surviving wi-fi interference in low power zigbee networks chieh-jan mike liang, nissanka bodhi...
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Surviving Wi-Fi Interference in Low Power ZigBee Networks
Chieh-Jan Mike Liang, Nissanka Bodhi Priyantha, Jie Liu, Andreas Terzis
Johns Hopkins University, Microsoft ResearchSensys 2010
Presenter: SY
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Outline
• Introduction• WiFi and Zigbee Interactions• Protecting 15.4 Packets• BuzzBuzz• Conclusion
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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
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Channel Utilization
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Real Measurement
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802.15.4
• Transmit 1 byte: 32 us• Max packet size: 133 bytes• Using CSMA/CA• Calculate hamming distance to detect valid
preamble
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802.11
• CSMA/CA
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Outline
• Introduction• WiFi and Zigbee Interactions• Protecting 15.4 Packets• BuzzBuzz• Conclusion
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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
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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
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Packet Reception Rate
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Overlay of 802.11 and 802.15.4
Why 802.11 back-off, interference still high
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Bit-error Distribution
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Zone In
Bit errors concentrated in the front part
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Varying Payload Size
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Asymmetric Region
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Outline
• Introduction• WiFi and Zigbee Interactions• Protecting 15.4 Packets• BuzzBuzz• Conclusion
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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
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Correlation Threshold
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Preamble Length
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Multi-Headers
• Send two packet back-to-back wouldn’t work• Two length field are different• Custom CRC• Performance:
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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
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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
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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
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RS Parity Size
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Outline
• Introduction• WiFi and Zigbee Interactions• Protecting 15.4 Packets• BuzzBuzz• Conclusion
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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
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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
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Evaluation
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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