# An Energy Efficient Sink Location Service for Continuous Objects in Wireless Sensor Networks
## 2 Related Work  
WSN is used for various applications such as forest fire monitoring, gas leakage monitoring, and radiation monitoring because it is easy to use and provides cost-efficiency and flexibility compared to wired networks [16]. For example, [PERSON] et al. proposed a forest fire detection system using WSN that consists of three functional blocks: fire detection, sending alerts, and intervention [17]. In the system, the wireless sensors detect ambient temperature and the value of CO2. When the sensor data exceed a predefined threshold, the alerts are sent to the base station thereby leading to the intervention of forest guards. Figure 3: Location update of a mobile sink in individual object tracking and continuous object tracking: **(left)** Individual location update and **(right)** Integrated location update. Therefore, for building the disaster management system, continuous object tracking is the fundamental functional requirements of the WSN. In wireless sensor networks, several protocols to detect and track continuous object have been developed. They focus on how to efficiently detect the continuous object and reduce the amount of reports about continuous object toward a static sink. On the other hands, many protocols to support sink mobility have been proposed with the aim of disseminating data to mobile sinks. In this chapter, we briefly introduce continuous object tracking schemes and evaluate the protocols for sink mobility support with features of continuous object tracking. According to [13; 18], sink mobility is the cause of balanced energy consumption of sensor nodes inside a network. Data sensed by distributed sensor nodes are transmitted through multi-hop communications of intermediate nodes between a source and a sink. The nearer sensor nodes to a sink receive, forward, and propagate a more amount of data if the location of the sink is fixed. The nearer sensor nodes therefore consume more energy and more rapidly die out. In other words, their lifetime is more shortened than that of nodes far from the sink. Consequently, died sensor nodes around the sink can not play their role of communications and sensor, and functions of the network can also stop. This phenomenon is called, the hot-spot problem [13; 14]. On the contrary, sensor nodes around sinks do not keep consuming energy if sinks change their location frequently. Such the hot-spot problem can be overcome by adopting mobile sink. The sink mobility also might be adopted by the nature of the deployed application [19]. For example, in the scenario in Figure 1, both the fire fighters and climbers act as mobile sinks. A number of WSN researchers have interested in supporting sink mobility [19]. They assume that sinks might be either stationary or mobile in the sensor field and mobile sinks might move around a sensor network with random mobility [20]. In other words, a static sink is a stationary object providing sensor nodes with its fixed location information as a priori knowledge, while a mobile sink is a moving object frequently updating its location to the sensor nodes. Figure 4 shows change of route for data dissemination before and after a sink movement. In such a context, the difficulty for sensor nodes is to efficiently update the mobile sink location to report sensor data. In many researches, overlaying a virtual infrastructure over the physical network has been investigated as an efficient strategy for an effective data dissemination to mobile sinks [21; 22]. They use the concept of virtual infrastructure, which acts as a rendezvous area for collected data from sources and location update messages from mobile sinks. The nodes belonging to the structure designated to store reported data from sources during the absence of a mobile sink. After the mobile sink updates its  
Figure 4: Data dissemination route before and after the mobile sink moves. current location to the structure, the designated nodes report data to the sink. Virtual infrastructures are divided into two categories: _source-based structure_ and _independent structure._