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Surviving Wi-Fi Interference in Low Power ZigBee Networks

Surviving Wi-Fi Interference in Low Power ZigBee Networks. Chieh -Jan Mike Liang, Nissanka Bodhi Priyantha , Jie Liu, Andreas Terzis Johns Hopkins University, Microsoft Research Sensys 2010 Presenter: SY. Outline. Introduction WiFi and Zigbee Interactions

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Surviving Wi-Fi Interference in Low Power ZigBee Networks

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  1. Surviving Wi-Fi Interference in Low Power ZigBee Networks Chieh-Jan Mike Liang, Nissanka Bodhi Priyantha, JieLiu, Andreas Terzis Johns Hopkins University, Microsoft Research Sensys 2010 Presenter: SY

  2. Outline • Introduction • WiFi and Zigbee Interactions • Protecting 15.4 Packets • BuzzBuzz • Conclusion

  3. About This Paper • WiFi interference on 802.15.4 network • Examines the interference • To bit-level granularity • Providing solutions for these interference • Show the solutions work

  4. Channel Utilization

  5. Real Measurement

  6. 802.15.4 • Transmit 1 byte: 32 us • Max packet size: 133 bytes • Using CSMA/CA • Calculate hamming distance to detect valid preamble

  7. 802.11 • CSMA/CA

  8. Outline • Introduction • WiFi and Zigbee Interactions • Protecting 15.4 Packets • BuzzBuzz • Conclusion

  9. Detect WiFi Interference • Use a sniffer • RFMD ML2724 narrow band radio • Fast RSSI output • Channel assignments • 802.11 -> channel 11 • 802.15.4 -> channel 22 • ML2724 -> 2465.792 MHz (equivalent of 15.4 channel 23) • Use Data Acquisition (DAQ) card • Record event timing

  10. Experiment • In Parking garage • 802.11 • 802.11 b/g access point and a laptop • A stream of 1,500-byte TCP segments • 802.15.4 • One sender, five receivers • Sends one max-size packet every 75 ms • Broadcast 2000 packets • Predefined byte pattern • Record every packets

  11. Packet Reception Rate

  12. Overlay of 802.11 and 802.15.4 Why 802.11 back-off, interference still high

  13. Bit-error Distribution

  14. Zone In Bit errors concentrated in the front part

  15. Varying Payload Size

  16. Asymmetric Region

  17. Outline • Introduction • WiFi and Zigbee Interactions • Protecting 15.4 Packets • BuzzBuzz • Conclusion

  18. Symmetric Region • Packet corrupted at front • Three techniques examined • Decrease correlation threshold • Reduce the constrain • Increase preamble length • Higher change to have valid preamble • Multi-header

  19. Correlation Threshold

  20. Preamble Length

  21. Multi-Headers • Send two packet back-to-back wouldn’t work • Two length field are different • Custom CRC • Performance:

  22. Asymmetric Region • Forward error correction (FEC) • Apply error-correction code (ECC) • Two ECCs • Hamming code • Adding extra parity bits • Can detect up to two bit errors and correct one bit error • Reed-Solomon Code • Block-based error-correction code • Divided message into x blocks of data and y blocks of parity

  23. Hamming Code • Hamming (12,8) • 4 parity bit in 8-bit data • Can detect and correct one bit error in 12-bit word • They use 72-byte data, result in 108-byte message • 754 bytes ROM, 82 bytes RAM • Encode: 1.4ms, decode: 1.8ms • Hamming (12,8) with interleaving • Interleave bits in message • 1.4 KB ROM, 100 bytes RAM • Encode: 6.7ms, decode: 9.2ms

  24. Reed-Solomon (RS) Code • Divided message into x blocks of data and y blocks of parity • Their implementation • 65 bytes data, 30 bytes parity • 2.9 KB ROM, 1.4 KB RAM • Execution time: • Result

  25. RS Parity Size

  26. Outline • Introduction • WiFi and Zigbee Interactions • Protecting 15.4 Packets • BuzzBuzz • Conclusion

  27. Techniques For Reliable Transmission • Three techniques • ARQ -- retransmission • Multi-header • TinyRS (Reed-Solomon coding) • Trade-off • Resource and computation time • TinyRS > Multi-header > ARQ • Performance • ARQ > Multi-header > TinyRS

  28. BuzzBuzz Protocol • Attempts to deliver using ARQ • If cannot delivered after 3 attempts • Adds TinyRS and Multi-header • Remember last setting for 60 seconds • After receive three consecutive packets that pass MH CRC • Go back to naïve approach

  29. Evaluation

  30. Conclusion • Examine interference between 802.11 and 802.15.4 • Found problems that previous research overlooked • Design and evaluated solutions • Multi-header • Reed-Solomon code • Implement TinyRS • Proposed BuzzBuzz protocol

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