Introduction to Sensor Networks

1. Introduction to Sensor Networks Rabie A. Ramadan, PhD Cairo University http://rabieramadan.org rabie@rabieramadan.org 3

2. 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

3. Localization Techniques in WSNs

4. 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.

5. 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.

6. Classifications of Localization Methods • Centralized vs Distributed • Anchor-free vs Anchor-based • Range-free vs Range-based • Mobile vs Stationary

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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, …

14. 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

15. 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

16. 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.

17. A 5.LocalizationTrilateration Example • Trilateration • A is 5m from B • A is 10m from C • A is 8m from D C B D

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

19. 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

20. L1L2: 2 hopL1L3: 6 hopL2L3: 5 hop 2 Hop 3 Hop 3 Hop • 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

21. Range-Free Localization • DV-hop • Advantages • Simplicity • Dose not depend on measurement error • Disadvantage • Only work for isotropic networks

22. 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

23. Location verification – SerLoc (Secure Range-independent localization)

24. 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

25. Secure Location Services • Secure Verification of Location Claims • [Sastry et al. WISE 2002]. • Location Privacy • Privacy-aware Location Sensor Networks [Gruteser et al. USENIX 2003]. • Secure Localization: Ensurerobust location estimationevenin the presence ofadversaries. • SeRLoc: [Lazos and Poovendran, WISE 2004]. • S-GPS: [Kuhn 2004]. • SPINE: [Capkun & Hubeaux, Infocom 2005].

26. SeRLoc • SeRLoc:SEcureRange-independent LOCalization. • SeRLoc features • Passive Localization. • No ranging hardware required. • Decentralized Implementation, Scalable. • Robust against attacks - Lightweight security.

27. Sensors: Randomly deployed, unknown location Omnidirectional Antennas Sensor range r Locators: Randomly deployed Directional Antennas r Known Location, Orientation R θ Beamwidth θ Locator range R Locator Sensor Two-tier network architecture (X2, Y2) (X4, Y4) (X3, Y3) (X1, Y1) (X5, Y5)

28. The Idea of SeRLoc ROI Locator Sensor L4 • 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. L1 L3 s L3 (0, 0)

29. 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

30. Weakness of SerLoc • Requires extra hardware, i.e., ultrasonic channel • Innocent victims may respond late due to backlog • Not location verification but range verification sink M’s claimed Location Verifier Oops... Verifier cannot tell the difference! Big trouble... M’s Real Location

31. 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

32. Next time Security in WSN