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Lower-Power Distributed Event Detection in WSNs. Y. Zhu, Y. Liu, M. Ni, Z. Zhang Presented by Shan Gao. Event Detection. Detect events which may occur momentarily. Two essential properties Physical events are usually persistent, which can last for seconds or even longer.
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Lower-Power Distributed Event Detection in WSNs Y. Zhu, Y. Liu, M. Ni, Z. Zhang Presented by Shan Gao
Event Detection • Detect events which may occur momentarily. • Two essential properties • Physical events are usually persistent, which can last for seconds or even longer. • Many applications accept a certain detection delay. • Duty cycling is a fundamental approach to conserving energy in WSNs.
Duty Cycle: δ = τon / τcycle • To conserve energy, • shorten τon • τon = τwarmup+ τsensing+ τprocessing+ τcommunication • Usually τon is fixed, tens of milliseconds • Lengthen τcycle • Problem • A longer τcycle leads to a longer delay and lower detectability.
Tradeoff • Longer τcycle • Longer delay • Lower detectability • Longer network lifetime • τcycle ≥ τevent • Detectability is 100% • τcycle < τevent • Some events possibly won’t be detected. • Users • Network lifetime Delay & detectability τcycle
RIW • Random independent wakeup • Simple but low efficient • The sensors which are close to each other may wakeup at about the same time due to the lack of awareness about their neighborhood.
CAS • Coordinated Wakeup Scheduling • Fully localized algorithm • The sensors reside closely should separate their wakeups as much as possible such that the detection delay can be minimized. • Stage 1: Initialization • Stage 2: Sensors wakeup and detect events at the time determined in Stage 1.
distributed scheduling coordination • Each sensor need to cooperate with neighbors to determine their wakeup time. • Tasks: • Identify neighbors, which are sensors within CR (cooperative range), 0 < CR <= 2Rs • Determine wakeup time, meanwhile reduce the detection delay.
Phase 1: Each sensor randomly picks up a wakeup time and broadcasts it to its neighbors. • Phase 2: Each sensor does multiple rounds of Aggressive Wakeup Adjustment. • For keeping synchronization, in each round, only one sensor can broadcast its adjustment and its neighbors update their own schedule table according to this adjustment.
Backoff technique • After using AWA to calculate the adjustment and waiting for a random time, one sensor broadcast this adjustment in a UPDT message. • Upon receiving this UPDT message, if the sender is in the receivers neighbor list, the receiver cancel sending its own adjustment request and update its own wakeup time table. • Multiple rounds of AWA is necessary to reach a reasonable schedule plan. • Usually each sensor need to send 2 UPDT messages successfully at least.
Aggressive Wakeup Adjustment • A sensor need to determine • whether it should adjust its wakeup time • and what the new wakeup is • For each sensor, it is desirable to evenly distribute the wakeups of its cooperative neighbors and itself over τcycle. • Wakeup Separation: t1, t2, t3 t1 t2 t3 t1 0 s1 s2 s3 τcycle
s1 • Variance: difference between wakeup separations • If the new variance is decreased, s1 broadcasts a UPDT message. t1 t1 t2 t3 t3 t1 t1 0 0 s2 s2 s3 s3 s1 τcycle τcycle