1. 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)

2. 2011-05-04 Identifying the Missing Tags in a Large RFID System2/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

3. 2011-05-04 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

4. 2011-05-04 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.

5. 2011-05-04 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.

6. 2011-05-04 Identifying the Missing Tags in a Large RFID System6/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Related Work Conclusion

7. 2011-05-04 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.

8. 2011-05-04 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

9. 2011-05-04 Identifying the Missing Tags in a Large RFID System9/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

10. 2011-05-04 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 )

11. 2011-05-04 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

12. 2011-05-04 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)

13. 2011-05-04 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.

14. 2011-05-04 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

15. 2011-05-04 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.

16. 2011-05-04 Identifying the Missing Tags in a Large RFID System16/41 Time Efficiency of TPP

17. 2011-05-04 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 95.04 seconds.

18. 2011-05-04 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.

19. 2011-05-04 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.

20. 2011-05-04 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

21. 2011-05-04 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.

22. 2011-05-04 Identifying the Missing Tags in a Large RFID System22/41 Time Efficiency of TPP/TR

23. 2011-05-04 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.

24. 2011-05-04 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.

25. 2011-05-04 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.

26. 2011-05-04 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

27. 2011-05-04 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.

28. 2011-05-04 Identifying the Missing Tags in a Large RFID System28/41 Time Efficiency of TPP/CSTR

29. 2011-05-04 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 = 38466.

30. 2011-05-04 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.

31. 2011-05-04 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).

32. 2011-05-04 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.

33. 2011-05-04 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.

34. 2011-05-04 Identifying the Missing Tags in a Large RFID System34/41 Time Efficiency of IIP

35. 2011-05-04 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

36. 2011-05-04 Identifying the Missing Tags in a Large RFID System36/41 Time Efficiency of IIP The optimal value the load factor is 1.516.

37. 2011-05-04 Identifying the Missing Tags in a Large RFID System37/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

38. 2011-05-04 Identifying the Missing Tags in a Large RFID System38/41 Simulation Results

39. 2011-05-04 Identifying the Missing Tags in a Large RFID System39/41 Simulation Results

40. 2011-05-04 Identifying the Missing Tags in a Large RFID System40/41 Outline Introduction System Model Missing-tag Detection Protocols Simulation Results Conclusion

41. 2011-05-04 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.