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  1. ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks[Sankarasubramaniam et. al, ACM MobiHoc 2003] Prepared by A. ABOUZEID RPI ECSE-6962

  2. Event Detection in a WSN A sensor node that can sense the event Event! Sink wants reliable event detection with minimum energy expenditure A sensor node RPI ECSE-6962

  3. Motivation • A sink is only interested in the collective information from a number of source nodes and not in individual sensor reports • Event-to-sink communication • Different from traditional notion of end-to-end communication • Energy-efficient • Congestion resolution RPI ECSE-6962

  4. Problem Statement • To configure the reporting rate f of source nodes so as to achieve the required event detection reliabilityRat the sink with minimum resource utilization • Also resolve congestion RPI ECSE-6962

  5. Linear increase with f Typical Behavior at a Sink Network gets congested sooner with increasing number of source nodes Congestion: Reliability level is always lower than the peak point RPI ECSE-6962

  6. Five characteristic regions Not Congested Congested Higher reliability than required Lower reliability than required Goal: To stay in OOR where energy expenditure is optimal OOR RPI ECSE-6962

  7. Congestion Detection • Congestion status is required at the sink to determine the network state • Based on expectation of buffer overflow at sensor nodes • During a single interval, f and n do not change much • If pending congestion is detected CN bit is set in event reports RPI ECSE-6962

  8. ESRT Actions RPI ECSE-6962

  9. ESRT State Diagram Not all transitions are possible (e.g. From (C,HR), ESRT cannot transition to (NC,LR)) RPI ECSE-6962

  10. Stability of ESRT • ESRT converges to OOR from any of four initial states {(NC,LR), (NC,HR), (C,HR), (C,LR)} • From (NC,HR), ESRT stays in the state until converges to OOR • Convergence time depends on ε – smaller ε causes longer convergence time RPI ECSE-6962

  11. Simulation Setup • Ns-2 simulator • 200 sensor nodes • 100m x 100m area • 40m transmission range • 30 byte packets • 65 packets IFQ • 10 sec decision interval (τ) RPI ECSE-6962

  12. From (NC,LR) Reaches OOR in two intervals RPI ECSE-6962

  13. From (NC,HR) ESRT stays in (NC,HR) until reaching OOR in five intervals RPI ECSE-6962

  14. (C,HR) to (NC,HR) then OOR RPI ECSE-6962

  15. (C,LR) to (NC,LR) then OOR RPI ECSE-6962

  16. Power savings from (NC,HR) Reporting rate gets reduced conservatively while maintaining reliability RPI ECSE-6962

  17. Conclusion • ESRT provides a reliable event-to-sink communication • Self-configuration • Energy awareness • Uses minimum energy while achieving required reliability • Congestion control • Collective identification • Individual sensor ID is not necessary • Biased implementation • Almost entirely in sink RPI ECSE-6962

  18. Questions • Definition of reliability as number of received packets? • Is ESRT congestion detection accurate and reliable? • ESRT action heavily depends on the congestion state • What if the congestion reports are inconsistent due to partial congestion or underlying path oscillation? • What is the effect of inaccurate congestion state detection on ESRT? • Is it reasonable assumption that a sink is capable of broadcasting to all the source nodes ? • what if R is higher than the peak point? RPI ECSE-6962

  19. Questions continued • Out of band? • If in-band, how to deal with congestion? • What if the density of source nodes is too low to meet the required reliability with fast reporting? • In other words, RPI ECSE-6962