ESRT: EventtoSink Reliable Transport in Wireless Sensor Networks

1
ESRT Event-to-Sink Reliable Transport in
Wireless Sensor Networks

• Y. Sankarasubramaniam, O. B. Zkan, I. F.
  Akyildiz,
• ACM MobiHoc 2003
• Presented by Yuyan Xue

2
Outline

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

3

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

4
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 can not sense the event
5
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 control

6
Basic Idea

• Design Idea
• Control method only care about event
  notification, but not individual data packets gt
• Define reliability with respect to number of data
  packets per event by the sink
• Control the transmission frequency at which
  sensors send packets
• Congestion detection

7
Controlling the frequency

• If receiving more packets than needed
• Have sensors reduce frequency
• Reduces probability of congestion
• Saves transmission energy in the network
• If receiving too few packets
• Have sensors increase sending frequency
• Unless there is congestion

8
Retransmissions?

• No need
• Individual packets are not important
• Only event notification
• Might be stale anyway
• Old sensor data possibly not useful
• Increases congestion
• If losses due to congestion, retransmission wont
  help

9
Congestion Control

• Sensor networks are usually idle
• Until an event occurs
• High probability of channel overload
• Information must reach users
• Solution congestion control

10

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

11
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.

12
ESRTs Definition of Reliability

• Reliability is measured by whether the sink can
  receive enough packets to make a decision on
  happening event.
• ESRT protocol uses periodical centralized control
  method, therefore the reliability can also be
  measured in terms of the number of packets per
  decision interval received by the sink.
• If omit other influences on traffic, such as node
  failure or link error, reliability can be
  controlled by regulating the reporting frequency
  of nodes.
• Observed reliability number of received data
  packets in decision interval at the sink.
• Desired reliability number of packets required
  for reliable event detection.
• Normalized reliability observed/desired.

13
Evaluation Environment (cont.)

• In order to solve this problem, we have to
  observe the relationship the observed reliability
  at the sink and the reporting frequency f of
  sensor nodes within event radius.
• NS-2 simulation basic parameters
• Area of sensor field 100x100 m2
• Number of sensor nodes 200
• Radio range of a sensor node 40 m
• Packet length 30 bytes
• IFQ length 65 packets
• Transmit Power 0.660 W
• Receive Power 0.395 W
• Decision interval ( t) 10 sec

14
Evaluation Environment

• Features
• Mirror typical value of Mica Mote
• Event center were randomly chosen
• All nodes within event radius behave as source
  nodes to the sink
• Employed CSMA/CA based MAC protocol and Dynamic
  Source Routing (assume ESRT performance are
  insensitive to underlying routing protocol)

15
Typical Behavior at a Sink
Network gets congested sooner with continuous
increasing of f
Congestion Reliability level is always lower
than the peak point
16
ESRT Overview

• Main goal Adjust reporting rate of sources to
  achieve optimal reliability requirements
• Places interest on events, not individual pieces
  of data
• Application-driven
• Application defines desired event reporting rate
• Includes a congestion-control element
• Runs mainly on the sink

17
Components of ESRT

• In sink
• Normalized reliability computation
• Broadcast the control signal to all source node
  to maintain the network staying in an optimal
  state
• In source
• Listen to sink broadcast
• Overhead free local congestion detection mechanism

18
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 ( traffic increment does not change much)
• Check the buffer fullness level at the end of
  each reporting interval (bk ?b gt B)
• If pending congestion is detected, CN (Congestion
  Notification) bit is set in event reports

19

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

20
Five characteristic regions (Cont.)
fmax
Congestion
No Congestion
Higher reliability than required
Goal To stay in OOR where energy expenditure is
optimal
Lower reliability than required
OOR
21
Five characteristic regions

• ? normalized reliability indicator
• (NC,LR) No congestion, Low reliability
• f lt fmax, ? lt 1-e
• (NC, HR) No congestion, High reliability
• f lt fmax, ? lt 1e
• OOR Optimal Operating Region
• f lt fmax, 1-e lt ? lt 1e
• (C, HR) Congestion, High reliability
• f gt fmax, ? gt 1
• (C, LR) Congestion, Low reliability
• f lt fmax, ? lt 1

22
Algorithm for ESRT (Cont.)

• If no congestion and low reliability increase
  reporting frequency aggressively ( fi1 fi / ?i
  )
• If no congestion and high reliability decrease
  reporting frequency cautiously (half the slope
  fi1 fi (?i 1) / 2?i )
• If optimal operating range reporting frequency
  left unchanged. Stay within e tolerance of
  optimal point.

23
Algorithm for ESRT

• If congestion and high reliability decrease
  reporting frequency to relieve congestion. No
  compromise on reliability (multiplicative
  decrease fi1 fi / ?i )
• If congestion and low reliability decrease
  reporting frequency aggressively. (exponential
  decrease fi1 fi ?i /k)

24
Stability of ESRT (cont.)

• ESRT converges to OOR from any of four initial
  states (NC,LR), (NC,HR), (C,HR), (C,LR)
• ESRT is self-configuring in this sense and can
  hence perform efficiently under random, dynamic
  topology.

25
Stability of ESRT

• Starting from no congestion, high reliability,
  and with linear reliability behavior when the
  network is not congested, the network state
  remains unchanged until ESRT converges
• Starting from no congestion, high reliability,
  and with linear reliability behavior when the
  network is congested, ESRT converges to optimum
  operating range in tlog2((?-1)/?)
• With linear reliability behavior when the network
  is not congested, the network state transition
  from congestion, high reliability to no
  congestion, low reliability is impossible.

26

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

27
Simulation Setup

• Ns-2 simulator
• 81 senders
• 100m x 100m area
• 40m event radius
• 30 byte packets
• ? 5
• 10 sec decision interval (t)

28
Performance Results (based on simulations)
please refer to the paper for graphs .. They may
not be legible here

• Starting with no congestion and low reliability
• Convergence is attained in a total of two
  decision intervals.

Reaches OOR in two intervals
29
Performance Results contd (based on
simulations)

• Starting with no congestion and high reliability

ESRT stays in (NC,HR) until reaching OOR in five
intervals
30
Performance Results contd (based on
simulations) please refer to the paper for
graphs

• Starting with congestion and high reliability

ESRT directly transits from (C,HR) to (NC,HR),
and then stays in (NC,HR) until converges at OOR
in five intervals
31
Performance Results contd (based on
simulations) please refer to the paper for
graphs

• Starting with congestion and low reliability

ESRT transits from (C,LR) to (NC,HR), and then
stays in (NC,HR) until converges at OOR in two
intervals. Prove the stability of ESRT protocol
start from (C,LR)
32
Performance Results contd (based on
simulations) please refer to the paper for
graphs

• Average power consumption while starting with no
  congestion and high reliability

ESRT stays in (NC,HR) until reaching OOR in five
intervals
33

• The reliable transport problem in WSN
• ESRT Overview
• ESRT Transport Protocol
• ESRT Performance
• Conclusion and Challenges

34
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

35
Challenges with ESRT (contd)

• How to solve multiple concurrent events?
• Is it a an appropriate 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?

36
Challenges with ESRT

• Is it reasonable assumption that a sink should
  broadcast to all the source nodes ?
• Sink must broadcast the updated reporting
  frequency at high energy so that all sources can
  hear it
• Ongoing event transmission would be disrupted
• Regulating all sensors to have the same reporting
  rate may not work well with heterogeneous sensors
• Assuming that sensors report periodically may not
  be true for all applications
• Congestion in WSN not just caused by frequent
  sensor reporting

37
Thank youQuestions!