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The Chinese Univ. of Hong Kong
Energy-Conserving Coverage Configuration for Dependable Wireless Sensor Networks
Chen XinyuTerm Presentation
2004-12-14
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Dept. of Computer Science and Engineering
Outline
MotivationCoverage configuration with Boolean
sensing modelCoverage configuration with general
sensing modelPerformance evaluations with ns-2Conclusions and future work
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Dept. of Computer Science and Engineering
Wireless Sensor Networks
Composed of a large number of sensor nodes
Sensors communicate with each other through short-range radio transmission
Sensors react to environmental events and relay collected data through the dynamically formed network
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Dept. of Computer Science and Engineering
Applications
Military reconnaissance Physical security Environment monitoring Traffic surveillance Industrial and
manufacturing automation
Distributed robotics …
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Dept. of Computer Science and Engineering
Requirements
Maintaining coverageEvery point in the region of interest should
be sensed within given parametersExtending system lifetime
The energy source is usually battery powerBattery recharging or replacement is
undesirable or impossible due to the unattended nature of sensors and hostile sensing environments
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Dept. of Computer Science and Engineering
Requirements (cont’d)
Fault toleranceSensors may fail or be blocked due to physical
damage or environmental interferenceScalability
High density of deployed nodesEach sensor must configure its own
operational mode adaptively based on local information, not on global information
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Dept. of Computer Science and Engineering
Approach: Coverage Configuration
Coverage configuration is a promising way to extend network lifetime by alternately activating only a subset of sensors and scheduling others to sleep according to some heuristic schemes while providing sufficient coverage in a geographic region
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Dept. of Computer Science and Engineering
Concerns
A good coverage-preserved and fault-tolerant sensor configuration protocol should have the following characteristics: It should allow as many nodes as possible to turn
their radio transceivers and sensing functionalities off to reduce energy consumption, thus extending network lifetime
Enough nodes must stay awake to form a connected network backbone and to preserve area coverage
Void areas produced by sensor failures and energy depletions should be recovered as soon as possible
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Dept. of Computer Science and Engineering
Two Sensing Models
Boolean sensing model (BSM)Each sensor has a certain sensing range,
and can only detect the occurrences of events within its sensing range
General sensing model (GSM)Capture the fact that signals emitted by a
target of interest decay over the distance of propagation
Exploit the collaboration between adjacent sensors
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Dept. of Computer Science and Engineering
Problem Formulation for the BSM
Each sensor node Ni knows its location (xi, yi), sensing radius ri, communication radius R
Sensors are deployed in a two-dimensional Euclidean plane
Responsible Sensing Region (RSR) i = { p | d(Ni,p) < ri }
A point is covered by a sensor node when this point is in the sensor's RSR
The one-hop neighbor set of Ni
N(i) = { Nj | d(Ni, Nj) ≤ R, j i }
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Dept. of Computer Science and Engineering
Some Definitions
Ni
Nj
Sponsored Sensing Arc (SSA) ij
Sponsored Sensing Region (SSR)
Sponsored Sensing Angle (SSG) ij
Covered Sensing Angle (CSG) ij
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Dept. of Computer Science and Engineering
Special Cases of SSR and SSA
d(Ni, Nj) ≥ ri + rj
Ni
Nj
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Dept. of Computer Science and Engineering
Special Cases of SSR and SSA
d(Ni, Nj) ≤ ri – rj
Ni
Nj
SSG ij =2CSG ij is not defined
Completely Covered Node (CCN) of Ni
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Dept. of Computer Science and Engineering
Special Cases of SSR and SSA
d(Ni, Nj) ≤ rj - ri
Ni
Nj
Complete-Coverage Sponsor (CCS) of Ni
Degree of Complete Coverage DCC i = | CCS(i) |
SSG ij is not defined
CSG ij =2
CCS(i)
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Dept. of Computer Science and Engineering
Minimum Partial Arc-Coverage (MPAC)
The minimum partial arc-coverage (MPAC) sponsored by node Nj to node Ni, denoted as ij, The number of Ni's non-CCSs covering the
point on the SSA ij that has the fewest nodes covering it.
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Dept. of Computer Science and Engineering
Derivation of MPAC ij
0 2ij
jljm
ij = 2ij = 1
Covered Sensing Angle (CSG)
Sponsored Sensing Angle (SSG) ij
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Dept. of Computer Science and Engineering
MPAC and DCC Based k-Coverage Sleeping Candidate Condition
K-coverageEvery point in the deployed area is
covered by at least k nodesTheorem
A sensor node Ni is a sleeping candidate while preserving k-coverage, iff i ≥ k or Nj N(i) - CCS(i), ij > k - i .
