Differentiated Surveillance for Sensor Networks

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Differentiated Surveillance for Sensor Networks

• Ting Yan, Tian He, John A. Stankovic

CS294-1 Jonathan Hui November 20, 2003
2
Idea

• Exploit node density/redundancy to maximize
  effective network lifetime.
• Degree of coverage matters!
• Sensing constraints
• Fault tolerance

3
Assumptions

• Static placement
• Localization
• Time Synchronization
• Uniform expected node lifetime
• For simplicity of describing protocol?
• Nodes on 2D plane
• Circular sensing radius r
• Communication range gt 2r

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Basic Protocol

• Initialization Phase
• Localization, Time Sync, Determine Working
  Schedule (T, Ref, Tfront, Tend)
• Sensing Phase
• Nodes power on and off based on working schedule

Round 0 (T)
Round 1 (T)
Round i (T)
Ref
Ref
Ref
t
Tfront
Tend
Tfront
Tend
Tfront
Tend
Sensing Phase
Init Phase
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Basic ProtocolDetermining Working Schedule

• Goal Each node determines its own working
  schedule such that all points within sensor
  coverage are covered for all time.
• Approach Represent sensor coverage with grid of
  points

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Basic ProtocolDetermining Working Schedule

• Reference Point Scheduling Algorithm
• Randomly choose Ref from 0, T) and broadcast to
  all nodes within 2r.
• For each discrete point
• Order neighboring Ref times and calculate
• Tfront Ref(i)-Ref(i-1)/2
• Tend Ref(i1)-Ref(i)/2
• Final schedule union of schedules for all points

RefA
RefB
RefC
Point 1
RefA
RefB
RefC
Node A
RefA
RefD
RefE
RefD
RefE
Point 2
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Enhanced Protocolwith Differentiation

• Working schedule for a desired coverage of degree
  a.
• (T, Ref, Tfront, Tend, a)
• Working period defined as
• Power On
• Power Off

Example (a 1)
RefB
RefC
RefA
A
B
C
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Design Issues

• Possible blind spots with discrete points
• Choose points within sensing range conservatively
• Offset in time synchronization
• Power on (off) slightly earlier (later)
• Irregular sensing regions
• Okay, as long as sensing regions of neighboring
  nodes are known
• But also requires knowledge of orientation
• Fault Tolerance
• Awake nodes use heartbeat messages to detect
  failed nodes
• If a node fails, wakeup all nodes within 2r and
  reschedule.
• What if communication range lt 2r?

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Extensions and Optimizations

• Second Pass Optimization
• After determining working schedule, broadcast
  schedule to all nodes within 2r.
• The node which has the longest schedule
• Minimize Tfront and Tend while maintaining
  sensing guarantee
• Rebroadcasts new schedule
• Done when every node has recalculated schedule or
  when no more can be done.

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Extensions and Optimizations

• Multi-Round Extension for Energy Balance
• Calculate M schedules each with different Ref
  values during Init Phase.
• Rotate schedules during Sensing Phase.

RefB
RefA
RefA
RefB
RefC
RefC
RefB
RefA
RefC
RefA
RefB
RefC
A
B
C
Schedule 1
Schedule 2
Schedule 3
Schedule 4
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Evaluation

• Simulation parameters
• Nodes distributed randomly with uniform
  distribution in 160mX160m field.
• Results taken from center 140mX140m to avoid edge
  effects
• Sensing range 10m
• Communication range 25m
• Ideal conditions
• Fault tolerance included?
• Compare against sponsored approach

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Evaluation

• Total energy consumption nearly constant with
  changes in density.

• Variation in total energy consumed decreases with
  greater densities.
• Whats happening with the sponsored approach?

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Evaluation

• Half-life increases linearly as density increases.

• Coverage provided for longer period of time.

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Evaluation

• Energy consumption increases linearly with
  different degrees.
• Energy consumption constant with different
  densities.

• Degree of coverage provided gt a.
• a only guarantees a lower bound.

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Comments

• Localized algorithm?
• But still requires time synchronization and
  doesnt support mobility
• Inflexible
• mobility not supported, schedules are fixed
• No notion of the goodness of a node
• Nodes that have more energy should take up a
  larger portion of the working schedule
• Difficult to reliably broadcast Ref values to all
  2r neighbors in a dense network
• Only have one chance to get it right!
• Worse in cases where communication range lt 2r
  (i.e. acoustic sensors)

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Comments

• Working schedules determined without taking other
  schedules and protocols into account
• How does it affect other protocols (i.e. TDMA)?
• Comparison to Sponsored Coverage unfair
• Sponsored Coverage supports fault tolerance,
  limited mobility, and is more adaptable
• Ability to specify degree of coverage
• But current algorithm doesnt correctly guarantee
  with a gt 2!
• Fault tolerance relies on communication range gt
  2r for heartbeat messages

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Conclusion

• Pros
• Improved performance in lifetime and workload
  balance
• Specify a degree of coverage
• Cons
• No upper bound on degree estimation
• Inflexible
• Static working schedule, static nodes, time
  synchronization, reliable communication