1. The Chinese Univ. of Hong Kong Energy-Conserving Coverage Configuration for Dependable Wireless Sensor Networks Chen Xinyu Term Presentation 2004-12-14

2. Dept. of Computer Science and Engineering Outline Motivation Coverage configuration with Boolean sensing model Coverage configuration with general sensing model Performance evaluations with ns-2 Conclusions and future work

3. Dept. of Computer Science and Engineering 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. Dept. of Computer Science and Engineering Applications Military reconnaissance Physical security Environment monitoring Traffic surveillance Industrial and manufacturing automation Distributed robotics …

5. Dept. of Computer Science and Engineering 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. Dept. of Computer Science and Engineering Requirements (cont’d) Fault tolerance Sensors may fail or be blocked due to physical damage or environmental interference Scalability High density of deployed nodes Each sensor must configure its own operational mode adaptively based on local information, not on global information

7. Dept. of Computer Science and Engineering 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. Dept. of Computer Science and Engineering 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. Dept. of Computer Science and Engineering 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. Dept. of Computer Science and Engineering Problem Formulation for the BSM Each sensor node N i knows its location (x i, y i ), sensing radius r i, communication radius R Sensors are deployed in a two-dimensional Euclidean plane Responsible Sensing Region (RSR)  i = { p | d(N i,p) < r i } A point is covered by a sensor node when this point is in the sensor's RSR The one-hop neighbor set of N i N(i) = { N j   | d(N i, N j ) ≤ R, j  i }

11. Dept. of Computer Science and Engineering Some Definitions NiNi NjNj Sponsored Sensing Arc (SSA)  ij Sponsored Sensing Region (SSR) Sponsored Sensing Angle (SSG)  ij Covered Sensing Angle (CSG)  ij

12. Dept. of Computer Science and Engineering Special Cases of SSR and SSA d(N i, N j ) ≥ r i + r j NiNi NjNj

13. Dept. of Computer Science and Engineering Special Cases of SSR and SSA d(N i, N j ) ≤ r i – r j NiNi NjNj SSG  ij =2  CSG  ij is not defined Completely Covered Node (CCN) of N i

14. Dept. of Computer Science and Engineering Special Cases of SSR and SSA d(N i, N j ) ≤ r j - r i NiNi NjNj Complete-Coverage Sponsor (CCS) of N i Degree of Complete Coverage DCC  i = | CCS(i) | SSG  ij is not defined CSG  ij =2  CCS(i)

15. Dept. of Computer Science and Engineering Minimum Partial Arc-Coverage (MPAC) The minimum partial arc-coverage (MPAC) sponsored by node N j to node N i, denoted as  ij, The number of N i 's non-CCSs covering the point on the SSA  ij that has the fewest nodes covering it.

16. Dept. of Computer Science and Engineering Derivation of MPAC  ij 0 22  ij  jl  jm  ij = 2  ij = 1 Covered Sensing Angle (CSG) Sponsored Sensing Angle (SSG)  ij

17. Dept. of Computer Science and Engineering MPAC and DCC Based k-Coverage Sleeping Candidate Condition K-coverage Every point in the deployed area is covered by at least k nodes Theorem A sensor node N i is a sleeping candidate while preserving k-coverage, iff  i ≥ k or  N j  N(i) - CCS(i),  ij > k -  i.

18. Dept. of Computer Science and Engineering Extended Sleeping Candidate Condition Constrained deployed area

19. Dept. of Computer Science and Engineering Node Scheduling Protocols Round-based Divide the time into rounds Approximately synchronized In each round, every live sensor is given a chance to be sleeping eligible Adaptive sleeping Let each node calculate its sleeping time locally and adaptively

20. Dept. of Computer Science and Engineering Round-Based Node Scheduling Protocol on sleeping ready-to- sleeping ready-to-on uncertain T round eligible / STATUS ineligible T round T wait eligible / STATUS ineligible / STATUS on-sleeping decision phase 1.Set a backoff timer T hello, a window timer T win, a wait timer T wait, and a round timer T round 2.Collect HELLO messages from neighbors 3.After T hello times out, broadcast a HELLO message to all neighbors 4.After T win expires, evaluate the sleeping eligibility according to sleeping candidate conditions

