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 2007 (TPDS 2007)

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

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

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

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

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

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

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

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

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?

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

Path Planning Algorithm

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

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

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.

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.

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

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

Performance evaluation

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

Performance evaluation

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

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