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Correlated Flooding in Low-Duty-Cycle Wireless Sensor Networks. Shuo Guo , Song Min Kim, Ting Zhu, Yu Gu , and Tian He. University of Minnesota, Twin Cities. Inventory Control. Traffic Control. Assisted Living. Environmental Monitoring. Interactive Gaming. Habit Monitoring.
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Correlated Flooding in Low-Duty-Cycle WirelessSensor Networks ShuoGuo, Song Min Kim, Ting Zhu, Yu Gu, and Tian He University of Minnesota, Twin Cities
Inventory Control Traffic Control Assisted Living Environmental Monitoring Interactive Gaming Habit Monitoring Target Tracking Precision Agriculture Space Monitor Border Control Infrastructure Integrity Analysis The Need for Long-Term Operation • For hostile environment • For unattended operations • For low deployment & maintenance cost
How to Prolong WSNs’ Lifetime? • Most effective solution: Low-Duty-Cycle WSN • Sensor nodes stay active for a very short interval and then turn off almost everything and go to sleep • A node periodically switches between active state and dormant state … Sender t Receiver … t Active State Dormant State
Existing Solutions • State-of-arts uses unicasts to do broadcasting/flooding • Lai et al., DCOSS’10, Sun et al., Sensys’09, Guo et al., MobiCom’09, • Inefficient use of energy! C B D C B D C D B A t 3 Transmissions in total! A Active State Dormant State
Existing Solutions • What if working schedules can be flexibly changed? C B D C B D C D B A t 3 Transmissions in total! A Active State Dormant State
Existing Solutions • What if working schedules can be flexibly changed? C B D C B D B+C+D A t 1 Transmissions in total! A Active State Dormant State
Basic Idea • Build an energy-efficient flooding tree as the basic flooding structure • Children of a common parent tune their working schedules to wake up simultaneously • Children send ACKs to acknowledge the reception
New Challenges • How to build an energy-efficient flooding tree? • Traditional way that only considers link quality is not sufficient • How to avoid ACK-implosion problem? • Should reduce ACKs sent back by the receivers, but still guarantee reliability RN S S S R1 R2 Collision! … S S ACK ACK ACK
Idea of Correlated Flooding • Let highly correlated nodes to wake up at the same time and receive flooding packets simultaneously • A flooding tree is built with the consideration of both link quality and link correlation • For the same sender, ACK is only sent by the node with the lowest link quality, to eliminate the ACK implosion problem. More Energy Efficient! Eliminate ACK-Implosion!
Link Correlation • Link Correlation is the phenomenon that the reception results of a broadcasting packets at different nodes are not independent Given that a broadcasting packet is successfully received by B, what is the probability that it is also received by A? ? Pr(A|B) Pr(A) = 95% =95%: no correlation! >95%: positive correlation! < 95%: negative correlation!
Existence of Link Correlation • Experiment: 1 sender, 40 receivers, 100 packets • Receivers record the reception status using a bitmap Link Quality Link Correlation? S S S R1 R2 R40 1110111111 90% Pr(R1|R2) = 100% 1100111111 80% Pr(R2|R40) = 86% … Pr(R1|R2R40) = 100% S S 1101111100 70%
Existence of Link Correlation • Experiment: 1 sender, 40 receivers, 100 packets • Blue: distribution of link quality, # of packets received • Red: distribution of # of packets received, given the successful reception at other links with lower link quality
Impact of Link Correlation on Flooding • Two-node case: • N-node case: With correlated links, q12 ↑, E(m) ↓ ↓ with correlation, E(m) ↓
Construction of Flooding Tree • Group Division • Senders divide their receivers into a number of groups with high correlation within the group • Sender Selection • Each receiver selects only one sender to optimize the overall performance
Construction of Flooding Tree • Group Division: nodes with higher correlation are divided into the same group, using k-mean
Construction of Flooding Tree • Sender Selection: nodes selects its flooding parent to make the worst link in each group as high as possible w/o link correlation: R1 chooses S2 S S S S S R1 S2 R2 R3 S1 85% 95% 70% 80% w/ link correlation: R1 chooses S1
Idea of Correlated Flooding • Let highly correlated nodes to wake up at the same time and receive flooding packets simultaneously • A flooding tree is built with the consideration of both link quality and link correlation • For the same sender, ACK is only sent by the node with the lowest link quality, to eliminate the ACK implosion problem.
Evaluation • Test-bed implementation • 20 MicaZ nodes form a 2-hop network • Simulation Setup • Randomly generated network, 200~1000 nodes • Baseline • Traditional Energy-Optimal Tree
Test-bed Results Flooding Delays Flooding Coverage Ratios Flooding Coverage Flooding Delay Group # Group #
Test-bed Results Energy Cost on Data Packets Energy Cost on ACKs Data Packets Sent ACKs Sent 50% 10% Group # Group #
Simulation Results Energy Cost on Data Packets Energy Cost on ACKs Data Packets Sent 25% ACKs Sent 70% Network Size Network Size
Summary of Contributions • Experimentally verify the existence of link correlation, followed by a theoretical study about its impact on broadcasting/flooding • Utilizes the information of both link quality and link correlation to build an energy efficient flooding tree that saves the energy cost on data packets • Receptions at highly correlated nodes are Acknowledged by only one ACK, saving the energy cost on control packets (ACKs)
Existence of Link Correlation • A test-bed experiment with 1 sender and 40 receivers • Sender broadcasts 100 packets while receivers recording the reception results 111110111111…11100010 Length=100
Existence of Link Correlation • Link quality: how many 1s are there in a single bitmap? • Link correlation: how many 1s are there in node A’s bitmap while the corresponding bit in node B’s bitmap is 1? A: 11011------80% link quality B: 11011------80% link quality Correlation: Pr(A|B) = 100% !
