Energy-Conserving Coverage Configuration for Dependable Wireless Sensor Networks

The Chinese Univ. of Hong Kong

Energy-Conserving Coverage Configuration for Dependable Wireless Sensor Networks

Chen XinyuTerm Presentation

2004-12-14

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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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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Applications

Military reconnaissance Physical security Environment monitoring Traffic surveillance Industrial and

manufacturing automation

Distributed robotics …

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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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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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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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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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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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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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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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Special Cases of SSR and SSA

d(Ni, Nj) ≥ ri + rj

Ni

Nj

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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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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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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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Derivation of MPAC ij

0 2ij

jljm

ij = 2ij = 1

Covered Sensing Angle (CSG)

Sponsored Sensing Angle (SSG) ij

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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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Extended Sleeping Candidate Condition

Constrained deployed area

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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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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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An Example of Sleeping Eligibility Evaluation

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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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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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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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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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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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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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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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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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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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NSFS-Based Pessimistic Sleeping Candidate Condition

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NSFS-Based Optimistic Sleeping Candidate Condition

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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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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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Bridge between BSM and GSM

Ensured-sensibility radius

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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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Performance Evaluation (1)

Sleeping sensor vs. communication radius

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Performance Evaluation (2)

Network topology

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Performance Evaluation (3)

Sleeping sensor vs. sensor number

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Performance Evaluation (4)

Sleeping sensor vs. sensibility threshold

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Performance Evaluation (5) Network lifetime vs. live sensor when the

MTBF is 800s, R is 12m

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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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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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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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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