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)

Identifying the Missing Tags in a Large RFID System2/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

Identifying the Missing Tags in a Large RFID System3/41 Introduction RFID (radio-frequency identification) Used in: –Warehouse management –Object tracking –Inventory control –Australia Goats

Identifying the Missing Tags in a Large RFID System4/41 Introduction Other topics: –Tag-collection problem –Tag-estimation problem This paper studies missing-tag problem. Few research papers has investigated this problem before.

Identifying the Missing Tags in a Large RFID System5/41 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.

Identifying the Missing Tags in a Large RFID System6/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Related Work Conclusion

Identifying the Missing Tags in a Large RFID System7/41 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.

Identifying the Missing Tags in a Large RFID System8/41 Time Slots Empty slot, singleton slot, collision slot. t tag for tag slot, t l for long-response slot, t s for short-response slot. t s < t l < t tag. t s = 0.4ms, t l = 0.8ms, t tag = 2.4ms

Identifying the Missing Tags in a Large RFID System9/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

Identifying the Missing Tags in a Large RFID System10/41 Baseline Protocol Because RFID reader has access to database, no need to read directly from tags. For each tag: –Broadcast the tag ID: t tag –Wait to receive short response: t s Total executing time: N(t tag +t s )

Identifying the Missing Tags in a Large RFID System11/41 Two-Phase Protocol (TPP) Reduce the number of tag IDs transmit. Two phase: –Frame phase –Polling phase

Identifying the Missing Tags in a Large RFID System12/41 Frame Phase of TPP RFID reader transmit –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 H(id, r)

Identifying the Missing Tags in a Large RFID System13/41 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 bit in the ring. –If necessary, the number of bits can be larger than 200.

Identifying the Missing Tags in a Large RFID System14/41 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 ID 2 ID 4 ID 1 ID 3

Identifying the Missing Tags in a Large RFID System15/41 Time Efficiency of TPP Set the value of the protocol parameter f. The executing time of TPP is T 1. The number of tags mapped to the collision slots is N 1.

Identifying the Missing Tags in a Large RFID System16/41 Time Efficiency of TPP

Identifying the Missing Tags in a Large RFID System17/41 Time Efficiency of TPP When N = 50,000, the optimal frame size is f = 104,028, and the minimum execution time of TPP is seconds.

Identifying the Missing Tags in a Large RFID System18/41 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.

Identifying the Missing Tags in a Large RFID System19/41 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.

Identifying the Missing Tags in a Large RFID System20/41 Tag Removal ID 3 is removed, it will be broadcast in polling phase. ID 4 will be checked in frame phase. Frame 0 to f-1 ID 2 ID 4 ID 1 ID 3

Identifying the Missing Tags in a Large RFID System21/41 Time Efficiency of TPP/TR Set the value of the protocol parameter f. The executing time of TPP/TR is T 2. The number of removal tags is N 2.

Identifying the Missing Tags in a Large RFID System22/41 Time Efficiency of TPP/TR

Identifying the Missing Tags in a Large RFID System23/41 Time Efficiency of TPP/TR When N = 50,000, the optimal frame size is f = 75,479.

Identifying the Missing Tags in a Large RFID System24/41 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.

Identifying the Missing Tags in a Large RFID System25/41 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.

Identifying the Missing Tags in a Large RFID System26/41 Tag Removal ID 3 is removed, it will be broadcast in polling phase 1. ID 4 and ID 5 will be checked in frame phase and polling phase 2. Frame 0 to f-1 ID 2 ID 4 ID 1 ID 3 ID 5

Identifying the Missing Tags in a Large RFID System27/41 Time Efficiency of TPP/CSTR Set the value of the protocol parameter f. The executing time of TPP/TR is T 3. The number of removal tags is N 3. The number of tags broadcast in polling phase 2.

Identifying the Missing Tags in a Large RFID System28/41 Time Efficiency of TPP/CSTR

Identifying the Missing Tags in a Large RFID System29/41 Time Efficiency of TPP/CSTR When N = 50,000, the optimal frame size is f =

Identifying the Missing Tags in a Large RFID System30/41 Iterative ID-free protocol (IIP) Transmitting tag IDs is an expensive operation. IIP remove all the polling phase, it iteratively performs the frame phase.

Identifying the Missing Tags in a Large RFID System31/41 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% probability to not participate in the current frame. –Using H’(id,r).

Identifying the Missing Tags in a Large RFID System32/41 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 t tag.

Identifying the Missing Tags in a Large RFID System33/41 Time Efficiency of IIP Set the value of the protocol parameter f. The executing time for a frame size of f of IIP is T 4. The expected number of tags whose presence will be verified by the frame is N’. N* tags not confirm.

Identifying the Missing Tags in a Large RFID System34/41 Time Efficiency of IIP

Identifying the Missing Tags in a Large RFID System35/41 Time Efficiency of IIP The average time for verifying the presence of one tag is

Identifying the Missing Tags in a Large RFID System36/41 Time Efficiency of IIP The optimal value the load factor is

Identifying the Missing Tags in a Large RFID System37/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

Identifying the Missing Tags in a Large RFID System38/41 Simulation Results

Identifying the Missing Tags in a Large RFID System39/41 Simulation Results

Identifying the Missing Tags in a Large RFID System40/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

Identifying the Missing Tags in a Large RFID System41/41 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.