Introduction to Sensor Networks

Rabie A. Ramadan, PhDCairo University

http://rabieramadan.org

rabie@rabieramadan.org

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Virtual grid architecture routing Utilizes data aggregation and in-network

processing to maximize the network lifetime

Inside each zone, a node is optimally selected to act as CH.

Data aggregation is performed at two levels:• Local: the set of CHs performing local

aggregation• Global: the selection of global

aggregation points is NP-hard Strategies for the selection of MAs:

• Exact alg: ILP• Approximate algs: genetics-based, k-

means, greedy-based

Localization Techniques in WSNs

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Why do I need localization ?

In sensor networks, nodes are deployed without priori knowledge about their locations.

Estimating spatial-coordinates of the node is referred to as localization.

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LocalizationGPS

Global Positioning System (GPS) is an immediate solution.

There some factors against the usage of GPS:• GPS can work only outdoors.• GPS receivers are too expensive to unsuitable for

wide-range deployment. • It cannot work in the presence of obstructions.

Classifications of Localization Methods

Centralized vs Distributed Anchor-free vs Anchor-based Range-free vs Range-based Mobile vs Stationary

Centralized versus Distributed Localization Algorithms

In centralized algorithms,• nodes send data to a central location where

computation is performed and the location of each node is determined and sent back to the nodes.

Drawbacks • high communication costs • intrinsic delay

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Centralized versus Distributed Localization Algorithms

In distributed algorithms,• each node determines its location by

communication with its neighboring nodes • robust and energy efficient

Drawback• Can be more complex to implement • At times may not be possible due to the limited

computational capabilities of sensor nodes

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Anchor-Free vs Anchor-Based Anchor Nodes:

• Nodes that know their coordinates a priori • By use of GPS or manual placement• For 2D three and 3D four anchor nodes are needed

Anchor-free• Relative coordinates

Anchor-based• Use anchor nodes to calculate global coordinates

Range-Free vs Range-Based Range-Free

• Range-free techniques use connectivity information between neighboring nodes to estimate the nodes‟ position

• Local Techniques• Hop-Counting Techniques

Range-Based• Received Signal Strength Indicator (RSSI)

• Attenuation• RF signal

• Time of Arrival (ToA) • time of flight

• Time Difference of Arrival (TDoA)• requires time synchronization• electromagnetic (light, RF, microwave)• sound (acoustic, ultrasound)

• Angle of Arrival (AoA)• RF signal

Range-Based Techniques

Time of Arrival• All sensors transmit a signal with a predefined

velocity to their neighbors. • Then, the nodes each send a signal back to their

neighbors• by using the transmission and received times each

node estimates its distance to its neighbor

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Range-Based Techniques

Received Signal Strength Indicator (RSSI) • The amount of power present in a received radio

signal. • Due to radio-propagation pathloss, received signal

strength (RSS) decreases as the distance of the radio propagation increases.

• The distance between two sensor nodes can be compared using the RSS value at the receiver, assuming that the transmission power at the sender is either fixed or known

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Range-Based Techniques

TDOA (Time Difference of Arrival)• Transmit both radio and ultrasonic signals at the

same time to observe the arrival time difference.• Extra hardware, i.e., ultrasonic channel, is

required• Not only radio but also sound signals have

multipath effects affected by humidity, temperature, …

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Range-Based Techniques

Angle of Arrival (AoA) • Gather data using either radio or microphone arrays.• Allow a receiving node determines the direction of a

transmitting node.• A single transmitted signal is heard by several spatially

separated microphones. • The phase or time difference between the signal‟s

arrival at different microphones is calculated and thus the AoA of the signal is found.

• Requires directional antennae

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Anchor‐Based versus Anchor‐Free Localization Techniques

Anchor-based methods• Anchor nodes have GPS;• Other nodes derive their locations by trilateration.

Anchor-free methods.• Connectivity only;• Distance estimation for all communication links.• Node locations that reflect the position of the sensor

nodes relative to each other

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Proximity base localization

Trilateration / Multilateration technique• Proximity based localization:

• Some nodes which can know their position through some technique (ex. GPS) broadcast their position information.

