1. 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

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