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
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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 Research
Sensys 2010
Presenter: SY<br>
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<br>
04
Channel Utilization<br>
05
Real Measurement<br>
06
802.15.4 Transmit 1 byte: 32 us
Max packet size: 133 bytes
Using CSMA/CA
Calculate hamming distance to detect valid preamble<br>
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<br>
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<br>
11
Packet Reception Rate<br>
12
Overlay of 802.11 and 802.15.4 Why 802.11 back-off, interference still high<br>
13
Bit-error Distribution<br>
14
Zone In Bit errors concentrated in the front part<br>
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<br>
19
Correlation Threshold<br>
20
Preamble Length<br>
21
Multi-Headers Send two packet back-to-back wouldn’t work
Two length field are different
Custom CRC
Performance:<br>
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<br>
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<br>
24
Reed-Solomon (RS) Code Divided message into x blocks of data and y blocks of parity
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<br>
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<br>
29
Evaluation<br>
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<br>
