## 1 Introduction  
Recent developments of wireless communication and Micro Electro Mechanical Systems (MEMS) technologies have made possible the deployment of large-scale Wireless Sensor Networks (WSNs) [1]. WSNs can be used for a wide range of applications in the scientific, commercial, medical, and military battlefield, industry control, traffic control, and ambient conditions detection areas [1; 2; 3]. Typical WSNs are composed of a large number of sensor nodes which transmit the sensed information to the sink. Since a sensor node is constrained by a device with limited power supply, recharging sensor nodes is often infeasible. Energy efficient algorithms are one of the most important challenges in large-scale WSNs, since sensor nodes have restricted energy. For example, WSNs may not fulfill their function normally if some sensor nodes fail due to insufficient power. Therefore, it is important to reduce the energy consumption of sensor nodes and to maximize the lifetime of the entire network [4; 5; 6; 7; 8; 9; 10]. Most existing studies on data dissemination protocols in WSNs are designed for stationary sensor nodes and sinks [4; 5; 6; 7; 8; 9; 10]. Although these protocols have not been designed to provide sink mobility, the mobile sink is used by current applications such as mobile phones and PDAs carried by itinerant users. In the mobile sink environment, the connection of the last hop to the mobile sink will be broken if it does not promptly reflect the updating of the relay path from the mobile sink. Many researchers have recently studied the problem of scalable and efficient data dissemination in large-scale WSNs from multiple sources to multiple mobile sinks [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Let us consider a monitoring application as an example. Assume that soldiers collect the information on the enemy's emergence from WSNs deployed in a battlefield (see Figure 1 for an example). A sink is a soldier that collects the sensed information from WSNs. WSNs may not execute their function normally if some sensor nodes fail due to energy exhaustion. Therefore, it is important  
Figure 1: A sensor network example. Soldiers use the sensor network to detect tank locations. to reduce the energy consumed by the energy-limited sensor nodes that continuously provide the data reports to soldiers in a battlefield. This scenario motivates us in this paper. In this paper, we propose a Sink-oriented Dynamic Location Service (SDLS) protocol to handle sink mobility. SDLS provides a complete solution to support both sink mobility and energy conservation. To the best of our knowledge, this is the first work on a Sink-oriented (user-oriented) data dissemination approach for supporting general data delivery to multiple, mobile sinks in WSNs. Our contribution is as follows:  
1. We propose a global-grid structure to avoid repetitively constructing an individual grid structure for each source node. The global grid structure helps reduce overall energy consumption. A sink can use the global grid structure to identify the location of the source node whenever it wants to do so. 2. We propose an Eight-Direction Anchor (EDA) system that acts as a location service server. EDA prevents intensive energy consumption at the border sensor nodes; consequently, the energy consumed is well balanced for all the sensor nodes. 3. We propose a Location-based Shortest Relay (LSR) that efficiently forwards (or relays) data from the source node to the sink with minimal path delay. Our protocol does not need a global grid structure anymore, once a sink identifies the location information of the source node by using global grid structure. 4. We evaluate performance on a JAVA implementation of SDLS. We demonstrate that the proposed protocol not only reduces communication costs, but also balances energy consumption. The remainder of the paper is organized as follows. Section 2 introduces related work supporting mobile sinks. Section 3 details the SDLS protocol and Section 4 evaluates performance. Finally, Section 5 concludes the paper.