1. Coverage Issues in WSNs
Presented by Ming-Tsung Hsu

2. Outline
Wireless Sensor Networks
Coverage Issues

3. Wireless Sensor Networks

4. Faster, Smaller, Numerous
Moore’s Law
“Stuff” (transistors, etc) doubling every 1-2 years
Bell’s Law
New computing class every 10 years
Streaming Data to/from the Physical World
log (people per computer)
year

5. Applications Environmental Monitoring Interactive and Control
Sample Rate & Precision
Disconnection & Lifetime
Density & Scale
Low Latency
Environmental Monitoring
Habitat Monitoring
Integrated Biology
Structural Monitoring
Interactive and Control
Pursuer-Evader
Intrusion Detection
Automation
Mobility

6. Fundamental Functionalities
Data collection - Sensor subsystem
Gathering information and controlling/monitoring environments
Data processing - Process subsystem
Performing local computations
Data transmission - Communication subsystem
Exchanging data

7. Characteristics A special wireless ad hoc network
Large number of nodes are deployed randomly and densely
Scalability & Self-Configuration
Battery powered
Energy Efficiency
Topology and density change
Adaptivity
Working for a common task
Data Centric
In-network data processing (Data aggregation)
Message-level Latency

8. Sensor Deployment How to deploy sensors over a field?
Deterministic, planned deployment
Random deployment
Desired properties of deployments?
Depends on applications
Connectivity
Coverage

9. Sensor Network Formation
Deployed densely and randomly
“Dense” means “exits redundant nodes”
 Density control
“Random” means “topology is indefinite”
 Topology control
Self-Configuration & Self-Organization
Scalability
Energy

10. Node’s Operations On-Duty (working) nodes Off-Duty (sleeping) nodes
Forming a sensor network
Am I redundant ?
Off-duty?
Energy Consideration
Role-change? Off-duty?
Off-Duty (sleeping) nodes
When to wakeup? On-duty?
Duty cycle policy
Scheduling vs. Adaptive
Duty period

11. Coverage, Connectivity
Is every point covered by 1 or K sensors
1-covered, K-covered
Is the sensor network connected
K-connected
Others 1 8 R 2 7 6 3 4 5

12. Coverage & Connectivity: not independent, not identical
If region is continuous & Rt > 2Rs
Region is covered sensors are connected
X. Wang (Sensys’03)
H. Zhang & J. Hou (2004) Rt Rs

13. Real Products

14. Problem Tree for Coverage and Connectivity Problems
deployment
connectivity
coverage
topology formation
blanket
barrier
network
network
density control
deterministic
# of sensors?
adaptive
K-connectivity
# of sensors
are needed?
K-coverage
topology control
Scheduling
ASCENT …
surveillance & exposure
LEACH …
adaptive
algorithmic
probabilistic
scheduling
per-node (Max Rt)
PEAS …
per-node
homo
homo
OGDC …
k-connected
Xue&Kumar …
Penrose …
various connected subgraphs

15. Coverage Issues

16. Related Geometric Problems

17. Surveillance the Voronoi diagram and the maximal breach path
the Delaunay triangulation and the maximal support path

18. Exposure
The exposure for an object in the sensor field during the interval
along a path
minimal exposure path  the worst coverage of a sensor network

19. Simple Coverage Problem
Given an area and a sensor deployment
Question: Is the entire area covered? 1 8 R 2 7 6 3 4 5

20. K-Coverage Problem Given: region, sensor deployment, integer k
Question: Is the entire region k-covered?
C.-F. Huang & Y.-C Tseng (WSNA’03) 1 8 R 2 7 6 3 4 5

21. Is the perimeter k-covered?

22. Is a belt region k-barrier covered?
Construct a graph G(V, E)
V: sensor nodes, plus two dummy nodes L, R
E: edge (u,v) if their sensing disks overlap
Region is k-barrier covered iff L and R are k-connected in G R L

23. Density Control Given: an area and a sensor deployment
Problem: turn on/off sensors to maximize the sensor network’s life time

24. Density Control (cont’d)
Nodes are on-duty or off-duty by Scheduling or Probing
Resulting monitoring area still covered
Sensing range
Determined (disc)
Irregular in shape, or even follow a probabilistic model

25. Approaches for Density Control
Adaptive
PEAS (ICNP’02 , ICDCS’03)
CCP (SenSys’03)
Scheduling
SET K-COVER (ICC’01)
Co-Grid (IPSN’04)
OGDC (International Workshop on Theoretical and Algorithmic Aspects of Sensor, Ad hoc Wireless and Peer-to-Peer Networks, 2004)

26. PEAS and OGDC
PEAS: A robust energy conserving protocol for long-lived sensor networks
Fan Ye, et al (UCLA), ICNP’02, ICDCS’03,
“Maintaining Sensing Coverage and Connectivity in Large Sensor Networks”
H. Zhang and J. Hou (UIUC), International Workshop on Theoretical and Algorithmic Aspects of Sensor, Ad Hoc Wireless, and Peer-to-Peer Networks (04), The Wireless Ad Hoc and Sensor Networks: An International Journal (05)

27. PEAS: basic ideas Probing Environment and Adaptive Sleeping
How often to wake up?
How to determine whether to work or not?
Wake-up rate?
yes
Sleep
Wake up
Go to
Work?
work no

28. How often to wake up?
Desired: the total wake-up rate around a node equals some given value

29. Inter Wake-up Time f(t) = λ exp(- λt) exponential distribution
λ = average # of wake-ups per unit time

30. Wake-up rates A B A + B: f(t) = (λ + λ’) exp(- (λ + λ’) t)

31. Adjust wake-up rates Working node knows Probing node adjusts its λ by
Desired wake-up rate λd
Measured wake-up rate (form working node) λm
Probing node adjusts its λ by
λ := λ (λd / λm)

32. Go to work or return to sleep?
Depends on whether there is a working node nearby. Rp
Go back to sleep go to work

33. Is the resulting network covered or connected?
If Rt ≥ (1 + √5) Rp and … then
P(connected) → 1
Simulation results show good coverage

34. Basic Idea of OGDC OGDC: Optimal Geographical Density Control
Minimize the number of working nodes ↔ Minimize the total amount of overlap

35. Minimum overlap D Rs

36. Minimum overlap (cont’d)

37. Minimum overlap

38. Near-optimal

39. OGDC: the Protocol Time is divided into rounds
In each round, each node runs this protocol to decide whether to be active or not
Select a starting node. Turn it on and broadcast a power-on message
Select a node closest to the optimal position. Turn it on and broadcast a power-on message. Repeat this.

40. Selecting starting nodes
Each node volunteers with a probability p.
Backs off for a random amount of time.
If hears nothing during the back-off time, then sends a message carrying
Sender’s position
Desired direction

41. Select the next working node
On receiving a message from a starting node
Each node computes its deviation D from the optimal position.
Sets a back-off timer inversely proportional to D.
On receiving a power-on message from a non-starting node

43. PEAS vs. OGDC
Complexity
Coverage
Time Sync

44. Blanket vs. Barrier Coverage
Blanket coverage
Every point in the area is covered (or k-covered)
Barrier coverage
Every crossing path is k-covered

45. Is a belt region k-barrier covered?
Construct a graph G(V, E)
V: sensor nodes, plus two dummy nodes L, R
E: edge (u,v) if their sensing disks overlap
Region is k-barrier covered iff L and R are k-connected in G. R L

46. Donut-shaped region
K-barrier covered iff G has k essential cycles.

47. Thanks