differentiated surveillance for sensor networks ting yan, tian he, john a. stankovic cs294-1...
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Differentiated Surveillance for Sensor Networks
Ting Yan, Tian He, John A. Stankovic
CS294-1Jonathan HuiNovember 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 > 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
t
Sensing PhaseInit Phase
Ref
Tfront Tend
Round 0 (T)Ref
Tfront Tend
Round 1 (T)Ref
Tfront Tend
Round i (T)
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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(i+1)-Ref(i)]/2
Final schedule = union of schedules for all points
RefA
RefA
RefBRefC
RefD RefE
Point 1
Point 2
RefA RefBRefC
RefD RefE
Node A:
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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:
aTRefiT front aTRefiT end
Example (a = 1)
A
B
C
RefA RefB RefC
Example (a = 2)
A
B
C
RefA RefB RefCExample (a = 3)
A
B
C
RefA RefB RefC
Uh-Oh!
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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 < 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.
A
B
C
RefA RefARefB RefBRefC RefCRefBRefARefC RefA RefB RefC
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.
What’s 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 >= a.
a only guarantees a lower bound.
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Comments Localized algorithm?
But still requires time synchronization and doesn’t 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 < 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 doesn’t correctly guarantee with a > 2!
Fault tolerance relies on communication range > 2r for heartbeat messages