1. The Chinese Univ. of Hong Kong Dept. of Computer Science & Engineering A Sensibility-Based Sleeping Configuration Protocol for Dependable Wireless Sensor Networks Chen Xinyu Group Meeting 2005-01-28

2. 2 Outline Introduction Neighboring-sensor field sensibility Sensibility-based sleeping configuration protocol Performance evaluations Conclusions

3. 3 Wireless Sensor Networks Composed of a large number of sensor nodes Sensors communicate with each other through short-range radio transmission Sensors react to environmental events and relay collected data through the dynamically formed network

4. 4 Applications Environment monitoring Military reconnaissance Physical security Traffic surveillance Industrial and manufacturing automation Distributed robotics … Ossama Younis and Sonia Fahmy: Distributed Clustering in Ad-hoc Sensor Networks: A Hybrid, Energy-Efficient Approach (InfoCom2004)

5. 5 Requirements Maintaining coverage Every point in the region of interest should be sensed within given parameters Extending system lifetime The energy source is usually battery power Battery recharging or replacement is undesirable or impossible due to the unattended nature of sensors and hostile sensing environments

6. 6 Requirements (Cont’d) Fault tolerance Sensors may fail or be blocked due to physical damage or environmental interference Produce some void areas which do not satisfy the coverage requirement Scalability High density of deployed nodes Each sensor must configure its own operational mode adaptively based on local information, not on global information

7. 7 Approach: Coverage Configuration Coverage configuration is a promising way to extend network lifetime by alternately activating only a subset of sensors and scheduling others to sleep according to some heuristic schemes while providing sufficient coverage in a geographic region

8. 8 Concerns A good coverage-preserved and fault-tolerant sensor configuration protocol should have the following characteristics: It should allow as many nodes as possible to turn their radio transceivers and sensing functionalities off to reduce energy consumption, thus extending network lifetime Enough nodes must stay awake to form a connected network backbone and to preserve area coverage Void areas produced by sensor failures and energy depletions should be recovered as soon as possible

9. 9 Two Sensing Models Boolean sensing model (BSM) Each sensor has a certain sensing range, and can only detect the occurrences of events within its sensing range General sensing model (GSM) Capture the fact that signals emitted by a target of interest decay over the distance of propagation Exploit the collaboration between adjacent sensors

10. 10 Discussions for the BSM Each sensor has a deterministic sensing radius Allow a geometric treatment of the coverage problem Miss the attenuation behavior of signals Ignore the collaboration between adjacent sensors in performing area sensing and monitoring

11. 11 Problem Formulation for the GSM Point Sensibility s(N i, p): the sensibility of a sensor N i for an event occurring at an arbitrary measuring point p  : the energy emitted by events occurring at point p  : the decaying factor of the sensing signal d(N i, p) : the distance between senosr N i and point p

12. 12 All-Sensor Field Sensibility (ASFS) Suppose we have a “background” distribution of n sensors, denoted by N 1, N 2, …, N n, in a deployment region A All-Sensor Field Sensibility for point p With a sensibility threshold , the point p is covered if S a (p) ≥ 

13. 13 Discussions for the ASFS Need a sink working as a data fusion center Produce a heavy network load in multi- hop sensor networks Pose a single point of failures

14. 14 Neighboring-Sensor Field Sensibility (NSFS) Treat each sensor as a sensing fusion center Each sensor broadcasts its perceived field sensibility Each sensor only collects its one-hop neighbors’ messages Transform the original global coverage decision problem into a local problem

15. 15 Responsible Sensing Region (RSR) Voronoi diagram Partition the deployed region into a set of convex polygons such that all points inside a polygon are closet to only one particular node The polygon in which sensor N i resides is its Responsible Sensing Region  i If an event occurs in  i, sensor N i will receive the strongest signal Open RSR and closed RSR

16. 16 Pessimistic Scan Region

17. 17 Connectivity Requirement Considering only the coverage issue may produce disconnected subnetworks Simple connectivity preservation Evaluating whether N i ’s one-hop neighbors will remain connected through each other or through its two-hop neighbors when N i is removed

18. 18 N i ’s Sleeping Candidate Condition : Responsible Sensing Region of N j : the two-hop confined region of N i : communication path between N j and N k

19. 19 Optimistic Scan Region

20. 20 uncertain I Sensibility-Based Sleeping Configuration Protocol (SSCP) on sleeping ready-to- sleeping ready-to-on T round eligible / STATUS ineligible T round T wait eligible / STATUS ineligible / STATUS uncertain II

21. 21 Performance Evaluation with ns-2 Boolean sensing model ESS: extended sponsored sector Proposed by Tian et. al. of Univ. of Ottawa, 2002 Consider only the nodes inside the RSR of the evaluated node General sensing model SscpP: SSCP with the pessimistic scan region SscpO: SSCP with the optimistic scan region

22. 22 Bridge between BSM and GSM Ensured-sensibility radius

23. 23 Default Parameters Setting The deployed area is 50m x 50m  = 1,  = 3,  = 0.001 (r = 10m) R = 12 m The number of deployed sensor: 120 Power Consumption: Tx (transmit) = 1.4W, Rx (receive) = 1W, Idle = 0.83W, Sleeping = 0.13W

24. 24 Performance Evaluation (1) Sleeping sensor vs. communication radius

25. 25 Performance Evaluation (2) Network topology

26. 26 Performance Evaluation (3) Sleeping sensor vs. sensor number

27. 27 Performance Evaluation (4) Sleeping sensor vs. sensibility threshold

28. 28 Performance Evaluation (5) Network lifetime vs. live sensor when the MTBF is 800s, R is 12m

29. 29 Performance Evaluation (6)  -coverage accumulated time The total time during which  or more percentage of the deployed area satisfies the coverage requirement

30. 30 Conclusions Propose NSFS with the GSM transform a global decision problem to a local one exploit the cooperation between adjacent sensors Develop SSCPs to build dependable wireless sensor networks

31. 31 Q & A