Shuo Guo, Song Min Kim, Ting Zhu, Yu Gu, and Tian He University of Minnesota, Twin Cities

Target Tracking Border Control Environmental Monitoring Habit Monitoring Precision Agriculture Infrastructure Integrity Analysis Inventory Control Traffic Control Assisted Living Interactive Gaming Space Monitor  For hostile environment  For unattended operations  For low deployment & maintenance cost 2

 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 3 t … Sender Active State Dormant State … Receiver t

 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! Active State Dormant State B C D A B C D A t B CD 3 Transmissions in total!

 What if working schedules can be flexibly changed? Active State Dormant State B C D A B C D A t B CD 3 Transmissions in total!

 What if working schedules can be flexibly changed? Active State Dormant State B C D A B C D A t B+C+D 1 Transmissions in total!

 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

 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 S S S R1R1 S R2R2 S RNRN … ACK Collision!

LLet highly correlated nodes to wake up at the same time and receive flooding packets simultaneously ◦A◦A flooding tree is built with the consideration of both link quality and link correlation FFor the same sender, ACK is only sent by the node with the lowest link quality, to eliminate the ACK implosion problem. 9 More Energy Efficient! Eliminate ACK-Implosion!

 Link Correlation is the phenomenon that the reception results of a broadcasting packets at different nodes are not independent 10 Given that a broadcasting packet is successfully received by B, what is the probability that it is also received by A? =95%: no correlation! >95%: positive correlation! < 95%: negative correlation! Pr(A|B)Pr(A) = 95% ?

 Experiment: 1 sender, 40 receivers, 100 packets  Receivers record the reception status using a bitmap 11 S S S R1R S R2R2 90% 80% S R 40 … % Pr(R 1 |R 2 ) = 100% Pr(R 2 |R 40 ) = 86% Pr(R 1 |R 2 R 40 ) = 100% Link QualityLink 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 12

 Two-node case:  N-node case: 13 With correlated links, q 12 ↑, E(m) ↓ ↓ with correlation, E(m) ↓

 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 14

 Group Division: nodes with higher correlation are divided into the same group, using k- mean 15

 Sender Selection: nodes selects its flooding parent to make the worst link in each group as high as possible 16 S R3R3 S R1R1 S R2R2 S S1S1 S S2S2 80% 85% 95% 70% w/o link correlation: R 1 chooses S 2 w/ link correlation: R 1 chooses S 1

 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. 17

 Test-bed implementation ◦ 20 MicaZ nodes form a 2-hop network  Simulation Setup ◦ Randomly generated network, 200~1000 nodes  Baseline ◦ Traditional Energy-Optimal Tree 18

Group # Flooding Delay Group # Flooding Coverage Flooding DelaysFlooding Coverage Ratios

Group # Data Packets Sent Group # ACKs Sent 50% 10% Energy Cost on Data PacketsEnergy Cost on ACKs

21 Data Packets Sent Network Size ACKs Sent Energy Cost on Data PacketsEnergy Cost on ACKs 70% 25%

 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)

 THE END

 A test-bed experiment with 1 sender and 40 receivers  Sender broadcasts 100 packets while receivers recording the reception results … Length=100

 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: % link quality B: % link quality Correlation: Pr(A|B) = 100% !