ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks[Sankarasubramaniam et. al, ACM MobiHoc 2003]

1
ESRT Event-to-Sink Reliable Transport in
Wireless Sensor NetworksSankarasubramaniam et.
al, ACM MobiHoc 2003

• Prepared by A. ABOUZEID

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
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

4
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

5
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
6
Five characteristic regions
Congested
Not Congested
Higher reliability than required
Lower reliability than required
Goal To stay in OOR where energy expenditure is
optimal
OOR
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

8
ESRT Actions
Network State Action
(NC,LR) Multiplicatively increase f Achieve required reliability ASAP
OOR Stay
(NC,HR) Decrease f conservatively Cautiously reduce energy consumption while not compromising reliability
(C,HR) Decrease f carefully but aggressively to (NC,HR) to relieve congestion Then, follow (NC,HR) behavior
(C,LR) Decrease f exponentially to relieve congestion ASAP
9
ESRT State Diagram
Not all transitions are possible (e.g. From
(C,HR), ESRT cannot transition to (NC,LR))
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 e smaller e causes
  longer convergence time

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 (t)

12
From (NC,LR)
Reaches OOR in two intervals
13
From (NC,HR)
ESRT stays in (NC,HR) until reaching OOR in five
intervals
14
(C,HR) to (NC,HR) then OOR
15
(C,LR) to (NC,LR) then OOR
16
Power savings from (NC,HR)
Reporting rate gets reduced conservatively while
maintaining reliability
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

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?

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,