RFID Readers Networks: A network layer perspective

1. RFID Readers Networks: A network layer perspective Ahmed Jedda

2. Outline • RFID Readers networks • Problems: • Readers to tags collisions • Tags discovery • Network planning algorithms • RFID coverage • Readers connectivity Wisense workshop

3. WSN vs. RFID • RFID offers much cheaper techniques when it comes to identification of physical objects • Similar to the radar problem found in the WWII • No need to guess your object, Wisense workshop

4. What is an RFID ? An RFID system consists: - RFID reader, - RFID tag, Procedure: • RFID reader sends a signal to an RFID tag, • An embedded circuit in the tag is energized, • A reply is sent back to the reader • The reply may have the identifier of the tag, Notes: No battery in RFID tags, (i.e. communication initiated by readers) Simple computation is allowed in tags, RFID readers may write in the memory of neighbor tags, Wisense workshop

5. Applications • Many applications • Children temperature monitoring in Hongkong hospitals (integration with sensors) • Assets monitoring: Indian TV station (assets worth millions of dollars) • Animal tracking (cows tracking in of the oldest applications of RFID) • Cars in parking (automatic payment) • Technology is sponsored by major companies with world-wide supply chains (Walmart, Gillete, DHL, etc ..) Wisense workshop

6. RFID readers networks models 1 reader, 1 tag Typical scenarios 1 reader --- 1 tag, 1 reader ---- multiple tags multiple readers ----- multiple tags Readers can interact with tags, Reader can interact with readers, = RFID reader networks ! 1 reader, many tags many readers, many tags As the number of readers and tags increase, we get dense RFID readers networks and we get many other problems as well ! Communication between readers is possible Wisense workshop

7. The readers collision problem

8. The readers collision problem Problem: Two types of collisions may be caused by readers: 1. Readers to readers (a wireless networking problem) 2. Readers to tags • Approaches: • Assign the readers time slots to transmit in them such that no collision happen • - Implemented by: Colorwave Wisense workshop

9. Colorwave Algorithm idea: 1. Create a graph G with vertices V = readers, E = the set of interference edges (i.e. if two reader may cause interference to each other, then add an edge between them in E.) 2. Color the vertices with colors {c1, …, ck} 3. Sequence of time slots are given colors {c1, …, ck} 4. A reader with color ci transmits at the time slot with color ci Colorwave: Random coloring - Each reader selects a random color, - If there is a collision, the reader and its neighbors select new colors, - to avoid collisions, set k to be high enough. Wisense workshop

10. The tag discovery (identification) problem

11. The tag identification problem Problem: A reader must find neighboring tags in the shortest possible time Basic solution: Broadcast and wait for replies. - The issue is the collisions caused by tags replying at the same time Approaches: Aloha-based (based on backoff timers) Tree-based (similar to tree traversals - divide tags according to their id’s prefixes) Wisense workshop

12. Aloha-based tags discovery Assumptions: A tag have timer, can generate pseudo-random numbers, may send multiple replies. Algorithm idea: 1. Readers broadcast inquiry messages, 2. Upon reception: 2.a: a tag backoff for a random time, 2.b: then, the tag sends back a reply 3. A tag must receive an acknowledgement from the reader. 3.a: if received: be quite in the future. 3.b: otherwise, repeat the same previous procedures Random backoff • Improvements: • Synchronization of readers and tags, • Provide tags with time frame for better backoff periods, • Tags send only once in a frame Wisense workshop

13. Tree-based tags discovery Assumptions: A reader may inquire all neighbor tags with id’s prefix [p1,p2, …, pk]. • Algorithm idea: • Send an inquiry message to tags with id = [p1,…,pk]XXX. • Tags with id prefix [p1,…,pk] reply to inquiry. • Case1: no answer [no tags with such prefix is nearby] • Case2: one answer [only one node exists with such prefix] • Case3: collision [there are more than one with such prefix] • In case of collision: • Generate pk+1 randomly – either 0 or 1 • Repeat Steps with id [p1, ….pk+1]XXX until no answer, or one exact answer ! This forms a binary tree Wisense workshop

14. Tree-based tags discovery Example 0010 0010 1101 1101 0100 0100 0011 0011 Reader inquires all tags Id = [XXXX] All tags reply, Collision is detected Wisense workshop

