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 [Kannan et al. Sensys’2009] IQ links Packet Reception Ratio (PRR)

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 3

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

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

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

7 src dst Problem: Current Approaches  Overhearing is required to identify the good phases of IQ links that are typically bursty

8 src dst Problem: Current Approaches  Overhearing is required to identify the good phases of IQ links that are typically bursty

9 src dst Problem: Current Approaches  Overhearing is required to identify the good phases of IQ links that are typically bursty

10 Problem: Current Approaches  Overhearing energy can be significantly more than the savings offered by the IQ links src

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

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

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

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

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

16 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 Requirements: Rate of Fluctuation of Channels Quality

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

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

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

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

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%)

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

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

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

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

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

27 Evaluation: ILTP

28 Evaluation: Channel Durations during Transformation

29 Evaluation: CTP+ILTP

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

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%

Questions ? 32

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

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

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

36 Evaluation of ILTP in Different Settings

Radio Power Consumption Data Rate 250 Kbps RX Power52.2 mW TX power56.4 mW TX energy/bit208 nJ RX energy/bit225 nJ CC2420 Radio Transceiver

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

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

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

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

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

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

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

45 ILTP: Channel Selection Strategy

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

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

48 src dst Problem: Current Approach

49 src dst Problem: Current Approaches

50 dst Problem: Current Approaches src