## 2. TYPES of Virtualization
### Network Virtualization  
Network virtualization is based on the virtual sensors and virtual links that allow multiple networks to coexist on the single physical platform. Sensor nodes can hardly perform any useful task if they are left isolated it is the overall collaboration between a number of devices that allows the system to accomplish a higher-level goal. Virtual networks can be formed by a subset of the physical network [27, 29]. Virtualization of sensor network could be the collaboration of different WSN using multiple radios. It is formed by the subset of versatile sensor nodes of the sensor network, with the subset or group of sensors being committed to certain tasks or facilitating certain application at a given time [29]. Network virtualization allows a subset of sensor nodes to execute an application, while at the same time another subset of sensor nodes execute a different application [1]. These subsets vary in number, size and processing capabilities with heterogeneous PHY and MAC layer protocols according to the application requirements. Network virtualization either combines many networks to act as one or shows one network to act as many in order to meet the service requirements of the users.  
Figure 3: Sensor virtualization: multiple applications using multiple services.  
Furthermore, as there are many stakeholders in WSNs with everyone aiming to achieve its goal thus heterogeneity has emerged as significant setback. However, on the other hand heterogeneity provides enhanced energy and communication capabilities [30,31]. Network virtualization promotes innovation and combination of heterogeneous node and radios. Moreover, sensor network virtualization opens new domains for business in this world of economic recession facilitating trading of sensor's infrastructure and network resources among different service providers [32].  
Network virtualization decouples both the Infrastructure provider from the traditional Service provider; thus virtualization provides flexibility and QoS opens new domains for business. The resources from the multiple infrastructure providers are aggregated on a single platform to facilitate the user with diverse application using a single service provider.  
Service providers can offer different services in a dynamic way on the virtual networks based the leased physical infrastructures, that is provided by the multiple infrastructure providers [27]. Furthermore, in the scenario of sensor networks the concept of sensor network virtualization is applied to separate service providers from the infrastructure providers to provide maximum flexibility, scalability and efficient utilization of resource. To achieve all this, research area for sensor network virtualization is still to be explored. Several challenges in the form of management and operations are being faced and the researchers round the globe are trying to solve these issues in an efficient way.  
Virtualization provides a platform upon which innovative sensor network architectures can be built, tested and assessed [21,27]. Network virtualization enables different multiple service providers to dynamically make multiple networks using multi radios that exist together, however separated from each other [27,33].  
Figure 4: Network Virtualization.  
Figure 4 shows multiple virtual sensors forming clusters based on the application demands. Each sensor is equipped with multiple radios. The infrastructure providers are responsible for the sensors and communication hardware. The service provider's job is to facilitate applications and user demands. The multiple infrastructure providers and service providers combine under SLA to facilitate users and applications. Furthermore, it shows multiple infrastructure providers and service providers running multiple applications and facilitating multiple users.