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Identifying the Missing Tags in a Large RFID SystemTao Li (University of Florida, US) Shigang Chen (University of Florida, US) Yibei Ling (Telcordia Technologies, US)
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Outline
• Introduction• System Model• Missing-tag Detection Protocols• Simulation Results• Conclusion
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Introduction
• RFID (radio-frequency identification)• Used in:
– Warehouse management– Object tracking– Inventory control– Australia Goats
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Introduction
• Other topics:– Tag-collection problem– Tag-estimation problem
• This paper studies missing-tag problem.• Few research papers has investigated
this problem before.
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Introduction
• The most important thing is to minimize the detection time.
• Two guidelines to achieve time efficiency:– Reduce radio collision– Report presence by each transmitting a bit
instead of a whole tag duel.
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Outline
• Introduction• System Model• Missing-tag Detection Protocols• Simulation Results• Related Work• Conclusion
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Problem and Assumption
• A large RFID system of N tags.• Each tag carries a unique ID.• Each tag can perform certain
computations and communicating with the RFID reader wirelessly.
• RFID reader has access to database that stores the ID of the tags.
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Time Slots
• Empty slot, singleton slot, collision slot.• ttag for tag slot, tl for long-response slot, ts f
or short-response slot. ts < tl < ttag.• ts = 0.4ms, tl = 0.8ms, ttag = 2.4ms
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Outline
• Introduction• System Model• Missing-tag Detection Protocols• Simulation Results• Conclusion
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Baseline Protocol
• Because RFID reader has access to database, no need to read directly from tags.
• For each tag:– Broadcast the tag ID: ttag
– Wait to receive short response: ts
• Total executing time: N(ttag+ts)
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Two-Phase Protocol (TPP)
• Reduce the number of tag IDs transmit.• Two phase:
– Frame phase– Polling phase
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Frame Phase of TPP
• RFID reader transmit <r, f>– r is a random number, f is the frame size– pseudo-randomly mapped H(id, r) -> [0, f-1]
Frame 0 to f-1
ID IDID
H(id, r) H(id, r) H(id, r)
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Frame Phase of TPP
• Hash function H(id, r):– String of 200 random bits stored in the tag– Return a certain number of bits after the rth b
it in the ring.– If necessary, the number of bits can be larger
than 200.
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Polling Phase of TPP
• Perform the baseline protocol on the tags that are mapped to the collision slots in the frame.
Frame 0 to f-1
ID2 ID4ID1 ID3
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Time Efficiency of TPP
• Set the value of the protocol parameter f.• The executing time of TPP is T1.• The number of tags mapped to the
collision slots is N1.
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Time Efficiency of TPP
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Time Efficiency of TPP
• When N = 50,000, the optimal frame size is f = 104,028, and the minimum execution time of TPP is 95.04 seconds.
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Two-Phase Protocol with Tag Removal (TPP/TR)• In TPP, two tags mapped to a collision
slot, we have to broadcast both tags.• The information in the collision slot is
totally unused.• Remove tags from collision slot to make it
a singleton slot.
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Two-Phase Protocol with Tag Removal (TPP/TR)• Tow Phase:
– Polling Phase (with tag removal)– Frame Phase
• For each k-collision slot, randomly removes k-1 tags to turn the slot a singleton.
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Tag Removal
• ID3 is removed, it will be broadcast in polling phase.
• ID4 will be checked in frame phase.
Frame 0 to f-1
ID2 ID4ID1 ID3
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Time Efficiency of TPP/TR
• Set the value of the protocol parameter f.• The executing time of TPP/TR is T2.• The number of removal tags is N2.
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Time Efficiency of TPP/TR
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Time Efficiency of TPP/TR
• When N = 50,000, the optimal frame size is f = 75,479.
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Three-Phase Protocol with Collision Sensitive Tag Removal (TPP/CSTR)• When f is reasonably large, most collision
slot are 2-collision slots.• Long response is needed.• The situation of the two tags response:
– Empty: both are missing– Singleton: one is missing, check later– Collision: neither is missing.
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Three-Phase Protocol with Collision Sensitive Tag Removal (TPP/CSTR)• Three Phase:
– Polling Phase 1 (with tag removal)– Frame Phase– Polling Phase 2
• For each k-collision slot with k>2, randomly removes k-2 tags to turn the slot 2-collision.
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Tag Removal
• ID3 is removed, it will be broadcast in polling phase 1.
• ID4 and ID5 will be checked in frame phase and polling phase 2.
Frame 0 to f-1
ID2 ID4ID1 ID3 ID5
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Time Efficiency of TPP/CSTR
• Set the value of the protocol parameter f.• The executing time of TPP/TR is T3.• The number of removal tags is N3. • The number of tags broadcast in polling
phase 2.
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Time Efficiency of TPP/CSTR
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Time Efficiency of TPP/CSTR
• When N = 50,000, the optimal frame size is f = 38466.
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Iterative ID-free protocol (IIP)
• Transmitting tag IDs is an expensive operation.
• IIP remove all the polling phase, it iteratively performs the frame phase.
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Frame Phase in IIP
• Transmitting a pre-frame vector:– ‘0’ for empty and singleton, ‘1’ for collision– A tag in a collision slot decide with 50% prob
ability to not participate in the current frame. – Using H’(id,r).
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Frame Phase in IIP
• Transmitting a post-frame vector:– ‘0’ for empty and collision, ‘1’ for singleton– A tag in a singleton slot will not participate in
the further execution.
• Pre-frame or post-frame will be divide into segments of 96 bits (length of ID), and transmit in ttag.
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Time Efficiency of IIP
• Set the value of the protocol parameter f.• The executing time for a frame size of f of
IIP is T4.• The expected number of tags whose
presence will be verified by the frame is N’.
• N* tags not confirm.
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Time Efficiency of IIP
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Time Efficiency of IIP
• The average time for verifying the presence of one tag is
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Time Efficiency of IIP
• The optimal value the load factor is 1.516.
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Outline
• Introduction• System Model• Missing-tag Detection Protocols• Simulation Results• Conclusion
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Simulation Results
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Simulation Results
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Outline
• Introduction• System Model• Missing-tag Detection Protocols• Simulation Results• Conclusion
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Conclusion
• Monitoring the set of tags in a large RFID system and identifying the missing ones.
• Proposing five missing-tag detection protocols with increasing time efficiencies.