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Recovering Data From Corrupt PacketsSENSYS 2013EWSN 2014
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Typical procedure of transmissionTechniques for overcoming bit errors
ARQ : Automatic Repeat reQuestFEC : Forward Error CorrectionOr the combination : HARQ
Interesting things happened when it finds a corrupted packet during the CRC detection process!All IS NOT LOST(ANL): a opportunistic approachTVA: an novel method of CRC correcting
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DSSS in 802.15.4 StandardA byte = 2 four-bit symbols
Echo symbol is finally represented by a 32-bit log chip sequence (i.e. the rightmost table)
What happened when takes noise into consideration ?Corruption may happen
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ANL : ObservationDeployment
16 nodesOne year from 2012 to 2013
Mutation Matrix:Each entry denotes a mutation frequency
One Observation:Mutations are not uniformly distributed
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ANL : Recovering DataConsider a received symbol
E.g s13,p(s5|s13)=
Consider a received sequence of symbolsE.g receive symbol sequence r = (s13,s3,s0,s11) a possible sent sequence t = (s5,s3,s1,s11)
Key idea: compute a probability distribution over the possible sent packets
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ANL : Left questionsHow to generate the mutation matrix?
Through Simulation
How to estimate pchip : using LQI
Is the actual sent word is assigned the highest rank?(95% of them are)
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TVA-A reliable protocol Procedure
Feature:A novel method of CRC error correctionCompact and computationally efficientDesigned to correct the most common error
patterns observed in WSNs
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CRC error correction TheoryTabular Method
m2 - the received msg ; m1 – the sent msg ; e – the error pattern we want to correct
Prebuild a table using CRCs of target error pattern P as indexIndex into the table with c and get error sequence e.The corrected message
Cyclic Method anyway,reduced memory requirement needed by single Tabular Method e.g 64 bytes to correct 4-bit-bursts in 40 bytes of data , while the tabular method uses 10KB
…… CRC(e1) CRC(e2) CRC(e3) ……
...... e1 e2 e3 ……
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Reliability of TVA:verifyCorrection process:m1
Risk : if CRC(e1) = CRC(e2) and
Verify : compare the sender side CRC and receiver side CRC
So what is this polynomial P?
Not the correcting polynomial G : since CRC(m1 e1 e2 ) = CRC(m1) CRC(e1) CRC(e2) = CRC(m1)
The key property of the polynomial P : it does not fail to detect a message error consisting of a common channel error XOR’d with a correction error
⊕e 1 e2…… CRC(e2)=c ……
…… e2 ……
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Searching for Polynomials:results
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Performance Comparison
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Conclusion of two workSolve the same issue
Recovering information from corrupt packetDifferent method
ANL : a probabilistic approach TVA : CRC error correcting
Combination Possibility?
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谢谢!
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Comparison with other protocol
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Outline1.All is not lost
the distinct pattern of how 802.15.4 packets are corrupted
an probabilistically approach infers the original content of a corrupt packet
2.Unlocking the full power of the CRC
a novel method of CRC error correction
a protocol TVA making use of the error correcting ability of CRC
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Comparision with ANL
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ObservationsObservation 1:Mutations are not uniformaly distributed
Observation 2:The most significant bit of a symbol is more stable than other bits
Observation 3:Symbols s0 to s7 are more stable than other symbols
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Explanation
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ExploitationCOMPUTE A DISTRIBUTION:
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