## 2 Related Work
### Location Service for a Location-based Approach with Mobile Sinks  
Several location-based routing approaches have been considered to address effectively the location tracking problem. However, most of the existing studies on location service protocols were designed for MANETs [17; 18]. Directly employing them in WSNs is ineffective. Location-based routing has been considered to address the issue of providing locations as an effective routing approach in WSNs [19; 20; 21; 22; 23; 24]. If the sensor node's location information is found, most applications can use this information to make routing decisions. Greedy Perimeter Stateless Routing (GPSR) [26] is a geographic routing system for multi-hop wireless networks. If a source node can identify a destination node's location, GPSR can forward a packet to the nearest neighbor node in order to reach the destination node. A locator [20] has been proposed as a data dissemination architecture to support mobile sinks. Once a sensor node wants to report its sensed data to sinks, the source can obtain the current sinks' locations from the locators. These locators are selected for the sensor fields using a deterministic geographic hash function and are replicated uniformly. Railroad [21] has proposed a data dissemination architecture by proactively exploiting a virtual infrastructure called rail. This rail is placed in the middle of the sensor field where all the metadata from the event data are stored. The rail acts as a meeting area for the events and queries. Once a source node detects an event, the data remain stored locally and only relevant metadata are sent to the rail. Then, the sink can retrieve metadata with the queries circulating around the rail. When corresponding metadata is found, the source node of the data transmits the corresponding data to the sink node that has issued the query. Hierarchical Location Service [22] has been proposed as a scalable hierarchical location service in which the location updating structure for the mobile sink is hierarchically built, which can reduce the updating cost. Further, the overall updating cost in WSNs with multiple sinks is greatly reduced by enforcing Voronoi scoping in which each mobile sink only updates its location to the location servers closest to it. Perhaps the most similar protocol to SDLS is the Anchor Location Service (ALS) protocol [23]. ALS is a grid-based protocol that provides the location information of the sink. ALS constructs a single global grid structure, assigning the sensors nearest the grid points as grid nodes. The sink selects the nearest grid node as its sink agent to distribute its location information. At the beginning of the process, the sink agent sends out four anchor setup messages in four straight orthogonal directions (North, South, East, and West) and all the grid nodes that lie along the routing path are recruited as anchors. Once an event occurs, the source node will register with the nearest grid node, known as the source agent. The source agent will then send four location query packets to find the location of the sink agent. It then receives the location of the sink agent. The source node finally sends the data packets to the sink agent using GPSR protocol. However, it is difficult to identify the location of a moving sink. The following problem occurs when the sink constructs an anchor system. First, the source node cannot identify the location of the sink if the sink does not construct an anchor system. Second, the sink has to wait until the sensor node detects the event. Third, when the sink has high mobility, the sink agent has to relay the data from the source node to the sink. Thus, the distance between the source node and the sink may increase significantly when a detour occurs. ALS also suffers from concentrated energy consumption because it always uses border nodes when it provides the sink location. Therefore, it is necessary to propose an energy efficient protocol to balance sensor node energy consumption.