# An Energy Efficient Sink Location Service for Continuous Objects in Wireless Sensor Networks
## 1 Introduction
* Reducing data report cost: Because a continuous object is a very large phenomenon and continuously diffuses, the detecting information generated by a number of sources brings considerable communication overhead for reporting data. The overhead might shorten network life time. To prolong network lifetime, the data report cost have to be reduced. Figure 1: An example of continuous object tracking: Forest fire monitoring system. The existing continuous object tracking studies [3; 4; 5; 6; 7; 8; 9; 10; 11] focus on energy-efficient reporting. The primary issue in the studies is reducing the number of reporting nodes without distortion and loss of detecting information. The studies exploit data aggregation by clustering or election of representative by local cooperation. However, the existing studies are inapplicable to the above-described scenario in terms of supporting sink mobility because they have not dealt with mobile sinks. The satisfaction of sink mobility means a mobile sink can collect data from sources despite the mobile sink moves continuously. To collect data, a mobile sink has to periodically notify its current location to the sources because it moves continuously. Intuitively, the most simple approach is the flooding. A mobile sink could notify its current location by flooding the location to whole network like Figure 2 left. However, periodic flooding for location notification of a mobile sink brings the waste of bandwidth and energy of the network called the broadcasting storming problem [12]. Instead of using flooding, a mobile sink can receive data from an intermediate agent like Figure 2 right. Assuming the agent is more powerful than sensor nodes and always knows the mobile sink's current location (e.g., a static sink can be the agent), the mobile sink can receive data from the agent without notifying its location to sources. The sources just forward data to the agent. However, in this case, a large number of data packets pass the area between the agent and the mobile sink. This concentration lead to both congestion and hot-spot problem [13; 14]. To avoid the problems in flooding-based or intermediary-based approach, the adoption of existing studies for supporting sink mobility is a reasonable alternative. One of the most important issues in sink mobility support studies is how to notify the current location of a mobile sink. However, the studies are not suitable in continuous object tracking because they have dealt with not continuous objects but individual objects. In case of tracking individual objects like Figure 3 left, the studies provide the location updates between a mobile sink and each source. Likewise, in case of tracking a continuous object like Figure 3 right, the studies provide the location updates between a mobile sink and each source detecting the continuous object individually. The individual location updates lead to significant energy consumption, and consequently, shortening the network lifetime. From the view of the continuous object tracking, the location updates between a mobile sink and a large number of sources are unnecessary because the sources detecting a continuous object are contiguous each other. Figure 2: Intuitive solutions for supporting sink mobility: (**left**) Flooding-based approach and (**right**) Intermediary-based approach. Our aim is to design an energy efficient sink mobility support scheme for continuous object tracking. The previous continuous object tracking researches focus on reducing communication cost of reporting data from a number of sources without considering of sink mobility. Unfortunately, the existing studies for supporting sink mobility are inappropriate in continuous object tracking because they have dealt with individual objects. To facilitate advanced continuous object tracking, the sink mobility support scheme being suitable for continuous object tracking is required. The main problem in existing sink mobility support studies is the individual location updates to the sources detecting a continuous object.