• Other nodes listen to these broadcast messages and calculate their own position.

• A simple method would be to calculate its position as the centroid of all the positions it has obtained.

• This method leads to accumulation of localization error.

5.LocalizationTrilateration Example

Trilateration• A is 5m from B• A is 10m from C• A is 8m from D

B

C

D

A

Range-Free Localization

DV-HOP• Similar to classical distance vector routing.• An anchor broadcasts a beacon to be flooded in the

area.

DV-Hop propagation method Each node maintains a table {Xi ,Yi ,hi} Updates only with its neighbors. Each landmark {Xi ,Yi}

• Computes a correction• And floods it into the network

Each node • Uses the correction from the closest landmark• Multiply its hop distance by the correction

jjih

YYXXc

i

jiji

i landmarks all , ,)()( 22

Corrections computed by the landmarksc1 c2 c3

Assume A gets its correction from L2

Its estimate distances to the three landmarks• To L1: 3×16.42• To L2: 2×16.42• To L3: 3×16.42

3 Hop

3 Hop2 Hop

L1L2: 2 hopL1L3: 6 hopL2L3: 5 hop

jjih

YYXXc

i

jiji

i landmarks all , ,)()( 22

Range-Free Localization

DV-hop • Advantages

• Simplicity• Dose not depend on measurement error

• Disadvantage• Only work for isotropic networks

APIT Overview

Anchors• Nodes equipped with high-

powered transmitter• Location information

obtained from GPS or other mechanism

Location estimation by isolating the environment into triangular regions between anchors

Location verification – SerLoc (Secure Range-independent

localization)

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What is location verification?

Different assumptions from general localization• What if some malicious nodes lie about their location?• Sample attack scenario

• Claim to be very close to the sink• Attract many packets• Drop some or all of them• Very easy DoS attack especially for geographic routing protocols

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• Secure Verification of Location Claims

[Sastry et al. WISE 2002].• Location Privacy

Privacy-aware Location Sensor Networks [Gruteser et al. USENIX 2003].

• Secure Localization: Ensure robust location estimation even in the presence of adversaries.

SeRLoc: [Lazos and Poovendran, WISE 2004].

S-GPS: [Kuhn 2004].

SPINE: [Capkun & Hubeaux, Infocom 2005].

Secure Location Services

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• SeRLoc: SEcure Range-independent

LOCalization.

• SeRLoc features

• Passive Localization.

• No ranging hardware required.

• Decentralized Implementation, Scalable.

• Robust against attacks - Lightweight

security.

SeRLoc

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Locators: Randomly deployed Known Location, OrientationDirectional Antennas

(X1, Y1)

(X3, Y3)

(X4, Y4)

(X5, Y5)

(X2, Y2)

N

S

EW

Two-tier network architectureSensors: Randomly deployed, unknown location

r

R

Locator range R

Beamwidth θ

θ

Omnidirectional AntennasSensor range r

Locator Sensor

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Locator Sensor

L1

L4

L3

(0, 0)

s

L3

ROI

The Idea of SeRLoc

• Each locator Li transmits

information that defines the

sector Si, covered by each

transmission.

• Sensor defines the region of

intersection (ROI) from all

locators it hears.

sLH

iiSROI

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How SerLoc works Node i claims its location is (x, y) Node i needs to send (x, y) a location verification request

msg to a nearby verifier• A verifier can be a normal sensor node

The verifier sends a random nonce to node i and start the clock

Node i has to immediately return the challenge through both radio and ultrasonic channels

The verifier measures the time for node i returning the challenge and take the difference between the radio & ultrasonic signal propagation. Based on this observation, verify the claimed location

Weakness of SerLoc

Requires extra hardware, i.e., ultrasonic channel Innocent victims may respond late due to backlog Not location verification but range verification

Verifier

M’s RealLocation

M’s claimedLocation

sink

Oops... Verifier cannot tellthe difference! Big trouble...

Possible Research Issues

Most localization work is mathematical and evaluated via (high level) simulations• More realistic work is needed

Indoor localization is harder• Look at CodeBlue project at Harvard

Location verification• Can’t trust sensors

Secure localization• Can’t trust anchors

Next time Security in WSN

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