Introduction to Sensor Networks

1. Introduction to Sensor Networks Rabie A. Ramadan, PhD Cairo University http://rabieramadan.org rabie@rabieramadan.org 2

2. Deployment, Clustering , and and Routing in WSN

3. Deployment Constraints • Sensor characteristics • Monitored field characteristics • Monitored/probed object

4. Deployment Parameters

5. Deployment Parameters

6. Deployment Parameters

7. Deployment Parameters

8. Deployment Problems and Solutions • Random Deployment • Virtual force Algorithm • Deterministic Deployment • Circle Packing • Energy Mapping • Movement-Assisted Sensor Deployment • Sink Placement Problem • Single node • Multiple sink deployment • Relay Node Placement in WSN

9. Random Deployment Virtual Force Algorithm

10. Virtual Force Algorithm • Sensors are initially deployed randomly • Objective: • To maximize the Coverage • Assume no prior knowledge about the monitored field • All nodes are mobile • Energy and obstacles might present in the field

11. Virtual Force Algorithm (Cont.) • Attractive and Repulsive forces • Sensors do not physically move • A sequence of virtual motion paths is determined for the randomly placed sensors. • Once the effective sensor positions are identified, a one-time movement is carried out to redeploy the sensors at these positions.

12. Virtual Force Algorithm (Semi Distributed.) • Assumptions: • Clustered network • All clustered heads are able to communicate with the sink node • The cluster head is responsible for executing the VFA and managing the one-time movement of sensors to the desired locations.

13. Virtual Force Algorithm (Cont.) • Each sensor behaves as a “Sourceof force” for all other sensors. • This force can be either positive (Attractive) or negative (Repulsive). • The closeness and wide distance between two sensors are measured using a predefined threshold.

14. Virtual Force Algorithm (Cont.) • Sensor Binary Model • Consider an n by m sensor field grid and assume that there are k sensors deployed in the random deployment stage. • Each sensor has a detection range r. Assume sensor siis deployed at point (xi , yi ). • For any point P at (x, y), we denote the Euclidean distance between si and P as d(si , P), • The coverage of a Grid Point P can be expressed by:

15. Virtual Force Algorithm (Cont.) • Virtual Forces • Attraction force  F12 • Repulsive force  F13 • Zero Force  F14 • Obstacle Force  • preferential coverage Force  Total Force on node i =

16. Virtual Force Algorithm (Cont.) • Energy Constraints • Using such forces , the cluster head runs the VFA • After stability occurs , Sensors are ordered to move to the new positions • Energy and Obstacles might be problems • Any sensor will not be able to move the required distance , the moving order is discarded • Obstacles need an obstacle avoidance algorithm

17. Think….. • If some sensors are stationary, does this affect the virtual force algorithm?

18. SENSOR REPLACEMENT BASED ENERGY MAPPING

19. The problem • A set of sensors S is deployed in a monitored field F(A)for a period of time T. • The field is divided into a grid of cells A. • Each cell is assigned a weight where represents the importance of the cell i. • The location of each sensor is assumed known; • More than one sensor could be deployed in one cell. • Sensors are assumed heterogeneous in terms of their energy and mobility.

20. Assumptions • A sensor could be in different states; • it could have its sensing off or on based on the field monitoring requirements. • Sensing off, radio off – (sleep mode) • Sensing off, radio receiving – (Receiving mode) • Sensing off, radio transmitting – (Routing mode) • Sensing on, radio receiving – (Sensing and Receiving mode) • Sensing on, radio transmitting – (Sensing and Transmitting mode) • Sensing on, radio off - (Sensing mode)

21. The main idea • Knowing the energy map of the network : • May lead to early detection to the uncovered areas. • Redeploy new sensors • Turn off some of the sensors due to their coverage redundancy • Wake up some of the nodes when needed • Move one or mobile nodes to cover the required uncovered spots

22. Redeployment based Energy map • Step 1: Energy dissipation rate prediction • Each sensor predicts its own energy rate based on its history (e.g. Markov Chain ..) • Step 2: sensors send their initial energy and the location, predicted energy dissipation rate to the sink node through a cluster head. • Sensors update their energy dissipation rate based on a specific threshold (if the new dissipation rate increased more than the given threshold , the node sends the new dissipation rate)

23. Redeployment based Energy map • Step 3: the sink node constructs the energy map based on the received dissipated energy rate from the sensors. • The sink may move one of the mobile sensors to the uncovered spot or wake up one of the sleeping sensors

24. Think ……. • What are the disadvantages of energy mapping algorithm ?

25. Movement-Assisted Sensor Deployment

26. The problem of sensor deployment • Given the target area, how to maximize the sensor coverage with less time, movement distance and message complexity • The importance of the problem • Distributed instead of centralized

27. Voronoi Diagram • Definition: • Every point in a given polygon is closer to the node in this polygon than to any other node.

