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ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks[Sankarasubramaniam et. al, ACM MobiHoc 2003]
Prepared by A. ABOUZEID
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Event Detection in a WSN
Event!
A sensor node
A sensor node that can sense the event
Sink wants reliable event detection with minimum energy expenditure
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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
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Problem Statement To configure the reporting rate f
of source nodes so as to achieve the required event detection reliability R at the sink with minimum resource utilization
Also resolve congestion
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Typical Behavior at a SinkNetwork gets congested sooner with increasing number of source nodes
Linear increase with f
Congestion:Reliability level is always lower
than the peak point
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CongestedNot Congested
Lower reliability than required
Higher reliability than required
OOR
Five characteristic regions
Goal: To stay in OOR where energy expenditure is optim
al
Rr
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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
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ESRT ActionsNetwork
StateAction
(NC,LR) Multiplicatively increase fAchieve required reliability ASAP
OOR Stay
(NC,HR) Decrease f conservativelyCautiously reduce energy consumption while not compromising reliability
(C,HR) Decrease f carefully but aggressively to (NC,HR) to relieve congestionThen, follow (NC,HR) behavior
(C,LR) Decrease f exponentially to relieve congestion ASAP
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ESRT State DiagramNot all transitions are possible
(e.g. From (C,HR), ESRT cannot transition to (NC,LR))
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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 cause
s longer convergence time
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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 )
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From (NC,LR)
Reaches OOR in two intervals
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From (NC,HR)
ESRT stays in (NC,HR) until
reaching OOR in five intervals
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(C,HR) to (NC,HR) then OOR
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(C,LR) to (NC,LR) then OOR
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Power savings from (NC,HR)
Reporting rate gets reduced conservatively
while maintaining reliability
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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
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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?
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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,