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
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Introduction
WiFi and Zigbee Interactions
Protecting 15.4 Packets
BuzzBuzz
Conclusion
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
Channel Utilization
Real Measurement
802.15.4
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Transmit 1 byte: 32 us
Max packet size: 133 bytes
Using CSMA/CA
Calculate hamming distance to detect valid
preamble
802.11
• CSMA/CA
Outline
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Introduction
WiFi and Zigbee Interactions
Protecting 15.4 Packets
BuzzBuzz
Conclusion
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
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
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One sender, five receivers
Sends one max-size packet every 75 ms
Broadcast 2000 packets
Predefined byte pattern
Record every packets
Packet Reception Rate
Overlay of 802.11 and 802.15.4
Why 802.11 back-off, interference still high
Bit-error Distribution
Zone In
Bit errors concentrated in the front part
Varying Payload Size
Asymmetric Region
Outline
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Introduction
WiFi and Zigbee Interactions
Protecting 15.4 Packets
BuzzBuzz
Conclusion
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
Correlation Threshold
Preamble Length
Multi-Headers
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Send two packet back-to-back wouldn’t work
Two length field are different
Custom CRC
Performance:
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
Hamming Code
• Hamming (12,8)
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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
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
RS Parity Size
Outline
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Introduction
WiFi and Zigbee Interactions
Protecting 15.4 Packets
BuzzBuzz
Conclusion
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
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
Evaluation
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