15. Tree-based tags discovery Example 0010 0010 1101 1101 0100 0100 0011 0011 Reader inquires all tags Id = [0XXX] Tags {0010, 0100, 0011} reply, Collision is detected Wisense workshop

16. Tree-based tags discovery Example 0010 0010 1101 1101 0100 0100 0011 0011 Reader inquires all tags Id = [01XX] Tag 0100 reply, Only one answer -- continue looking for other tags ! 1101 1 0100 1 0011 1 0 1 0010 0 0 Wisense workshop 0 No answer

17. RFID network planning

18. RFID Networks planning • Problem: • What are the best positions for the readers in a network that let every tag covered by at least one reader? • Other performance measurements could be considered - minimize interference, balance load etc .. • Approaches: • Known-area topology: if the area is already known, then the optimal placement of readers is possible such that every point is cover. • Examples: grid, honey grid. • Heuristic search algorithms (Genetic algorithms, Swarm intelligence etc ..): • solve a set of objective formulas (usually complex ones) • Most of them assume previous knowledge of tags positions ! [what if tags move then ?] • Set-cover based (3D) • Random placement Wisense workshop

19. Set-cover based network planning • Assumptions: • previous knowledge of tags locations, • Tags may exist in a cube, whereas readers only on borders. • Reading range is R Observation: 1. A tag centered at point (xt,yt,zt) can be covered by any reader located in a sphere centered at (xt,yt,zt) with radius R 2. The intersection of the sphere with the walls is a circle, 3. Put readers in the tags circles intersection The problem is discrete now ! Solve it with a greedy approximation set-cover algorithm T2 T1 T3 Placing one reader will cover the tree tags Wisense workshop

20. RFID Coverage Or simple, redundant readers elimination …

21. RFID Coverage Problem: Given an RFID network such that every tag is covered, there may be more readers that what is required. - Find the minimum amount of readers that cover every tag. Solutions of the problem may be used to: 1) increase network lifetime, 2) fault-tolerance, 3) load balancing, and 4) data redundancy elimination Approaches: Set-cover based, - GREEDY, NTE Readers compete for tags ownership (assume tags are writable), -RRE, GDE, LEO, RANDOM and others .. Wisense workshop

22. First algorithm: RRE • RRE: (Algorithm idea): • Every reader writes (tag count, identifier) in the memory of all its neighboring tags. • Wait for all readers to write, • Read memory contents of neighbor tags, • A reader owns a tag if it has the largest (tag count, identifier). (tag count, id) (1,1) {(3,2), (1,1)} Non-redundant readers are t2 and t3 (3,2) {(3,2), (1,4)} (2,3) {(3,2), (2,3)} (1,4) {(2,3), (1,4)} Other similar solutions: LEO: the first reader to write owns the tag, DRRE: the reader with maximum number of reader neighbors owns the tag Wisense workshop

23. Improvement on RRE - GREEDY Assumptions: centralized algorithm – a server exists GREEDY: (Algorithm idea): Let T = {t1, …., tm} : set of tags Let N = {N(r1), N(r2) …., N(rn)} : the set of tags covered by a reader r, Find a subset N’ of N of minimum size such that the union of N’ is T. Set-cover approximation algorithm: Always select the set N(ri) that minimize the number of uncovered tags. NTE is a similar algorithm, Maximum weight depends on tags count, and number of reader neighbors, Better than RRE, (but not GREEDY) Wisense workshop

24. Improvements on RRE - GDE (tcount, id) (2,4) GDE- Algorithm idea: 1. Runs in iterations, 2. A reader owns a tag only if it owns every other active neighbor tags. (that is, if it has the largest (tag count, identifier)) 3. Deactivate all owned tags, and repeat with new network until all tags are deactivated. (2,3) (2,2) (1,1) (2,5) (2,3) (2,2) (1,4) Wisense workshop

25. Improvements on RRE - GDE GDE- Algorithm idea: 1. Runs in iterations, 2. A reader owns a tag only if it owns every other active neighbor tags. (that is, if it has the largest (tag count, identifier)) 3. Deactivate all owned tags, and repeat with new network until all tags are deactivated. (1,3) (2,2) (1,1) (1,3) (2,2) (1,4) Wisense workshop

