Chen Xinyu Group Meeting 2004-10-25

1. Node Scheduling Schemes for Coverage Preservation and Fault Tolerance in Wireless Sensor Networks Chen Xinyu Group Meeting 2004-10-25 The Chinese Univ. of Hong Kong

2. Outline • Motivation • K-coverage sleeping candidate condition • Node scheduling schemes • Round-based • Adaptive sleeping • Performance evaluations • Conclusions 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 • Traffic surveillance • Industrial and manufacturing automation • Distributed robotics • Environment monitoring • … Dept. of Computer Science and Engineering

5. Problems • 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 • Sensors may fail or be blocked due to physical damage or environmental interference Dept. of Computer Science and Engineering

6. Concerns • A good coverage-preserved and fault-tolerant node scheduling scheme 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

7. Outline • Motivation • K-coverage sleeping candidate condition • Node scheduling schemes • Round-based • Adaptive sleeping • Performance evaluations • Conclusions Dept. of Computer Science and Engineering

8. Problem Formulation • Each sensor node Ni knows its location (xi, yi), sensing radius ri, communication radiusR • Sensing region SRi = { p | dip< ri } • The neighbor set of Ni, N(i) = { Nj S | dij≤ R, j  i } • Assuming that  Nj N(i), R ≥ ri + rj • Ensures that coverage implies connectivity Dept. of Computer Science and Engineering

9. 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

10. Special Cases of SSR and SSA • dij≥ ri + rj Ni Nj Dept. of Computer Science and Engineering

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

12. Special Cases of SSR and SSA • dij≤ 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

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

14. 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

15. 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

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

17. Outline • Motivation • K-coverage sleeping candidate condition • Node scheduling schemes • Round-based • Adaptive sleeping • Performance evaluations • Conclusions Dept. of Computer Science and Engineering

18. ineligible / STATUS eligible / STATUS eligible / STATUS ineligible Twait Twait Tround Tround Round-based Node Scheduling Scheme • Approximately synchronized • 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

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

20. Adaptive Sleeping Node Scheduling Scheme • 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

21. Adaptive Sleeping Node Scheduling Scheme • 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

22. Performance Evaluation • ESS: extended sponsored sector • Proposed by Tian et. al. of Univ. of Ottawa, 2002 • Consider only the nodes inside the SR of the evaluated node • Mpac: round-based scheme with elementary MPAC condition • MpacB: round-based scheme with extended MPAC condition in constrained area • MpacBAs: adaptive sleeping scheme with MpacB Dept. of Computer Science and Engineering

23. Performance Evaluation (1) • Sensor number vs. sensing radius Dept. of Computer Science and Engineering

24. Performance Evaluation (2) • Standard deviation of sensing radius Dept. of Computer Science and Engineering

25. Performance Evaluation (3) • Required coverage degree Dept. of Computer Science and Engineering

26. Performance Evaluation (4) • Fault tolerance approaches • Adaptive sleeping scheduling • (k+1)-coverage scheduling • Provide one more coverage degree than the design requirement k • -coverage accumulated time • The total time during which  percentage of the deployed area satisfies the coverage requirement Dept. of Computer Science and Engineering

27. Performance Evaluation (4) Dept. of Computer Science and Engineering

28. Performance Evaluation (7) • System lifetime vs. live sensor Dept. of Computer Science and Engineering

29. Conclusions • Develop MPAC-based node sleeping eligibility conditions • achieve k-coverage degree • can be applied with different sensing radii • Propose two fault tolerant approaches: • Adaptive sleeping scheduling • (k+1)-coverage scheduling • Identify that a tradeoff exists between sensing coverage and network lifetime Dept. of Computer Science and Engineering