28. Overview of the proposed algorithm • Sensors broadcast their locations and construct local Voronoi polygons • Find the coverage holes by examine Voronoi polygons • If holes exist, reduce coverage hole by moving • VOR : VORonoi-based • Pull sensors to the sparsely covered area

29. Part of Assignment 2 • Implement both Virtual Force algorithm and Voronoi based algorithm ? Report your experience and algorithms efficiency? • Given a set of sensors with limited amount of energy. Some of these sensors are assumed mobile and others are assumed stationary. Assume similar sensing and communication ranges for all sensors. Sensors are allowed to move from one place to another iff they have enough energy to move to the required destination. In addition , the borders of the monitored area is assumed known in terms of 2D coordinates. Borders may be found in the monitored area. Advice a suitable deterministic deployment algorithm for efficient deployment to the sensors given that the deployed sensors have to be connected and important areas in the field are covered. In addition , your algorithm must guarantee the coverage of the monitored field for certain period of time. • You may look for an already given solution or come up with a convincing one .

30. Deterministic Deployment Deployment Using Circle Packing

31. Deployment Using Circle Packing • Deployment of homogenous sensors • Full Coverage Deployment • Deployment of connected heterogeneous sensors 31

32. Deployment of homogenous sensors 32

33. Full Coverage Deployment 33

34. Sequential Packing-based Deployment Algorithm (SPDA) • Given • Sensors Sensing Ranges • Sensors Communication Ranges • Bounded Monitored Field • Objective • Best Connected Deployment Scheme • Max. Coverage. • Min. Overlapped Areas • Benefit from the properties learned from the optimal deployment 34

35. Sequential Packing-based Deployment Algorithm

36. Sequential Packing-based Deployment Algorithm

37. Potential Points 37

38. Think ….. How do you guarantee connectivity ?

39. Correctness of the Algorithm 39

40. Sink Placement Problem

41. Potential benefits of sink relocation • Increased network longevity: shortened data paths can safe the total energy consumed to data collection and extend the life of relaying nodes. • Improved timeliness: involves fewer relays leading to avoidance of large packet backlogs • Enhanced safety: moves the sink away from harmful events without damaging network performance

42. Sink node Inactive Sensor Active Sensor One hop Sensor Dead Sensor Energy-Based Relocation -- Motivation Normal Operational Mode: • Sensors pursue multi-hop paths to communicate with the sink node Issues: • When the sink is stationary, nearby sensors get involved in heavy packet forwarding and die quickly Can repositioning the sink node help? To where ? • Nodes further away are picked as substitute relays • Consequence: • Increase in total transmission power  rapid energy depletion • Effect grows spirally outward

43. Moving the Sink • Where to go • Towards the region, whose sensors generate the most number of packets • Centroid of the last-hop nodes that route the largest traffic (use a distance * traffic metric)

44. Think…. • What about putting the sink node initially in the center of all nodes? Does this will be the best position for the sink node?

45. Part of your assignment • Device an algorithm for Multiple Sink Network Design Problem in Large Scale Wireless Sensor Networks? • You may look at : • E. IlkerOyman and CemErsoy, Multiple Sink Network Design Problem in Large Scale Wireless Sensor Networks,, IEEE International Conference onCommunications, 2004

46. Relay Node Placement in WSNClustering Algorithms

47. Clustering Facts • Clustering plays a dominant role in delaying the first node death, while aggregation plays a dominant role in delaying the last node death • In each cluster one node acts as a cluster head which is in charge of coordinating with other cluster heads

48. LEACH Algorithm • The LEACH Network is made up of nodes, some of which are called cluster-heads • The job of the cluster-head is to collect data from their surrounding nodes and pass it on to the base station • LEACH is dynamic because the job of cluster-head rotates • LEACH is considered as clustering and routing protocol

49. The Amount of Energy Depletion • This is the formula for the amount of energy depletion by data transfer:

50. LEACH’s Two Phases • The LEACH network has two phases: the set-up phase and the steady-state • The Set-Up Phase • Where cluster-heads are chosen • The Steady-State • The cluster-head is maintained • When data is transmitted between nodes