# An Energy-Efficient Approach to Enhance Virtual Sensors Provisioning in Sensor Clouds Environments
## 1 Introduction  
A Wireless Sensor Network (WSN) can be described as a set of small electronic devices (called sensor nodes) capable of sensing environment conditions (such as temperature, humidity, and light), processing and transmitting data wirelessly through a transceiver unit. These devices can also actuate on the environment, according to its programming. WSN has successfully been applied in several areas, such as healthcare [1, 2], military [3], infra-structure surveillance [4, 5, 6], and environmental monitoring [7, 8, 9, 10]. However, WSN have several constraints, mainly due to the nodes limitations [11]. In most cases, the sensors nodes have limited processing capability, memory, and bandwidth, and usually are battery powered [12]. These constraints hinder the development of WSN applications [13]. For example, WSN generally perform a specific task and the data monitored cannot be shared between users from different WSN. As a result, in recent years, researchers are taking advantages of cloud computing [14] to leverage the full potential of the WSN. The joint of these two technologies have given birth to the sensor cloud field. According to [PERSON] et al. [15], a sensor cloud can be defined as a computing environment comprised of several wireless sensor networks (WSN) from different providers (owners), which may be distributed over a large geographical area. The cloud acts decoupling the physical sensors nodes from the users interested in the monitored data by managing virtual sensors. A virtual sensor is an emulation of a sensor that obtains its measurements from the physical sensor nodes present in the sensor cloud environment. Hence, multiple WSN can interoperate in order to attend single or multiple applications, and sensing activities can be provided as services for users, allowing them to acquire/purchase services on demand [16]. For example, a user interested in temperature and humidity measurements from a specific geographic location can request an instance of a virtual sensor in the sensor cloud and deactivate that instance when it is no longer needed. Thus, the user pays only for the resources they use. Figure 1 shows a typical sensor cloud infrastructure formed by three layers: the user-layer, middleware, and sensor-layer [15]. The user-layer serves as a gateway point for users while the middleware is responsible for managing the interactions between these users and virtual sensors. In Figure 1, three users interact with the sensor cloud by their instance of a virtual sensor. The sensor-layer is responsible for the registration and maintenance of sensor networks and the collection of data/measurements transmitted by the sensor nodes of these networks. Each WSN has an owner (organizations and people) and when a new owner decides to join the cloud, he must first publish his sensors nodes via some kind of metadata (for instance SensorML ([[http://www.opengeospatial.org/standards/sensorml](http://www.opengeospatial.org/standards/sensorml)]([http://www.opengeospatial.org/standards/sensorml](http://www.opengeospatial.org/standards/sensorml)))). In the same sense, the owner can stop the publishing when he does not want to share the sensor nodes anymore. The middleware layer is usually represented by an application server that receives all user queries (from the user-layer). The middleware reviews the user-defined parameters (such as sensing frequency, region of interest) in the query and checks for physical sensors (present in the sensor layer) capable of meeting these parameters. In the case of success, the middleware must decide, by a process called provisioning [15], which physical sensors should be allocated to form the virtual sensor. The virtual sensor is then bound to the query. If no sensors are available to process the query, the user should be informed. In this way, the user can try a new query based on different parameters. Figure 1: Conceptual model of a sensor cloud. Over the years, several proposals for specific middleware for sensor clouds environments have been published.