Expecting the Unexpected: Fast and Reliable Detection of Missing RFID Tags
in the Wild
Dept. of Computer Science and EngineeringMichigan State University
East Lansing, Michigan, 48824, USA
Muhammad Shahzad Alex X. Liu
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Motivation
Shop Lifting Employee Theft
2011: Retailers lost 34.5 billion USD
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Problem Statement Input
─ Set of IDs of expected tags─ RFID tag population containing:
● some or all expected tags● unexpected tags
─ Threshold on number of missing tags, ─ Required reliability,
Objective─ Detect the event: missing tags ─ Event detection probability ─ Minimize detection time
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Limitations of Prior Art Assume there are no unexpected tags
ICDCS 2008: How to monitor for missing RFID tags; Tan, Sheng, and LiMobiHoc 2010: Identifying the missing tags in a large RFID system; Li, Sheng, and LinSECON 2011: Fast identification of the missing tags in a large RFID system; Zhang, Liu, and SunIEEE ToC 2013: Completely pinpointing the missing RFID tags in a time-efficient way; Liu et. al.
However, in reality, there are unexpected tags
Airline baggage Multi-tenant warehouse
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Naïve Solutions Identification protocol
─ Slow: fastest RFID identification protocol is 14.3 times slower compared to our scheme
● SIGMETRICS 2013: Probabilistic Optimal Tree Hopping for RFID Identification; Shahzad and Liu
Estimation protocol─ Inaccurate: if new tags join, can not tell whether some
tags went missing
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Communication Protocol Overview
0 1 1 C 0 1 1
Frame size
Seed
3 2 6 4 47
Faster to distinguish between empty and non-empty slots Singleton and collision » non-empty At the end of frame, reader gets a sequence of 0s and 1s
─ 011C011 becomes 0111011
1 2 3 4 5 6 7
0 1 1 C 0 1 1
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RUN: Missing Tags Detection1 1 4 11 65
1 2 3 4 5 6 7 8 9 10 11
0 1 0 1 0 1 0 1 101
1048 10
0 1 1 1 0 0 0 0 101Frame size
Seed
Expected tags to be monitored
Missing tag event
detected
Unexpected falsepositive
Pre-computed frame
Executed frame
Unexpected tags
Unexpected tagdetected
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RUN: Handling Unexpected FPsRepeat frame times
0 1 1 1 0 1 0 1 101
0 1 1 1 0 0 0 0 101
0 1 0 1 1 1 0 0 111
0 0 0 1 1 0 0 0 011
0 1 1 0 1 0 0 1 010
1 0 0 0 1 0 0 1 010
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RUN: Parameter Selection Three unknown parameters
─ Frame size ─ Number of frames ─ Persistence probability
Two equations
● where ● obtained using the expression of false positive probability● : number of expected tags● : number of unexpected tags
● Obtained using the required reliability condition
Need the number of unexpected tags
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RUN: Estimating Unexpected Tags
1 4 11 5
1 2 3 4 5 6 7 8 9 10 11
1048 10
1 6
Number of total slots in frame Number of grey slots in frame Number of white slots that become green slots:
|𝑈|=− 𝑓𝑝
ln {1− 𝑁 01
𝑓 −𝑘 }
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RUN: Experimental Evaluation Implemented 4 protocols in addition to RUN
1. TRP (ICDCS, 2008)2. IIP (MobiHoc, 2010)3. MTI (SECON, 2011)4. SFMTI (IEEE ToC, 2013)5. TH (SIGMETRICS, 2013)
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Actual Reliability vs. Missing Tags Number of expected tags = 1,000 Number of unexpected tags = 10,000
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Number of expected tags = 1,000 Number of missing tags = 200
Actual Reliability vs. Unexpected Tags
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Effect of Threshold T Number of expected tags = 1,000 Number of unexpected tags = 10,000 Threshold = 200 Required reliability = 0.99
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RUN vs. RFID Identification Compared RUN with TH (SIGMETRICS 2013) RUN is 14.3 times faster than TH for
─ Number of expected tags = 1,000─ Number of unexpected tags = 10,000─ Threshold = 200─ Required reliability = 0.99
TH is faster than RUN when─ Required reliability > 0.99999, OR─ Threshold < 0.001 tags, which is impossible
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Conclusion Proposed a protocol to reliably detect missing tags
in presence of unexpected tags─ Reliable─ Fast─ C1G2 compliant─ Handles multiple readers
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Questions?
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