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Improving Link Quality by Exploiting Channel Diversity in Wireless Sensor Networks. Manjunath D, Mun Choon Chan, and Ben Leong National University of Singapore. Background: Low-Power Wireless Links. Categorization of the low-power wireless links. Packet Reception Ratio (PRR). IQ links.
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Improving Link Quality by Exploiting Channel Diversity in Wireless Sensor Networks Manjunath D, Mun Choon Chan, and Ben Leong National University of Singapore
Background: Low-Power Wireless Links • Categorization of the low-power wireless links Packet Reception Ratio (PRR) IQ links [Kannan et al. Sensys’2009] 2
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
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
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
Problem • Current approaches to exploit IQ links require overhearing • Overhearing energy can be significantly more than the savings offered by the IQ links 6
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
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
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
Problem: Current Approaches • Overhearingenergy can be significantly more than the savings offered by the IQ links src src src 10
Our Solution • Transform IQ links into good links (PRR > 0.9) using channel diversity • Transformation eliminates the need for overhearing 11
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
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
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
Requirements: Correlation • Pearson’s correlation coefficient at different granularities • Coefficient values are small: no positive correlation
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
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
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
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
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
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
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
ILTP: Integration with CTP Operation of CTP+ILTP 9 8 23
ILTP: Integration with CTP • Typically, a considerable number of nodes in a routing tree are leaf nodes 24
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
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
Evaluation: CTP+ILTP • Dynamic channel switching does not trade end-to-end reliability • CTP+ILTP: 99.7%, CTP: 97.6% 30
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
Emulation: Settings for Implementation Parameters • CST (Channel Switching Threshold) • PRRWND (PRR Window) 33
Emulation: Settings for Implementation Parameters • CST (Channel Switching Threshold) • PRRWND (PRR Window) 34
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
ILTP: Channel Selection Strategy Working set Transient set S R 38
ILTP: Channel Selection Strategy Working set Transient set S R X 39
ILTP: Channel Selection Strategy Working set Transient set S R 40
ILTP: Channel Selection Strategy Working set Transient set S R 41
ILTP: Channel Selection Strategy Working set Transient set S R 42
Emulation: Rate of Fluctuation of Channel Quality 10 switches/hour 39 switches/hour This gap can be reduced on excluding poor channels 43
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
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
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
Problem: Current Approach dst 2 1 4 3 src 48
Problem: Current Approaches dst 2 1 4 3 src 49
Problem: Current Approaches dst 2 1 4 3 src 50