Sensing Coverage for Surveillance in Wireless
Sensor Networks
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Contents
D. Tian and N. D. Georganas, A Coverage-Preserving
Node Scheduling Scheme for Large Wireless Sensor
Networks, Proc. ACM Workshop on Wireless Sensor
Networks
T. Yan, T. He, J. A. Stankovic, Differentiated
Surveillance for Sensor Networks, ACM SenSys'03,
Los Angeles, CA, November 2003
X. Wang, G. Xing, Y. Zhang, C. Lu, R. Pless, and C.
Gill, Integrated Coverage and Connectivity
Configuration in Wireless Sensor Networks, ACM
SenSys'03, Los Angeles, CA, November 2003
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A Coverage-Preserving Node Scheduling Scheme
for Large Wireless Sensor Networks
Proc. ACM Workshop on Wireless Sensor
Networks
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Sponsored Coverage Algorithm
Self-scheduling
Each node advertises its position and maintains a neighbor
table.
Determines work status by calculating the sponsored coverage
by its neighbors.
Back-off scheme to avoid collision and redundancy.
Sensing phase
Non-eligible nodes do the sensing, collecting and delivering
data to sink nodes.
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Differential Surveillance for Sensor Networks
ACM SenSys'03, Los Angeles, CA,
November 2003
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Overview
Exploit node density/redundancy to maximize effective
network lifetime.
Degree of coverage matters!
Sensing constraints
Fault tolerance
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Basic Idea: Integrating Point Coverage
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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Assumptions
Assumptions
Localization, nodes are immobile.
Sensing area.
Neighboring nodes are time-synchronized.
Rc > 2r.
• Rc: Communication radius
• r: Sensing radius
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Goal of the basic design?
Goals
Reduce total energy consumed and energy imbalance.
Reduce communication overhead.
Support flexible degrees of coverage – Differentiation.
Method: To maximize the number of sleeping nodes, lifetime
of the network.
Constraint: To guarantee 100% sensing coverage.
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Node’s working schedule
Initialization phase
Localization.
Time synchronization.
Sensing phase
Divided into rounds of equal duration.
Each node establishes a working schedule.
Sensing schedules for all grid points integrated.
Enhancements done to achieve optimization.
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Node Working Schedule (1/2)
A Schedule is comprised of the following four parameters:
T – round duration, constant for all nodes.
Ref – random time reference point from [0, T).
Tfront – time duration prior to Ref.
Tend – time duration after Ref.
Conditions
The chosen parameters do not change throughout the sensing
phase.
• Exception: Rescheduling for fault-tolerance purposes.
Tfront and Tend should be [0, T).
• Tfront + Tend < T.
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Node Working Schedule (2/2)
Round 0
(Duration T)
Round 1
(Duration T)
Ref
Ref
T front Tend
Init Phase
Round k
(Duration T)
...
T front Tend
Ref
T front Tend
Sensing Phase
A node wakes up at time (T*I + Ref - Tfront).
A node goes to sleep at time (T*I + Ref + Tend).
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Setting Tuple Parameters (T, Ref, Tfront,Tend)
r
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Integrating the schedules
Union of the schedules for all the grid points in each sensor.
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Rationale for Schedule Integration
A node usually has the same schedule for grid points
near to each other.
Normally, the integration will not increase the length
of integrated Tfront and Tend when the union operation
takes place.
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Advantages
Communication energy is minimized.
Energy consumed for communication purpose is less.
Random time reference point helps in balancing the
energy consumption across nodes.
Total energy consumption increases slowly with
increasing node density.
This in turn increases the system lifetime.
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Discussion
Given the basic design, what are the
practical issues need to be addressed?
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Design Issues for Blind Points
Bind points in space due to the Grid size’s granularity.
Control the size of grid, or
Determine the Conservative sensing rage.
Blind points in time due to the Time synchronization skew.
Use the additional sensing time to fill up the possible time gap.
Blind points in space due to the Irregularity in sensing patterns.
Use the conservative sensing range considering the irregular sensing shape.
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Reschedule for Fault Tolerance
Problem and Idea
Problem: Sensors can die any time.
Idea: The working sensor broadcasts the Heartbeat signal or
Hello message.
Detection of Node Failure
Heartbeat timeout happens.
• The related node is regarded as failed node.
The detecting node wakes up the neighbors of the failed node
within 2r distance.
The neighbors re-compute their schedules to cover the area that
belonged to the failed node.
Disadvantage
Communication overhead increase due to heartbeat signals.
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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.
RefC RefA RefB
RefB
RefC RefA RefA RefB
RefCRefA
RefC
RefB
A
B
C
Schedule 1
Schedule 2
Schedule 3
Schedule 4
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Differentiation
Differentiation done to facilitate reconfiguration to different
coverage degrees after deployment.
Extend or shrink Tfront and Tend proportional to the desired degree of
coverage α.
Minimum bound for coverage area for each grid point is α.
No rescheduling required for differentiation.
Disseminating the new α enough to achieve a different desired
coverage degree.
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Conclusions
The proposed scheme is advantageous in following ways:
Reduced communication overhead.
Lesser energy consumption.
Lesser energy variation-increased lifetime.
Optimizations improve performance even more.
