet delivery probability 70-100% 30-70% 1-30% 1 kilometer

  6. Delivery probabilities are uniformly distributed Broadcast Packet Delivery Probability > two-thirds of links deliver less than 90% Node Pair

  7. Protocols should exploit intermediate-quality links • Link-quality-aware routing (ETX, LQSR) • 802.11 transmit bit-rate selection • Multicast data distribution • Opportunistic protocols (OMAC, ExOR) This talk investigates the causes…

  8. Rest of the talk: Hypotheses for intermediate delivery rates • Marginal signal-to-noise ratios • Interference: Long bursts • Interference: Short bursts (802.11) • Multi-path interference

  9. Methodology: Link-level measurements of packet loss • Goal: all-pairs loss rates • Each node broadcasts for 90 seconds • All other nodes listen • Raw link-level measurements: • No ACKs, retransmissions, RTS/CTS • No other Roofnet traffic • No 802.11 management frames • No carrier sense

  10. Hypothesis 1: Marginal S/N • Simplified model for packet loss: • P(delivery) = f(signal/noise) • Signal strength reflects attenuation • Noise reflects interference • Perhaps marginal S/N explains intermediate delivery probabilities

  11. Delivery vs. S/N with a cable and attenuator Broadcast packet delivery probability Laboratory Signal-to-noise ratio (dB)

  12. Laboratory Roofnet Delivery vs. S/N on Roofnet Broadcast packet delivery probability Signal-to-noise ratio (dB) S/N does not predict delivery probability for intermediate-quality links

  13. Hypothesis 2: long bursts of interference A B Bursty noise might corrupt packets without affecting S/N measurements

  14. Loss over time on two different Roofnet links avg: 0.5 stddev: 0.28 Delivery probability avg: 0.5 stddev: 0.03 Time (seconds) The top graph is consistent with bursty interference. The bottom graph is not.

  15. Most links aren’t bursty Cumulative fraction of node pairs Std. dev. of one-second delivery averages

  16. Hypothesis 3: short bursts of interference (802.11) A B • MAC doesn’t prevent all concurrent sends • Outcome depends on relative signal levels • Hypothesis: When a nearby AP sends a packet, we lose a packet.

  17. Methodology: record non-Roofnet 802.11 traffic • Goal: measure non-Roofnet traffic • Before the broadcast experiments • Each node records all 802.11 traffic

  18. No correlation between foreign traffic observed and packets lost Experiment packets lost per second Non-Roofnet packets observed per second (before the experiment)

  19. Hypothesis 4: Multi-path interference B B A Reflection is a delayed and attenuated copy of the signal

  20. Receiver Sender A channel emulator to investigate multi-path effects delay attenuation

  21. A reflection can cause intermediate packet loss Delivery probability Delay of second ray (nanoseconds or feet)

  22. Roofnet links are long Cumulative fraction of links Link distance (feet or nanoseconds) It’s reasonable to expect delays >500 ns

  23. Related Work • Measurements of AP networks: Eckhardt and Steenkiste 1996; Kotz 2003 • Sensor net measurements: Ganesan 2002; Woo 2003 • Protocol design: Lundgren 2002; Yarvis 2002; De Couto 2003; Woo 2003; Draves 2004

  24. Summary • Most Roofnet links have intermediate loss rates • S/N does not predict delivery probability • Loss is not consistent with bursty interference • Multi-path is likely to be a major cause

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