21. Dept. of Computer Science and Engineering An Example of Sleeping Eligibility Evaluation

22. Dept. of Computer Science and Engineering Connectivity Requirement Considering only the coverage issue may produce disconnected subnetworks Simple connectivity preservation If a sensor is sleeping eligible, evaluating whether its one-hop neighbors will remain connected through each other when the considered sensor is removed

23. Dept. of Computer Science and Engineering Adaptive Sleeping Node Scheduling Protocol A node may suffer failures or deplete its energy  loss of area coverage Round-based: timer T round is a global parameter and not adaptive to recover a local area loss Letting each node calculate its sleeping time locally and adaptively

24. Dept. of Computer Science and Engineering Adaptive Sleeping Node Scheduling Protocol 1.Set a timer T sleeping 2.When T sleeping times out, broadcast a PROBE message 3.Each neighbor receiving the PROBE message will return a STATUS message to the sender 4.Evaluate sleeping eligibility. If eligible, set T sleeping according to the energy information collected from neighbors

25. Dept. of Computer Science and Engineering 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

26. Dept. of Computer Science and Engineering Problem Formulation for the GSM The sensibility of a sensor N i for an event occurring at an arbitrary measuring point p is defined by  : the energy emitted by events occurring at point p  : the decaying factor of the sensing signal

27. Dept. of Computer Science and Engineering 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) ≥ 

28. Dept. of Computer Science and Engineering 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

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

30. Dept. of Computer Science and Engineering Responsible Sensing Region 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

31. Dept. of Computer Science and Engineering NSFS-Based Pessimistic Sleeping Candidate Condition

32. Dept. of Computer Science and Engineering NSFS-Based Optimistic Sleeping Candidate Condition

33. Dept. of Computer Science and Engineering 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 uncertain I

34. Dept. of Computer Science and Engineering Performance Evaluation with ns-2 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 Mpac: round-based protocol with elementary MPAC condition MpacB: round-based protocol with extended MPAC condition in constrained area MpacBAs: adaptive sleeping protocol with MpacB SscpP: Sscp with the pessimistic sleeping condition SscpO: Sscp with the optimistic sleeping condition

35. Dept. of Computer Science and Engineering Bridge between BSM and GSM Ensured-sensibility radius

36. Dept. of Computer Science and Engineering 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

37. Dept. of Computer Science and Engineering Performance Evaluation (1) Sleeping sensor vs. communication radius

38. Dept. of Computer Science and Engineering Performance Evaluation (2) Network topology

39. Dept. of Computer Science and Engineering Performance Evaluation (3) Sleeping sensor vs. sensor number

40. Dept. of Computer Science and Engineering Performance Evaluation (4) Sleeping sensor vs. sensibility threshold

41. Dept. of Computer Science and Engineering Performance Evaluation (5) Network lifetime vs. live sensor when the MTBF is 800s, R is 12m

42. Dept. of Computer Science and Engineering Performance Evaluation (6)  -coverage accumulated time The total time during which  or more percentage of the deployed area satisfies the coverage requirement

43. Dept. of Computer Science and Engineering Approaches to Build Dependable Wireless Sensor Networks Decreasing the communication radius or increasing the coverage degree is equivalent in providing fault tolerance Detecting sensor failures and recovering the area loss as quick as possible: adaptive sleeping configuration Exploiting the cooperation between neighboring sensors: general sensing model

44. Dept. of Computer Science and Engineering Conclusions Develop MPAC-based node sleeping eligibility conditions for the BSM achieve k-coverage degree can be applied with different sensing radii Develop SSCPs for the GSM exploit the cooperation between adjacent sensors Suggest three effective approaches to build dependable sensor networks

45. Dept. of Computer Science and Engineering Future Work Exploit algorithms to identify node redundancy without location information Study the network behavior with node failures Build dependable sensor networks both on area coverage and network connectivity

46. Dept. of Computer Science and Engineering Performance Evaluation (5) Distribution of coverage degree

47. Dept. of Computer Science and Engineering Performance Evaluation (4) MpacBCa sleeping sensor