1. Repairing Sensor Network Using Mobile Robots Y. Mei, C. Xian, S. Das, Y. C. Hu and Y. H. Lu Purdue University, West Lafayette ICDCS 2006 Speaker ： Shih-Yun Hsu

2. Outline  Introduction  Goal  Assumptions  Coordination Algorithm  Simulation  Conclusions

3. Introduction  Many sensor networks studies focus on  Effectively collect data and transfer data  Energy-aware  Fault tolerant routing techniques  Sensor deployment  Sensor nodes may fail and leave holes in coverage  Sensor replacement

4. Introduction  Using some redundant mobile sensors relocating themselves to fill the holes  Mobility is an expensive feature, as mobile sensors need to have motors, motion control, and GPS modules

5. Goal  Using robots to assist sensor replacement  Characteristics of robots  Mobile  Can pick, carry, and unload sensor nodes  Minimize the motion overhead and the messaging overhead  Energy  Time require

6. Assumptions  Sensor nodes are randomly uniformly distributed deployed  Location-aware  Limited lifetime

7. Assumptions  Robots are initially randomly uniformly distributed deployed  Each robot carries a certain number of functional nodes  Travel at a constant speed  Location-aware  The number of robots is much smaller than the number of sensor nodes

8. Assumptions  Robots’ transmission range are larger than sensor nodes’ transmission range

9. Algorithm overview guardee guardian detect sensors Robot report

10. Algorithm concepts  Initialization  Setting up the roles of robots  Manager  Maintainer  Setting up the initial relationship between the sensor nodes and the robots  Setting up the guardian-guardee relationship among sensor nodes  Failure detection and reporting  Failure handling

11. Coordination Algorithm  Centralized algorithm  Fixed algorithm  Dynamic algorithm

12. Centralized algorithm  Initialization  Failure detection and reporting  Failure handling

13. Sensor Robot Manager Centralized algorithm - Initialization Phase - Manager move to the center of sensor network Maintainer

14. Centralized algorithm - Initialization Phase - Sensor Robot Manager broadcast to all sensors and maintainers

15. Centralized algorithm - Initialization Phase - Sensor Robot Maintainer sends its location to manager

16. Centralized algorithm - Initialization Phase - Sensor Robot Maintainer sends its location to one-hop neighbor sensor nodes

17. Centralized algorithm - Initialization Phase - Sensor Robot Each sensor node sends its location to one-hop neighbor

18. Centralized algorithm - Initialization Phase - Sensor Robot Each sensor will pick its nearest neighbor Sensor as its guardian guardian guardee

19. Centralized algorithm - Failure detection - Sensor Robot Guardian conceives that the guardee has failed for a certain amount of time guardian guardee Each sensor node periodically sends beacon messages to its one-hop neighbor nodes

20. Centralized algorithm - Failure reporting - Sensor Robot Guardian sends a failure message with the failed node's location to the manager guardian guardee

21. Centralized algorithm - Failure handling - Sensor Robot Manager selects the robot whose current location is the closest to the failure guardian guardee Robot move to the failure location

22. Centralized algorithm - Failure handling -  When the robot is moving towards a failed sensor node, it still can receive failure repair request forwarding from the manager  First-come-first-serve

23. Centralized algorithm - Failure handling -  Robot has moved a certain threshold distance on its way to repair a failed robot, it sends an update of its location to the manager via geographic routing and to the one-hop neighbor sensor nodes via broadcast  Threshold is a distance that should be decided based on the transmission range

24. Fixed algorithm Each grid has one robot All robots are both the manager and the maintainer in their own grid

25. Fixed algorithm

27. Dynamic algorithm

28. The overhead of maintain voronoi graphs is too heavy Each sensor selects the nearest robot to be the maintainer

29. Dynamic algorithm

32. Simulation  Simulator ： GlomoSim  Maintenance robots ： k 2 = 4, 9, and 16  Total area ： 200 × 200 × k 2 m 2  Total sensors ： 50 × k 2  Robot transmission range ： 250 m  Sensor transmission range ： 63 m  Sensor nodes’ beaconing period ： 10 sec  Geographic Routing ： GPSR  Threshold distance ： 20 m

33. Motion overhead

34. Message overhead

36. Repair time

37. Conclusions  This paper presented three different algorithms using mobile robots for sensor network repairing  The three algorithms have different properties, and the optimal choice of the coordination algorithm depends on specific scenarios and objectives being optimized