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

Outline

• 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

• 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

• 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– 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

• 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

• 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)– 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

• 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