Improving Link Quality by Exploiting Channel Diversity in Wireless Sensor Networks

1. Improving Link Quality by Exploiting Channel Diversity in Wireless Sensor Networks Manjunath D, Mun Choon Chan, and Ben Leong National University of Singapore

2. Background: Low-Power Wireless Links • Categorization of the low-power wireless links Packet Reception Ratio (PRR) IQ links [Kannan et al. Sensys’2009] 2

3. Background: Intermediate Quality (IQ) Links • More than one-third of the links in practical sensor networks are of intermediate quality • IQ links are deemed unstable and are typically ignored by routing protocols • BUT IQ links offer substantial progress due to their longer range

4. Background: Importance of IQ Links • IQ links can reduce significant number of packet transmissions thus energy in WSNs 40% dst A src 100% 100% [Biswas et al. SIGCOM’2005] 4

5. Background: Importance of IQ Links • Using IQ links may be inevitable 100% 50% 100% 50% 50% 100% [Biswas et al. SIGCOM’2005] • Packet receptions may be correlated [Kannan et al. Mobicom’2010] 5

6. Problem • Current approaches to exploit IQ links require overhearing • Overhearing energy can be significantly more than the savings offered by the IQ links 6

7. Problem: Current Approaches • Overhearing is required to identify the good phases of IQ links that are typically bursty dst 1 2 4 3 src 7

8. Problem: Current Approaches • Overhearing is required to identify the good phases of IQ links that are typically bursty dst 1 2 4 3 src 8

9. Problem: Current Approaches • Overhearing is required to identify the good phases of IQ links that are typically bursty dst 1 2 4 3 src 9

10. Problem: Current Approaches • Overhearingenergy can be significantly more than the savings offered by the IQ links src src src 10

11. Our Solution • Transform IQ links into good links (PRR > 0.9) using channel diversity • Transformation eliminates the need for overhearing 11

12. Our Solution • Overhearing is not required as transformed IQ links are used constantly as part of routes rather being exploited opportunistically Channel A E default channel (25%) src A B C dst Channel C Channel B 12

13. Our Solution: Requirements • Packet receptions across different channels on an IQ link should NOT be positively correlated • Rate of fluctuation of quality of channels on IQ links should NOT be rapid 13

14. Requirements: An Empirical Study • IEEE 802.15.4 supports two sets of orthogonal channels with eight channels in each set Receiver Sender Channel 1 Mote 1 Mote 9 Channel 2 Mote 2 Mote 10 Channel 3 Mote 3 Mote 11 Channel 4 Mote 4 Mote 12 traces traces Channel 5 Mote 5 Mote 13 Channel 6 Mote 6 Mote 14 Channel 7 Mote 7 Mote 15 Channel 8 Mote 8 Mote 16 Location 1 Location 2

15. Requirements: Correlation • Pearson’s correlation coefficient at different granularities • Coefficient values are small: no positive correlation

16. Requirements: Rate of Fluctuation of Channels Quality Sufficient number of channels on IQ links change in quality on the time scale of a few minutes PRR=0.96, 26  20  24  20  26 16

17. IQ Link Transformation Protocol (ILTP) • Four main components of ILTP • Identification and filtering of poor channels • Strategy to select channels for operation • Coordinating channel switching • Integration of ILTP with Routing 17

18. ILTP: Identify and Filter Poor Channels • Increases the probability of finding a good channel as typically poor channels remain poor for long durations PRR for 5 hours = 0.01 • Poor channels can be identified either in advance or on-the-fly

19. ILTP: Channel Selection Strategy • Random channel selection works !!! • Number of available channels is a small value of 16 • The number is further reduced by filtering poor channels • ILTP identifies and avoids using transient channels on-the-fly 19

20. ILTP: Coordinating Channel Switching • Nodes switch to the same channel by using a common random seed • Nodes switch channels at the same time • Transmissions are regular and rate-controlled • The receiver accurately infers the bi-directional PRR perceived at the sender 20

21. Coordination: Overhead • Synchronization requirement is local not global • Rate-controlling does not impose any penalty • Control of overhead of the ILTP is low (about 0.18%) 21

22. ILTP: Integration with CTP • Why CTP? • ILTP is a layer between routing and MAC layers • ILTP identifies IQ links by accessing CTP’s neighbor table 22

23. ILTP: Integration with CTP Operation of CTP+ILTP 9 8 23

24. ILTP: Integration with CTP • Typically, a considerable number of nodes in a routing tree are leaf nodes 24

25. Evaluation Evaluations on three large-scale testbeds Motelab (Harvard University) • 85 TmoteSky devices Twist(Berlin Institute of Technology) • 90 TmoteSky devices Indriya (National University of Singapore) • 125 TelosB devices 25

26. Evaluation: Experimental Settings • Transmission powers: 0 dBm, -15 dBm, and -7 dBm • Experimental duration for each data point is 30 min and IPI is 250 ms • The PRR metric is bi-directional • ILTP and ILTP+CTP are evaluated separately 26

27. Evaluation: ILTP 27

28. Evaluation: Channel Durations during Transformation 28

29. Evaluation: CTP+ILTP 29

30. Evaluation: CTP+ILTP • Dynamic channel switching does not trade end-to-end reliability • CTP+ILTP: 99.7%, CTP: 97.6% 30

31. Conclusion • A new approach to exploit IQ links that eliminates the need for overhearing • IQ links are transformed into good links by switching among different channels • Channels on IQ links are generally not correlated and they change minutes-wise • Transformed IQ links reduce packet transmissions by 24% to 58% at a reliability of above 99% 31

32. Questions ?

33. Emulation: Settings for Implementation Parameters • CST (Channel Switching Threshold) • PRRWND (PRR Window) 33

34. Emulation: Settings for Implementation Parameters • CST (Channel Switching Threshold) • PRRWND (PRR Window) 34

35. Reducing Number of Overhearing Nodes Does Not Help Default route: 300 TXs + 300 RXs Total = 600 TXs/RXs Opportunistic route: 70*3 + 30*2 = 270 TXs + 270 RXs Overhearing = 70 extra RXs Total = 610 TXs/RXs dst src

36. Evaluation of ILTP in Different Settings 36

37. Radio Power Consumption

38. ILTP: Channel Selection Strategy Working set Transient set S R 38

39. ILTP: Channel Selection Strategy Working set Transient set S R X 39

40. ILTP: Channel Selection Strategy Working set Transient set S R 40

41. ILTP: Channel Selection Strategy Working set Transient set S R 41

42. ILTP: Channel Selection Strategy Working set Transient set S R 42

43. Emulation: Rate of Fluctuation of Channel Quality 10 switches/hour 39 switches/hour This gap can be reduced on excluding poor channels 43

44. Evaluation Over a Duty-cycled MAC Protocol (Preliminary Results) • BoX-MAC with polling interval of 500 milliseconds • Experimental duration and IPI: 24 hours and 10 seconds 44

45. ILTP: Channel Selection Strategy 45

46. Proposed Solution: An Empirical Study • IEEE 802.15.4 supports 16 non-overlapping channels in 2.4 GHz band • Adjacent channels interfere with each other Sender Receiver Parallel communication on 8 orthogonal channels on an IQ link 46

47. Emulation of Transformation of IQ Links • Optimal and random channel selection strategies • Both the strategies transformed all the IQ links into good links (PRR > 0.9) on at least one of the orthogonal channels sets 47

48. Problem: Current Approach dst 2 1 4 3 src 48

49. Problem: Current Approaches dst 2 1 4 3 src 49

50. Problem: Current Approaches dst 2 1 4 3 src 50