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

Improving Link Quality by Exploiting Channel Diversity in Wireless Sensor Networks

Manjunath D, Mun Choon Chan, and Ben LeongNational University of Singapore

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Background: Low-Power Wireless Links

Categorization of the low-power wireless links

[Kannan et al. Sensys’2009]

IQ links

Packet Reception Ratio (PRR)

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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

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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 dst100% 100%

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Background: Importance of IQ Links

[Biswas et al. SIGCOM’2005]

50%100%

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50%

100%

100%

Using IQ links may be inevitable

Packet receptions may be correlated [Kannan et al. Mobicom’2010]

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Problem

Current approaches to exploit IQ links require overhearing

Overhearing energy can be significantly more than the savings offered by the IQ links

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Problem: Current Approaches

Overhearing is required to identify the good phases of IQ links that are typically bursty

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Overhearing energy can be significantly more than the savings offered by the IQ links

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Our Solution

Transform IQ links into good links (PRR > 0.9) using channel diversity

Transformation eliminates the need for overhearing

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Our Solution

src A B C dst

Edefault channel (25%)

Channel AChannel BChannel C

Overhearing is not required as transformed IQ links are used constantly as part of routes rather being exploited opportunistically

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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 802.15.4 supports two sets of orthogonal channels with eight channels in each set

Mote 1 Mote 9Channel 1

Location 1








Location 2

Sender Receiver

traces traces

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Requirements: Correlation

Pearson’s correlation coefficient at different granularities

Coefficient values are small: no positive correlation

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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

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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 = 0.01

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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

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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

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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%)

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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

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Operation of CTP+ILTP

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Typically, a considerable number of nodes in a routing tree are leaf nodes

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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

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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

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Evaluation: ILTP

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Evaluation: Channel Durations during Transformation

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Evaluation: CTP+ILTP

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Evaluation: CTP+ILTP

Dynamic channel switching does not trade end-to-end reliability

CTP+ILTP: 99.7%, CTP: 97.6%

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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%

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Questions ?

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Emulation: Settings for Implementation Parameters

CST (Channel Switching Threshold) PRRWND (PRR Window)

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Emulation: Settings for Implementation Parameters

CST (Channel Switching Threshold) PRRWND (PRR Window)

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 src

dst

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Evaluation of ILTP in Different Settings

Radio Power Consumption

Data Rate 250 Kbps

RX Power 52.2 mW

TX power 56.4 mW

TX energy/bit 208 nJ

RX energy/bit 225 nJ

CC2420 Radio Transceiver

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Working set

S R

Transient set

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Working set

S R

Transient set

X

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Working set

S R

Transient set

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Working set

S R

Transient set

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Working set

S R

Transient set

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Emulation: Rate of Fluctuation of Channel Quality

This gap can be reduced on excluding poor channels

10 switches/hour

39 switches/hour

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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

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Proposed Solution: An Empirical Study

Sender Receiver

Parallel communication on 8 orthogonal channels on an IQ link

IEEE 802.15.4 supports 16 non-overlapping channels in 2.4 GHz band

Adjacent channels interfere with each other

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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

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Problem: Current Approach

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Documents

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