Identifying the Missing Tags in a Large RFID System

Identifying the Missing Tags in a Large RFID System

Tao Li (University of Florida, US) Shigang Chen (University of Florida, US) Yibei Ling (Telcordia Technologies, US)

2011-05-04 Identifying the Missing Tags in a Large RFID System 2/41

Outline

• Introduction

• System Model

• Missing-tag Detection Protocols

• Simulation Results

• Conclusion


Introduction

• RFID (radio-frequency identification)

• Used in:– Warehouse management– Object tracking– Inventory control– Australia Goats


Introduction

• Other topics:– Tag-collection problem– Tag-estimation problem

• This paper studies missing-tag problem.

• Few research papers has investigated this problem before.


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.


Outline

• Introduction

• System Model



• Related Work

• Conclusion


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.


Time Slots

• Empty slot, singleton slot, collision slot.

• ttag for tag slot, tl for long-response slot, ts for short-response slot. ts < tl < ttag.

• ts = 0.4ms, tl = 0.8ms, ttag = 2.4ms


Outline

• Introduction

• System Model



• Conclusion


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)


Two-Phase Protocol (TPP)

• Reduce the number of tag IDs transmit.

• Two phase:– Frame phase– Polling phase


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)


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.


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


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.



• When N = 50,000, the optimal frame size is f = 104,028, and the minimum execution time of TPP is 95.04 seconds.


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.


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.


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


Time Efficiency of TPP/TR


• The executing time of TPP/TR is T2.

• The number of removal tags is N2.



• When N = 50,000, the optimal frame size is f = 75,479.


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.


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.


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


Time Efficiency of TPP/CSTR


• The executing time of TPP/TR is T3.

• The number of removal tags is N3.

• The number of tags broadcast in polling phase 2.



• When N = 50,000, the optimal frame size is f = 38466.


Iterative ID-free protocol (IIP)

• Transmitting tag IDs is an expensive operation.

• IIP remove all the polling phase, it iteratively performs the frame phase.


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).


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.


Time Efficiency of IIP


• 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.



• The average time for verifying the presence of one tag is



• The optimal value the load factor is 1.516.


Outline

• Introduction

• System Model



• Conclusion


Simulation Results


Outline

• Introduction

• System Model



• Conclusion


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.

Identifying the Missing Tags in a Large RFID System

Documents

Transcript of Identifying the Missing Tags in a Large RFID System