The Chinese Univ. of Hong KongDept. of Computer Science & Engineering

A Sensibility-Based Sleeping Configuration Protocol for

Dependable Wireless Sensor Networks

Chen XinyuGroup Meeting

2005-01-28

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Outline

Introduction Neighboring-sensor field sensibility Sensibility-based sleeping configuration

protocol Performance evaluations Conclusions

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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 Environment

monitoring Military

reconnaissance Physical security Traffic surveillance Industrial and

manufacturing automation

Distributed robotics …

Ossama Younis and Sonia Fahmy: Distributed Clustering in Ad-hoc Sensor Networks: A Hybrid, Energy-Efficient Approach (InfoCom2004)

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Requirements

Maintaining coverage• Every point in the region of interest should be

sensed within given parameters

Extending system lifetime• The energy source is usually battery power

• Battery 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 tolerance• Sensors may fail or be blocked due to physical

damage or environmental interference• Produce some void areas which do not satisfy

the coverage requirement Scalability

• High density of deployed nodes• Each 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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Discussions for the BSM

Each sensor has a deterministic sensing radius

Allow a geometric treatment of the coverage problem

Miss the attenuation behavior of signals Ignore the collaboration between adjacent

sensors in performing area sensing and monitoring

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Problem Formulation for the GSM

Point Sensibility s(Ni, p): the sensibility of a sensor Ni for an event occurring at an arbitrary measuring point p

: the energy emitted by events occurring at point p : the decaying factor of the sensing signal d(Ni, p) : the distance between senosr Ni and point p

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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 only collects its one-hop neighbors’

messages

Transform the original global coverage decision problem into a local problem

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Responsible Sensing Region (RSR)

Voronoi diagram• Partition 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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Pessimistic Scan Region

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Connectivity Requirement

Considering only the coverage issue may produce disconnected subnetworks

Simple connectivity preservation

• Evaluating whether Ni’s one-hop neighbors will remain connected through each other or through its two-hop neighbors when Ni is removed

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Ni’s Sleeping Candidate Condition

: Responsible Sensing Region of Nj

: the two-hop confined region of Ni

: communication path between Nj and Nk

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Optimistic Scan Region

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uncertain I

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

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Performance Evaluation with ns-2

Boolean sensing model• 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

General sensing model• SscpP: SSCP with the pessimistic scan region

• SscpO: SSCP with the optimistic scan region

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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: 120 Power 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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Conclusions

Propose NSFS with the GSM• transform a global decision problem to a

local one

• exploit the cooperation between adjacent sensors

Develop SSCPs to build dependable wireless sensor networks

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Q & A