1. IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS 2007 (TPDS 2007)
SenCar: An Energy-Efficient Data Gathering Mechanism for Large-Scale Multihop Sensor Networks
Ming Ma, and Yuanyuan Yang
Department of Electrical and Computer Engineering, State University of New York at Stony Brook
IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS (TPDS 2007)

2. Outline Introduction Data gathering scheme for a connected network
Motivations and main idea
Data gathering scheme for a connected network
Data gathering in a disconnected network
Performance evaluation
Conclusions

3. Introduction Static network architecture SenCar network architecture:
Sensors close to the BS consume much more energy than sensors at the margin of the network
SenCar network architecture:
To balance loading of each sensor node by sensor car can prolong network lifetime
Static network architecture
SenCar network architecture

4. Data gathering scheme for a connected network
Assumption
A SenCar will be sent out to gather data from sensors periodically.
Each sensor may turn on their transceivers only when SenCar moves close to its cluster, all sensors belonging to the same cluster will be woken up and will send or relay packet
a moving path of SenCar consists of a series of connected line segments

5. Data gathering scheme for a connected network
The problem is divided into three part:
Load balancing v.s network lifetime
Given a moving path of SenCar, the load balancing alg. can calculate the network lifetime
Path planning algorithm v.s network lifetime
Given set of candidate paths, we can choose the best path with maximum network lifetime by load balancing alg.
Clustering v.s network lifetime
The clustering algorithm is used to divide the network into clusters such that each node knows packet relaying path

6. Data gathering scheme for a connected network – Load Balance
A sensor network can be modeled as a directed graph :
:sensor set
A: all directed link:
, if si can reach sj in 1-hop
, if the moving path of SenCar traverses the transmission range of si, or, equivalently, si can reach SenCar in one hop while SenCar is moving
C: SenCar

7. Load balancing
A corresponding flow graph is constructed as follows: j

8. Load balancing : data generating rate of node si
The number of unit of traffic that si can relay
: data generating rate of node si
: energy limit of node si
:power consumption for generating a unit of traffic
:power consumption for relaying a unit of traffic
T : network lifetime
Total generate unit of traffic

9. Load balancing
For any given T, this problem is a regular maximum flow problem
When maximum flow means:
Until time T, all generated traffic from n sensor nodes is received by SenCar
i.e., all sensors must be alive until T
How to calculate network lifetime?
Incremental alg. to find max T:
We can increase T until the maximum flow less than :
Some nodes have failed before time T
How to increase T?
Since SenCar gathers data periodically, every time
we can set at the beginning and increase T by every time
The value T of the last run, which the maximum flow = ,is the network lifetime of the corresponding SenCar path
Ie.Car最多可跑幾次

10. Path planning
We assume that each sensor forwards one packet to SenCar, whereas SenCar moves from A to B
Node 1 is bottleneck: it must forward 8 packet toSenCar
A straight path is not well enough (X)
How to find some turning points of SenCar such the network lifetime can be prolong?

11. Path planning Determining Turning Points of the Moving Path:
Given the starting point A to the end point B:
To select a best path with the max network lifetime
Grid size is a fixed parameter

12. Path Planning Algorithm

13. Clustering
Clustering the Network along the Segments of the Moving Path:
To determine the direction of packet forwarding
Root只有一個和turning node無關
Shortest path tree

14. Moving path determination
Merge the approach of the three part by divide and conquer
clustering
determining the turning point
An example of 4 iteration:

15. Real world constraints
The sensed data must be gathered by SenCar before the sensor’s buffer overflows.
the maximum moving distance of SenCar without recharging may be limited by its battery capacity.
Delay sensitivity application
Solution:
For each turning point is added into path, all constraints must be satisfied.
The recursive moving path alg. could be terminated before the above bound is reached.

16. Avoiding Obstacles in the Sensing Field
For each candidate location of a turning point:
If the next turning point are blocked by the obstacles, then the candidate location is not eligible to be the turning point.

17. Data gathering in a disconnected network
Intra-cluster solutions + inter-cluster solution
Inter-cluster problem is NP-Complete
Inter-cluster solution can be found by exhaustive search or TSP solution
cluster1

18. Performance Evaluation
800 sensor nodes
Scheme 1—a static BS placed at the center of the network (at point (500 m, 250 m))
Scheme 2—a SenCar moves alg. straight line between (0 m, 250 m) and (1,000 m, 250 m)
Scheme3-well-planned

19. Performance evaluation

20. Performance evaluation
X percent network lifetime
(100-x)percent sensors either run out of battery or cannot send the data to the sink due to the failure of the relaying nodes

21. Performance evaluation

22. Conclusion
This paper proposed a new data collection mechanism to prolong network lifetime
This paper presented a heuristic algorithm for planning the moving path/circle of SenCar and balancing the traffic load

23. Thank You!!