26. Improvement on GDE – GDE-k GDE-K Algorithm idea: • Run GDE for k iterations, • At the k+1 iteration, run RRE Wisense workshop

27. Algorithm RANDOM+ RANDOM+: Algorithm idea • Each reader weight is (rand(r), id(r)), where rand(r) is a random number selected by the reader at the beginning of an algorithm. • A tag is owned by a neighbor reader with maximum weight. Benefits: Can be run multiple iterations: - execution on non-redundant readers. - each iteration a new rand value is selected. Random-6 (i.e. with 6 iterations) perform somehow similar (or even better sometimes) compared to GDE [with the same amount of write/read rounds] We can also run RANDOM+ after GDE – (GDE-RANDOM+)  not much improvement though ! Wisense workshop

28. Problems with writable tags Observation: • the read range Rr is larger than the write range Rw. Problem: • How to design algorithms that consider this case ? Case: Rw < Rr: there is always a topology organization such that no tag is writable, Therefore: an algorithm should consider writing is not allowed (if Rw < Rr is to be considered). Can we do RRE without writing ? [relax to tag reporting problem] • there must be links between readers, otherwise impossible. • readers should be connected - otherwise impossible. • If there are links, --> at least worse than election ! • that is, no localized algorithm • A lower bound: O(nlogn + m) in term message complexity. In fact, RRE = directed graph construction GDE = topological sorting Wisense workshop

29. Readers connectivity

30. How to connect all readers ? Problem: • Find a connected multihop network that contains minimum amount of readers to perform a certain task. • In our case, task = coverage. Assumptions: • Readers can communicate with each other Wisense workshop

31. How to connect all readers ? Problem: • Given a graph G = (V,E) and a set V’ of V (called necessary nodes), find an minimum sized V’’ (called relays) in V such that the induced graph of V’ and V’’ is connected ---- very similar to the minimum steiner tree problem • Approaches: • Local rules: delete a node if it does’nt affect connectivity (usually the k-hop neighborhood graph is considered) – • Minimum spanning trees • Shortest path trees: • Virtual trees • Connected dominating set Wisense workshop

32. Minimum spanning tree based Assume that this tree is the minimum spanning tree of graph G= (V,E) Necessary nodes Relay nodes others Wisense workshop

33. Minimum spanning tree based Leaves that are not necessary delete themselves Necessary nodes Relay nodes others Wisense workshop

34. Minimum spanning tree based Leaves that are not necessary delete themselves Necessary nodes Relay nodes others Wisense workshop

35. shortest path tree based Algorithm idea: • Start with a tree that T contains a single necessary node • Find the closest necessary v node to the tree T, • Add the nodes of the path between v and the tree T Wisense workshop

36. shortest path tree based Algorithm idea: • Start with a tree that T contains a single necessary node • Find the closest necessary v node to the tree T, • Add the nodes of the path between v and the tree T Wisense workshop

37. shortest path tree based Algorithm idea: • Start with a tree that T contains a single necessary node • Find the closest necessary v node to the tree T, • Add the nodes of the path between v and the tree T Wisense workshop

38. shortest path tree based Algorithm idea: • Start with a tree that T contains a single necessary node • Find the closest necessary v node to the tree T, • Add the nodes of the path between v and the tree T Wisense workshop

39. shortest path tree based Algorithm idea: • Start with a tree that T contains a single necessary node • Find the closest necessary v node to the tree T, • Add the nodes of the path between v and the tree T Wisense workshop

40. Virtual trees Algorithm idea: • Assume knoweldge of tags positions, • Build a tree of tags, • This facilitates the tree construction process Wisense workshop

41. Connected dominating sets Recall: A dominating set of a graph G = (V,E) is a set V’ such that every node in V is either in V’ or neighbor to a node in V’. A connected dominating set V’ restrict that the induced graph of V’ is connected Algorithm idea: • Use a localized algorithm to build a connected dominating set, • - we use the multiset relay algorithm • Then execute RRE, GDE or any other --- but give priority to nodes already in CDS. Wisense workshop

42. Thanks ! Wisense workshop