Coverage Issues in WSNs

Presented by Ming-Tsung Hsu

Outline

Wireless Sensor Networks Coverage Issues

Wireless Sensor Networks

Faster, Smaller, Numerous

Moore’s Law “Stuff” (transistors, etc)

doubling every 1-2 years

Bell’s Law New computing class

every 10 years

year

log

(p

eo

ple

pe

r c

om

pu

ter)

Streaming Data to/from the

Physical World

Applications

Environmental Monitoring Habitat Monitoring Integrated Biology Structural Monitoring

Interactive and Control Pursuer-Evader Intrusion Detection Automation

Density & Scale

Sample Rate & Precision

MobilityLow Latency

Disconnection & Lifetime

Fundamental Functionalities

Data collection - Sensor subsystem Gathering information and controlling/monitoring

environments Data processing - Process subsystem

Performing local computations Data transmission - Communication

subsystem Exchanging data

Characteristics

A special wireless ad hoc network Large number of nodes are deployed randomly and densely

Scalability & Self-Configuration Battery powered

Energy Efficiency Topology and density change

Adaptivity Working for a common task

Data Centric In-network data processing (Data aggregation)

Message-level Latency

Sensor Deployment

How to deploy sensors over a field? Deterministic, planned deployment Random deployment

Desired properties of deployments? Depends on applications Connectivity Coverage

Sensor Network Formation

Deployed densely and randomly “Dense” means “exits redundant nodes”

Density control “Random” means “topology is indefinite”

Topology control

Self-Configuration & Self-Organization Scalability Energy

Node’s Operations

On-Duty (working) nodes Forming a sensor network Am I redundant ?

Off-duty? Energy Consideration

Role-change? Off-duty?

Off-Duty (sleeping) nodes When to wakeup? On-duty?

Duty cycle policy Scheduling vs. Adaptive Duty period

Coverage, Connectivity

Is every point covered by 1 or K sensors 1-covered, K-covered

Is the sensor network connected K-connected

Others

6

54

3

2

1

7

8 R

Coverage & Connectivity: not independent, not identical If region is continuous & Rt > 2Rs Region is covered sensors are connected

X. Wang (Sensys’03) H. Zhang & J. Hou (2004)

Rs

Rt

Real Products

Problem Tree for Coverage and Connectivity Problems

coverage connectivity

homo

probabilistic

per-node

k-connected

blanket

deployment

adaptive

algorithmic

homo

per-node (Max Rt)

deterministic

barrier

surveillance & exposure

networktopology formation

networkdensity control

adaptive

# of sensorsare needed?

scheduling

K-coverage K-connectivitytopology control

Scheduling ASCENT …

LEACH …

various connected subgraphs

Penrose …Xue&Kumar …

# of sensors?

PEAS …

OGDC …

Coverage Issues

Related Geometric Problems

Surveillance

(a)the Voronoi diagram and the maximal breach path(b)the Delaunay triangulation and the maximal support path

Exposure

2

1

1 2, , ,

t

t

dp tE p t t t I F p t dt

dt

The exposure for an object in the sensor field during the interval 1 2,t t

along a path p t

minimal exposure path the worst coverage of a sensor network

Simple Coverage Problem

Given an area and a sensor deployment Question: Is the entire area covered?

6

54

3

2

1

7

8 R

K-Coverage Problem

Given: region, sensor deployment, integer k Question: Is the entire region k-covered? C.-F. Huang & Y.-C Tseng (WSNA’03)

6

54

3

2

1

7

8 R

Is the perimeter k-covered?

Is a belt region k-barrier covered? Construct a graph G(V, E)

V: sensor nodes, plus two dummy nodes L, R E: edge (u,v) if their sensing disks overlap

Region is k-barrier covered iff L and R are k-connected in G

L R

Density Control

Given: an area and a sensor deployment Problem: turn on/off sensors to maximize the

sensor network’s life time

Density Control (cont’d)

Nodes are on-duty or off-duty by Scheduling or Probing Resulting monitoring area still covered

Sensing range Determined (disc) Irregular in shape, or even follow a probabilistic

model

Approaches for Density Control Adaptive

PEAS (ICNP’02 , ICDCS’03) CCP (SenSys’03)

Scheduling SET K-COVER (ICC’01) Co-Grid (IPSN’04) OGDC (International Workshop on Theoretical

and Algorithmic Aspects of Sensor, Ad hoc Wireless and Peer-to-Peer Networks, 2004)

PEAS and OGDC

PEAS: A robust energy conserving protocol for long-lived sensor networks Fan Ye, et al (UCLA), ICNP’02, ICDCS’03,

“Maintaining Sensing Coverage and Connectivity in Large Sensor Networks” H. Zhang and J. Hou (UIUC), International Workshop on

Theoretical and Algorithmic Aspects of Sensor, Ad Hoc Wireless, and Peer-to-Peer Networks (04), The Wireless Ad Hoc and Sensor Networks: An International Journal (05)

PEAS: basic ideas

Probing Environment and Adaptive Sleeping How often to wake up? How to determine whether to work or not?

Sleep Wake up Go to Work?

workyes

no

Wake-up rate?

How often to wake up?

Desired: the total wake-up rate around a node equals some given value

Inter Wake-up Time

f(t) = λ exp(- λt)

• exponential distribution• λ = average # of wake-ups per unit time

Wake-up rates

f(t) = λ exp(- λt)

f(t) = λ’ exp(- λ’t)

A

B

A + B: f(t) = (λ + λ’) exp(- (λ + λ’) t)

Adjust wake-up rates

Working node knows Desired wake-up rate λd

Measured wake-up rate (form working node) λm

Probing node adjusts its λ byλ := λ (λd / λm)

Go to work or return to sleep? Depends on whether there is a working node

nearby.

Go back to sleep go to work

Rp

Is the resulting network covered or connected?

If Rt ≥ (1 + √5) Rp and … then

P(connected) → 1

Simulation results show good coverage

Basic Idea of OGDC

OGDC: Optimal Geographical Density Control

Minimize the number of working nodes ↔ Minimize the total amount of overlap

Minimum overlap

RsD 3 distance Optimum

overlap minimize To

D

Rs

Minimum overlap (cont’d)

D

RsD 3 distance Optimum

overlap minimize To

Minimum overlap

Near-optimal

OGDC: the Protocol

Time is divided into rounds In each round, each node runs this protocol to

decide whether to be active or not Select a starting node. Turn it on and broadcast a power-

on message Select a node closest to the optimal position. Turn it on

and broadcast a power-on message. Repeat this.

Selecting starting nodes

Each node volunteers with a probability p. Backs off for a random amount of time. If hears nothing during the back-off time, then sends

a message carryingSender’s positionDesired direction

Select the next working node

On receiving a message from a starting node Each node computes its deviation D from the

optimal position. Sets a back-off timer inversely proportional to D.

On receiving a power-on message from a non-starting node

PEAS vs. OGDC

Complexity Coverage Time Sync

Blanket vs. Barrier Coverage

Blanket coverage Every point in the area is covered (or k-covered)

Barrier coverage Every crossing path is k-covered

Is a belt region k-barrier covered? Construct a graph G(V, E)

V: sensor nodes, plus two dummy nodes L, R E: edge (u,v) if their sensing disks overlap

Region is k-barrier covered iff L and R are k-connected in G.

L R

Donut-shaped region

K-barrier covered iff G has k essential cycles.

Thanks