Differentiation facilitates flexible degrees of coverage and
hence increases the range of applications.
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Integrated Coverage and Connectivity Configuration
in Wireless Sensor Networks
ACM SenSys'03, Los Angeles, CA, November 2003
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Outline
Motivation
Coverage vs. Connectivity: Geometric Analysis
Coverage Configuration Protocol (CCP)
CCP + SPAN
Simulation results
Conclusion
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Motivation
Challenge in sensor networks
Achieve long life time on limited energy
Approach:
Schedule unnecessary nodes to sleep
Use active nodes provide “sufficient” service
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“Sufficient” Service
Sensing
N-Coverage: every point in a region is covered (monitored)
by at least N sensors
Different applications require different degree of coverage!
• e.g., distributed detection vs. object tracking
Communication
K-Connectivity: communication graph is connected if (K-1)
nodes fail
Other metrics: throughput, delay
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Goals
Design an energy conservation protocol that guarantees
desired coverage and connectivity.
Requirements
Integrated: Must guarantee both coverage and connectivity
Flexible: can re-configure the network to different coverage
degrees and connectivity
• Meet diverse application requirements
Decentralized: achieve scalability
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Limitations of Existing Protocols
Treat connectivity and coverage in isolation
Connectivity only: ASCENT, SPAN, AFECA, GAF, …
Coverage only: exposure, Ottawa’s protocol, …
Density: PEAS
Lack flexibility: only provide fixed degree of coverage
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Assumptions
The region to be covered is convex
Disc models for coverage and communication
A point p is covered by a node v if |pv| < Rs
• Rs: Sensing range
Nodes u and v are connected if |uv| < Rc
• Rc: Communication range
Intuition: Rc/Rs is important!
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Connectivity  Coverage?
A connected network does not guarantee coverage
Connectivity does not require “connection” with a location
where there is no node
Coverage must cover all locations in a region
This result holds for any Rc/Rs
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Coverage  Connectivity
If Rc/Rs  2 A covered network is always
connected
A K-covered network is also K-connected
A K-covered network has an interior connectivity of 2K
Interior node: a node whose sensing circle locates inside the coverage region
Interior connectivity: the minimum number of nodes that need to be removed in
order to disconnect two interior nodes
Interior connectivity is important because interior nodes carry more communication
load
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Implication of Geometric Analysis
Given a required coverage degree of Ks, and a
required connectivity of Kc
If Rc  2Rs, the protocol only needs to guarantee
max(Ks. Kc) coverage configuration
Solution: Coverage Configuration Protocol (CCP)
If Rc < 2Rs, the protocol must address both coverage
and connectivity.
Solution: CCP + SPAN
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A Sufficient Condition for K-Coverage
A convex region B is K-covered if all the intersection points
among sensing circles and/or B’s boundary inside B are Kcovered
Implication: a coverage configuration protocol only needs to
worry about intersection points!
Intuition: All points in a
same “patch” surrounded
by sensing circles share
the same coverage degree
S
p
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K-Coverage Eligibility Rule
A node is eligible to become active iff there exists an
intersection point inside its sensing circle that is not
K-covered
To evaluate eligibility, a node only needs to know the
locations of active nodes within 2Rs
on?
Active nodes
Sleeping nodes
Intersection point
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Coverage Configuration Protocol
Sleeping node periodically wake up
Listen to neighbors’ location beacons and announcements
If eligible:
• Set a random timer
• Turn active and broadcast JOIN if it is still eligible when
timer expires
Active node
Listen to neighbors’ location beacons and announcements
If ineligible:
• Set a random timer
• Broadcast WITHDRAW and sleep if still ineligible when
timer expires
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Simulation: Coverage Configurability
Min-500,700,900
Average-500
Achieved Coverage degree
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Average-700
Average-900
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6
4
2
0
0
1
2
3
4
5
6
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Required Coverage degree
CCP strictly enforces desired coverage degrees!
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CCP + SPAN
When Rc < 2Rs, CCP cannot guarantee connectivity.
Solution: integrate CCP with SPAN (a connectivity
maintenance protocol from MIT)
Combined eligibility rules
A sleeping node is eligible if it satisfies is eligible under SPAN OR CCP
An active node becomes ineligible if it is ineligible under both SPAN
AND CCP
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Simulation:
Coverage+Connectivity (Rc = 1.5Rs)
Combination of SPAN & CCP is necessary for desired coverage and
connectivity when Rc < 2Rs
SPAN
CCP
SPAN+CCP
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Conclusion
Geometric analysis on relationship between coverage
and connectivity
Only need to worry about coverage when Rc  2Rs
Must worry about both when Rc < 2Rs
Coverage Configuration Protocol: configure network
to any (feasible) degree of coverage requested by the
application
Integrate of CCP with SPAN for networks with Rc <
2Rs
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Pros and Cons
Three papers
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Research problems
Event, Dimension, Space, Time, Requirement, Objective
0-D, 1-D, 2-D and 3 –D coverage
Full coverage in space and time
Partial Coverage in time
Partial Coverage in space
Guaranteed Detection Delay
Guaranteed Stealth Distance/Breach path/Breach Area
Static Event Coverage
Mobile Event Coverage
Everlasting, ephemeral events
Event Contour Mapping
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