1. KAIST Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks Suho Yang (CS710: November 4, 2008) Kyle Jamieson, Hari Balakrishnan, Y.C. Tay LNCS Springer 2006

2. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Contents Introduction Motivation Our proposal: Sift Performance Evaluation Conclusion & Discussion 2

3. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Event-driven WSN Report an event information when an phenomenon occurs ↔ periodic traffic Latency-sensitive applications 3 Introduction

4. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 4 Sift Focus Designing MAC protocol to handle event-driven traffic Challenges Low latency Good throughput Good fairness Introduction

5. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 5 Problems of traditional contention-based MAC in WSN Spatial correlated contention Not suitable for bursty traffic Motivation: Problems of Traditional CSMA

6. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 6 Problems of traditional non-persistent CSMA Timeslot: opportunity for a node to begin transmitting Pick a timeslot chosen uniformly in [0, CW] Listen up to chosen slot Transmit if nobody else started transmitting Wait more if somebody else started transmitting Motivation: Problems of Traditional CSMA Busy Medium When the channel is idle, 1-persistent CSMA: transmit immediately with 100% probability p-persistent CSMA : transmit immediately with p*100% probability Non-persistent CSMA: transmit after waiting for a random amount of time and checks again

7. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 7 Problems of traditional non-persistent CSMA Successful transmission case Collision case Motivation: Problems of Traditional CSMA Node A: Node B: Slot choice (slot #4) Slot choice (slot #8) Node A: Node B: Slot choice (slot #4)

8. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 8 Problems of traditional non-persistent CSMA High contention causes collisions in CSMA Motivation: Problems of Traditional CSMA Unacceptable collision rate above ~15 transmitting sensors Due to uniform distribution!

9. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 9 Solution for this problem Create more slots Conventional approach Called “binary exponential backoff” (BEB) Motivation: Problems of Traditional CSMA Acknowledgement? Reduce CW Double CW and resend YesNo

10. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 10 Problem of BEB Takes time for every node to increase CW Especially if traffic is spatially-correlated and bursty Waste backoff slots if collisions cause CW to increase We are interested in the collision-free transmission of only the first R of N potential reports of some event Motivation: Problems of Traditional CSMA sink

11. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 11 Sift is a MAC protocol for... Event-driven traffic Low-latency requirements Sift’s properties Extremely simple Offers up to 7-fold lower latency Goal Design a MAC protocol that minimizes the latency taken to send R of without collisions Our proposal: Sift

12. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 12 Main Idea Change the way we pick slots Instead of uniform distribution Use small and fixed-size contention window No BEB Not all sensing nodes need to report an event It is enough for a subset of the event reports to reach the data sink Out of N nodes, only the first R nodes report (the remainder are suppressed) Changing the Distribution for picking transmission slot Use an geometrically-increasing probability distribution → Reduce the chance of collisions → Reduce wastage of backoff slots Our proposal: Sift

13. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 13 Sift’s slot selection distribution Increasing exponential distribution Our proposal: Sift

14. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 14 Why use this pdf? Our proposal: Sift A Bins represent backoff slots → Nodes choosing each slot →

15. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 15 Optimal non-persistent CSMA performance With knowledge of number of nodes (IEEE JSAC ’04) Our proposal: Sift

16. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Sift keeps success rate above this unacceptable range 16 Sift approaches Optimal Sift needs no knowledge of the number of nodes Our proposal: Sift

17. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation-based NS-2 Comparisons with 802.11 (BEB), 802.11/copy (=copy overheard CW+countdown timer) Experiment Setup (Event-driven traffic pattern) Topology: Single-hop to one base station N nodes sense and report an event R (≤ N ) reports are required If a node hears ≥ R reports then it suppresses its own event report 17 BS E.g. N=4, R=3

18. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation_1-1: Latency Sift outperforms when N is large 18 R=16 R=1

19. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation_1-2: Latency Sift outperforms as R Increases 19

20. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation_2: Fairness Sift outperforms 802.11 in terms of fairness 20 Eight nodes 64 nodes

21. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation_3: Hidden terminal experiment setup Separate 128 sensors into mutually-hidden clusters Nodes in one cluster cannot hear nodes in another All nodes send to the base station Result: hidden terminal collisions at the base station 21 Base Station

22. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Performance Evaluation Simulation_3: Hidden terminal experiment setup 22

23. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 23 Sift is ideal for sensor networks, where... it is often sufficient that any R of N sensors that observe an event report it spatially-correlated contention occurs sudden changes in the number of sensors that are trying to send data Key idea Use a geometrically-increasing probability distribution for picking a transmission slot within a fixed-size contention window Sift is a latency-enhancing MAC for event-driven sensor networks Conclusion

24. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 24 The lack of mentions about... Energy consumption No attention about... How to determine R? Only manual setting Too strong assumption Single-hop communication to one base station Discussion

25. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 Thank you 25

26. Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24 26 Existing MAC protocols in WSN Let s be a slot number, assume N ≥ 2 sensors transmitting. Define: Appendix A: Optimal Non-Persistent CSMA “Collision Minimizing CSMA and its Applications to Wireless Sensor Networks.” IEEE JSAC, 2004