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 1.Physical events are usually persistent, which can last for seconds or even longer. 2.Many applications accept a certain detection delay. Duty cycling is a fundamental approach to conserving energy in WSNs.

Duty Cycle: δ = τ on / τ cycle To conserve energy, – shorten τ on τ on = τ warmup + τ sensing + τ processing + τ communication Usually τ on is fixed, tens of milliseconds – Lengthen τ cycle Problem – A longer τ cycle leads to a longer delay and lower detectability.

Tradeoff Longer τ cycle – Longer delay – Lower detectability – Longer network lifetime τ cycle ≥ τ event – Detectability is 100% τ cycle < τ event – Some events possibly won’t be detected. Users – Network lifetime   Delay & detectability  τ cycle

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: CR – Identify neighbors, which are sensors within CR (cooperative range), 0 < CR <= 2R s – Determine wakeup time, meanwhile reduce the detection delay.

Phase 1: Each sensor randomly picks up a wakeup time and broadcasts it to its neighbors. Aggressive Wakeup Adjustment 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 τ cycle. Wakeup Separation: t1, t2, t3 t1t2t3t1 0τ cycle s1s1 s2s2 s3s3

s 1 Variance: difference between wakeup separations If the new variance is decreased, s 1 broadcasts a UPDT message. t1t3t1 0τ cycle s2s2 s3s3 t1t2t3t1 0τ cycle s1s1 s2s2 s3s3

Performance Evaluation CR

Τ cycle = 10