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

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

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 Traffic surveillance Industrial and manufacturing automation Distributed robotics Environment monitoring …

5. Dept. of Computer Science and Engineering 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

6. Dept. of Computer Science and Engineering 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

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

8. Dept. of Computer Science and Engineering Problem Formulation Each sensor node N i knows its location (x i, y i ), sensing radius r i, communication radius R Sensing region SR i = { p | d ip < r i } The neighbor set of N i, N(i) = { N j  S | d ij ≤ R, j  i } Assuming that  N j  N(i), R ≥ r i + r j Ensures that coverage implies connectivity

9. 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

10. Dept. of Computer Science and Engineering Special Cases of SSR and SSA d ij ≥ r i + r j NiNi NjNj

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

12. Dept. of Computer Science and Engineering Special Cases of SSR and SSA d ij ≤ 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)

13. 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 j 's non-CCSs covering the point on the SSA  ij that has the fewest nodes covering it.

14. 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

15. 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.

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

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

18. Dept. of Computer Science and Engineering Round-based Node Scheduling Scheme 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 Approximately synchronized

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

20. Dept. of Computer Science and Engineering Adaptive Sleeping Node Scheduling Scheme 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

21. Dept. of Computer Science and Engineering Adaptive Sleeping Node Scheduling Scheme 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

22. Dept. of Computer Science and Engineering 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

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

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

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

26. Dept. of Computer Science and Engineering 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

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

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

29. Dept. of Computer Science and Engineering 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

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

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