The Chinese Univ. of Hong Kong Node Scheduling Schemes for Coverage Preservation and Fault Tolerance in Wireless Sensor Networks Chen Xinyu Group Meeting

The Chinese Univ. of Hong Kong

Node Scheduling Schemes for Coverage Preservation and Fault Tolerance in Wireless Sensor Networks

Chen XinyuGroup Meeting

2004-10-25

Dept. of Computer Science and Engineering

Outline

MotivationK-coverage sleeping candidate conditionNode scheduling schemes

Round-basedAdaptive sleeping

Performance evaluationsConclusions


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


Applications

Military reconnaissance Physical security Traffic surveillance Industrial and

manufacturing automation

Distributed robotics Environment monitoring …


Problems

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

Sensors may fail or be blocked due to physical damage or environmental interference


Concerns

A good coverage-preserved and fault-tolerant node scheduling scheme 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


Outline





Problem Formulation

Each sensor node Ni knows its location (xi, yi), sensing radius ri, communication radius R

Sensing region SRi = { p | dip < ri }

The neighbor set of Ni, N(i) = { Nj S | dij ≤ R, j i }

Assuming that Nj N(i), R ≥ ri + rj

Ensures that coverage implies connectivity


Some Definitions

Ni

Nj

Sponsored Sensing Arc (SSA) ij

Sponsored Sensing Region (SSR)

Sponsored Sensing Angle (SSG) ij

Covered Sensing Angle (CSG) ij


Special Cases of SSR and SSA

dij ≥ ri + rj

Ni

Nj



dij ≤ ri - rj

Ni

Nj

SSG ij =2CSG ij is not defined

Completely Covered Node (CCN) of Ni



dij ≤ 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)


Minimum Partial Arc-Coverage (MPAC)

The minimum partial arc-coverage (MPAC) sponsored by node Nj to node Ni, denoted as ij, the number of Nj's non-CCSs covering the

point on the SSA ij that has the fewest nodes covering it.


Derivation of MPAC ij

0 2ij

jljm

ij = 2ij = 1

Covered Sensing Angle (CSG)

Sponsored Sensing Angle (SSG) ij


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 .


Extended Sleeping Candidate Condition

Constrained deployed area


Outline





Round-based Node Scheduling Scheme

onsleeping

ready-to-sleeping

ready-to-on

uncertain

Tround

eligible / STATUSineligible

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

Approximately synchronized


An Example of Sleeping Eligibility Evaluation


Adaptive Sleeping Node Scheduling Scheme

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


Adaptive Sleeping Node Scheduling Scheme

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


Performance Evaluation

ESS: extended sponsored sector Proposed by Tian et. al. of Univ. of Ottawa, 2002 Consider only the nodes inside the SR of the

evaluated node Mpac: round-based scheme with elementary

MPAC condition MpacB: round-based scheme with extended

MPAC condition in constrained area MpacBAs: adaptive sleeping scheme with

MpacB


Performance Evaluation (1)

Sensor number vs. sensing radius



Standard deviation of sensing radius



Required coverage degree



Fault tolerance approachesAdaptive sleeping scheduling (k+1)-coverage scheduling

Provide one more coverage degree than the design requirement k

-coverage accumulated timeThe total time during which percentage of

the deployed area satisfies the coverage requirement





System lifetime vs. live sensor


Conclusions

Develop MPAC-based node sleeping eligibility conditionsachieve k-coverage degreecan be applied with different sensing radii

Propose two fault tolerant approaches:Adaptive sleeping scheduling (k+1)-coverage scheduling

Identify that a tradeoff exists between sensing coverage and network lifetime

Documents

The Chinese Univ. of Hong Kong Node Scheduling Schemes for Coverage Preservation and Fault Tolerance in Wireless Sensor Networks Chen Xinyu Group Meeting