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Dept. of Computer Science and Engineering
Extended Sleeping Candidate Condition
Constrained deployed area
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Dept. of Computer Science and Engineering
Node Scheduling Protocols
Round-based Divide the time into roundsApproximately synchronized In each round, every live sensor is given a
chance to be sleeping eligibleAdaptive sleeping
Let each node calculate its sleeping time locally and adaptively
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Dept. of Computer Science and Engineering
Round-Based Node Scheduling Protocol
onsleeping
ready-to-sleeping
ready-to-on
uncertain
Tround
eligible / STATUS
ineligible
Tround
TwaitTwait
eligible / STATUS
ineligible / STATUS
on-sleeping decision phase1. Set a backoff timer Thello, a window timer Twin,
a wait timer Twait, and a round timer Tround
2. Collect HELLO messages from neighbors3. After Thello times out, broadcast a HELLO
message to all neighbors4. After Twin expires, evaluate the sleeping
eligibility according to sleeping candidate conditions
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Dept. of Computer Science and Engineering
An Example of Sleeping Eligibility Evaluation
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Dept. of Computer Science and Engineering
Connectivity Requirement
Considering only the coverage issue may produce disconnected subnetworks
Simple connectivity preservation If a sensor is sleeping eligible, evaluating
whether its one-hop neighbors will remain connected through each other when the considered sensor is removed
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Dept. of Computer Science and Engineering
Adaptive Sleeping Node Scheduling Protocol
A node may suffer failures or deplete its energy loss of area coverage
Round-based: timer Tround is a global parameter and not adaptive to recover a local area loss
Letting each node calculate its sleeping time locally and adaptively
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Dept. of Computer Science and Engineering
Adaptive Sleeping Node Scheduling Protocol
1. Set a timer Tsleeping 2. When Tsleeping times out, broadcast a PROBE
message3. Each neighbor receiving the PROBE message will
return a STATUS message to the sender4. Evaluate sleeping eligibility. If eligible, set Tsleeping
according to the energy information collected from neighbors
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Dept. of Computer Science and Engineering
Discussions for the BSM
Each sensor has a deterministic sensing radius
Allow a geometric treatment of the coverage problem
Miss the attenuation behavior of signalsIgnore the collaboration between
adjacent sensors in performing area sensing and monitoring
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Dept. of Computer Science and Engineering
Problem Formulation for the GSM
The sensibility of a sensor Ni for an event occurring at an arbitrary measuring point p is defined by
: the energy emitted by events occurring at point p
: the decaying factor of the sensing signal
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Dept. of Computer Science and Engineering
All-Sensor Field Sensibility (ASFS)
Suppose we have a “background” distribution of n sensors, denoted by N1, N2, …, Nn, in a deployment region A
All-Sensor Field Sensibility for point p
With a sensibility threshold , the point p is covered if Sa(p) ≥
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Dept. of Computer Science and Engineering
Discussions for the ASFS
Need a sink working as a data fusion center
Produce a heavy network load in multi-hop sensor networks
Pose a single point of failures
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Dept. of Computer Science and Engineering
Neighboring-Sensor Field Sensibility (NSFS)
Treat each sensor as a sensing fusion center Each sensor broadcasts its perceived field
sensibility Each sensor collects its one-hop neighbors’
messages
Transform the original global coverage decision problem into a local problem
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Dept. of Computer Science and Engineering
Responsible Sensing Region
Voronoi diagramPartition the deployed region into a set of
convex polygons such that all points inside a polygon are closet to only one particular node
The polygon in which sensor Ni resides is its Responsible Sensing Region i
If an event occurs in i, sensor Ni will receive the strongest signal
Open RSR and closed RSR
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Dept. of Computer Science and Engineering
NSFS-Based Pessimistic Sleeping Candidate Condition
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Dept. of Computer Science and Engineering
NSFS-Based Optimistic Sleeping Candidate Condition
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Dept. of Computer Science and Engineering
Sensibility-Based Sleeping Configuration Protocol (SSCP)
onsleeping
ready-to-sleeping
ready-to-on
Tround
eligible / STATUS
ineligible
Tround
TwaitTwait
eligible / STATUS
ineligible / STATUS
uncertain II
uncertain I
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Dept. of Computer Science and Engineering
Performance Evaluation with ns-2
ESS: extended sponsored sector Proposed by Tian et. al. of Univ. of Ottawa, 2002 Consider only the nodes inside the RSR of the evaluated
node Mpac: round-based protocol with elementary MPAC
condition MpacB: round-based protocol with extended MPAC
condition in constrained area MpacBAs: adaptive sleeping protocol with MpacB SscpP: Sscp with the pessimistic sleeping condition SscpO: Sscp with the optimistic sleeping condition
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Dept. of Computer Science and Engineering
Bridge between BSM and GSM
Ensured-sensibility radius
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Dept. of Computer Science and Engineering
Default Parameters Setting
The deployed area is 50m x 50m = 1, = 3, = 0.001 (r = 10m)R = 12 m The number of deployed sensor: 120Power Consumption:
Tx (transmit) = 1.4W, Rx (receive) = 1W, Idle = 0.83W, Sleeping = 0.13W
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Dept. of Computer Science and Engineering
Performance Evaluation (1)
Sleeping sensor vs. communication radius
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Dept. of Computer Science and Engineering
Performance Evaluation (2)
Network topology
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Dept. of Computer Science and Engineering
Performance Evaluation (3)
Sleeping sensor vs. sensor number
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Dept. of Computer Science and Engineering
Performance Evaluation (4)
Sleeping sensor vs. sensibility threshold
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Dept. of Computer Science and Engineering
Performance Evaluation (5) Network lifetime vs. live sensor when the
MTBF is 800s, R is 12m
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Dept. of Computer Science and Engineering
Performance Evaluation (6)
-coverage accumulated time•The total time during which or more percentage of the deployed area satisfies the coverage requirement
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Dept. of Computer Science and Engineering
Approaches to Build Dependable Wireless Sensor Networks
Decreasing the communication radius or increasing the coverage degree is equivalent in providing fault tolerance
Detecting sensor failures and recovering the area loss as quick as possible: adaptive sleeping configuration
Exploiting the cooperation between neighboring sensors: general sensing model
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Dept. of Computer Science and Engineering
Conclusions
Develop MPAC-based node sleeping eligibility conditions for the BSMachieve k-coverage degreecan be applied with different sensing radii
Develop SSCPs for the GSMexploit the cooperation between adjacent
sensorsSuggest three effective approaches to
build dependable sensor networks
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Dept. of Computer Science and Engineering
Future Work
Exploit algorithms to identify node redundancy without location information
Study the network behavior with node failures
Build dependable sensor networks both on area coverage and network connectivity