Energy-Conserving Coverage Configuration for Dependable Wireless Sensor Networks

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

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

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 Dept. of Computer Science and Engineering

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

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 Dept. of Computer Science and Engineering

6. 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 Dept. of Computer Science and Engineering

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 Dept. of Computer Science and Engineering

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 Dept. of Computer Science and Engineering

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 Dept. of Computer Science and Engineering

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

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

12. Special Cases of SSR and SSA • d(Ni, Nj)≥ ri + rj Ni Nj Dept. of Computer Science and Engineering

13. Special Cases of SSR and SSA • d(Ni, Nj)≤ ri – rj SSG ij =2 CSG ij is not defined Ni Nj Completely Covered Node (CCN) of Ni Dept. of Computer Science and Engineering

14. Special Cases of SSR and SSA • d(Ni, Nj)≤ rj - ri Ni SSG ijis not defined CSG ij=2 Nj Complete-Coverage Sponsor (CCS) of Ni CCS(i) Degree of Complete Coverage DCC i= | CCS(i) | Dept. of Computer Science and Engineering

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

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

17. 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 Ni is a sleeping candidate while preserving k-coverage, iff i ≥ k or  Nj  N(i) - CCS(i),ij > k - i . Dept. of Computer Science and Engineering

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

19. 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 Dept. of Computer Science and Engineering

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

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

22. 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 Dept. of Computer Science and Engineering

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

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

25. 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 Dept. of Computer Science and Engineering

26. Problem Formulation for the GSM • The sensibility of a sensor Ni 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 Dept. of Computer Science and Engineering

27. All-Sensor Field Sensibility (ASFS) • Suppose we have a “background” distribution of n sensors, denoted by N1, N2, …, Nn, in a deployment region A • All-Sensor Field Sensibility for point p • With a sensibility threshold , the point p is covered if Sa(p) ≥ Dept. of Computer Science and Engineering

28. 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 Dept. of Computer Science and Engineering

29. 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 Dept. of Computer Science and Engineering

30. 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 Ni resides is its Responsible Sensing Region i • If an event occurs in i, sensor Ni will receive the strongest signal • Open RSR and closed RSR Dept. of Computer Science and Engineering

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

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

33. ineligible / STATUS eligible / STATUS eligible / STATUS Twait Twait ineligible Tround Tround Sensibility-Based Sleeping Configuration Protocol (SSCP) ready-to-on ready-to- sleeping uncertain II uncertain I sleeping on Dept. of Computer Science and Engineering

34. 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 Dept. of Computer Science and Engineering

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

36. 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 Dept. of Computer Science and Engineering

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

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

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

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

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

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

43. 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 Dept. of Computer Science and Engineering

44. 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 Dept. of Computer Science and Engineering

45. 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 Dept. of Computer Science and Engineering