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?