## 2 Virtualization in Wireless Sensor Networks  
Technically, virtualization is set up by decoupling the application services from the underlying physical network so that the application services are not directly connected to their corresponding network elements, but are rather connected via logical/virtual linkages spread across the entire network. In most traditional schemes, the middleware and virtual machine approaches have been mostly used in WSNs to achieve virtualization at the network and node level, respectively. Virtualized WSN (VWSN) are typically applied in some recent application areas such as in smart industrial applications [14]. Due to the scale of these applications areas, for example in smart industry applications, it has become pertinent for resource sharing to be the most likely approach for deploying WSN services. Sharing of resources with less individual ownership has become the global trend [15; 16], thus giving way to more resources being efficiently managed by on-demand platforms. Such shared resource management schemes have been found to be successful, for example with the Uber scheme [17], and with the hospitality scheme using Airbnb [18]. These schemes, some of which depend on sensor network infrastructures, are normally shared by a variety of users and are accessed using different platforms, for example via specialized operating systems, virtual machines, middleware and other sharing platforms [4]. In most application areas, the ubiquity of the deployed sensor nodes places a great demand on the available shared resources. Thus, this growing demand has made the concept of virtualization of these services a highly beneficial technology for enhancing resource sharing among different stakeholders. An application area that may greatly benefit from VWSN is the smart industry application area. An example of the smart industrial concept is shown in Figure 1. It is seen from Figure 1 that by introducing a heterogeneous sensing layer, the network virtualization layer is thus able to provide fewer nodes for deployment in different sensing applications while meeting all the necessary sensing requirements. In developing resource sharing frameworks, lightweight architectures are normally used to initiate VWSN for deployment in smart industrial applications. Essentially, these resource sharing frameworks normally include a sensing layer, a virtualization layer, cloud services and an end-user access. The sensing layer typically consists of a number of different sensing scenarios under different application domain areas [19]. The concept of VWSN in smart IIoT presented in Figure 1 aims to reduce the number of nodes required for sensing deployment. As such, the gateways are equipped with virtualization layers, which enable several application tasks to run on the gateway instead of on the sensor nodes. Thus, a great deal of load balancing is provided by the resource rich gateway for the sensor nodes. The virtualization layer consists of resource rich gateways for the WSN. These gateways consist of a various communication standards based on IEEE 802.15.4 [20], 802.11 [21], 802.3 MAC and sub-GHz open and proprietary standards, which include 6 LoWPAN [22], ZigBee [23], Bluetooth Low Energy [24], Wi-Fi, Thread [25], and ZWave [26]. The gateway is also responsible for running multiple instances of the various sensing applications on the node, while giving the illusion that only one application is accessing the gateway at a time. The gateway (or sink) is often able to provide load balancing, resource management, network discovery and able to offer a secure platform for shared infrastructure. Enterprises, governments, municipalities and civilians can thus share the data obtained from the different WSNs. Furthermore, in order to perform a particular sensing function, the number of deployed sensor nodes at the sensing layer will reduce because they are able to share a common aggregated VWSN without changing the underlying hardware network. The also supports improved scalability and reliability of the network [27].