# An Energy Efficient Sink Location Service for Continuous Objects in Wireless Sensor Networks
## 2 Related Work
### Independent Structure  
Meanwhile, several researches exploit independent structure for supporting sink mobility [22; 23; 24; 25; 26]. The researches assume that each sensor node knows its geographic location as well as the network geographic boundaries. A node position can be obtained through Global Positioning System (GPS) or  
Figure 5: Examples of object tracking in TTDD: (**left**) Individual object tracking and (**right**) continuous object tracking. some virtual coordinate system [27; 28]. Eligibility of nodes comprising the structure is easily performed based on this geographic information. Thus, the overhead for building the virtual infrastructure is avoided. The location of independent structure is known as a priori knowledge to sensor nodes and sinks so all nodes including both sensors and sink might easily access it. The most representative is A Line-Based Data Dissemination protocol for wireless sensor networks with mobile sink (LBDD) [22]. In LBDD, the independent structure is vertical virtual \(line\) which divides the sensor field into two parts, as shown in Figure 6 left. The line has \(w\) wide and consists of \(h\) sized groups. The line is placed in the center of the sensor field so that sensor nodes and a mobile sink might easily access it. The nodes within the line are \(inline-\)\(nodes\), while the other nodes are referred to as ordinary nodes. This line acts as a rendezvous region for data storage and lookup. When an ordinary sensor node detects an individual object and generates new data, it forwards the data to the nearest inline-node(1) in Figure 6 left). The inline-node shares the data with the other inline-nodes in its group. In order to collect data, a mobile sink sends a query including its current location towards a horizontal direction(2) in Figure 6 left). The first inline-node which receives the query propagates it to both vertical directions along the line until it reaches the inline-node storing the data. The data is then sent directly to the sink(3) in Figure 6 left). Even though LBDD has the possibility of detouring, it is very simple and efficient protocol for supporting sink mobility in individual object tracking. However, in continuous object tracking, LBDD might lead to a serious concentration of messages. Inherently, all messages generated in the sensor field, including data from sources and queries from sinks, pass through the line area like in Figure 6 right. The sensor nodes on the structure receive, forward, and propagate a more amount of messages because the structure is fixed as a certain shape. The sensor nodes therefore consume more energy and more rapidly die out. In other words, their lifetime is more shortened than other nodes. Consequently, died sensor nodes on the structure do not play their role of communications and sensor, and functions of the network can also stop. Namely, the hot-spot problem is occurred. Existing continuous object tracking studies focus on reducing reporting cost considering a static sink. To support sink mobility, the adoption of existing sink mobility support researches is a reasonable solution. The researches provide sink mobility efficiently in individual object tracking by overlaying virtual structures. However, in continuous object tracking, they cause critical problems shown as following Table 1. In continuous object tracking, source-based structure consumes enormous energy for constructing the structures proportional to the number of sources. On the other hands, indepen  
Figure 6: Examples of object tracking in LBDD: (**left**) individual object tracking and (**right**) continuous object tracking. one structure so it consumes constant energy for constructing the structure. Therefore, independent structure is appropriate in terms of the construction cost for structures. Meanwhile, independent structure might lead to the hot-spot problem because the all messages generated in the sensor field are passing through the structure.