## 1 Introduction  
The revolution in modern communication produced an innovative technology, the Internet of Things (IoT) [1; 2; 3], which can connect any physical objects all over the world and exchange data among them. These objects can be sensed and controlled through its infrastructure, which allows users to own all services across the world through small electronic devices. This technology is radically based on Wireless Sensor Networks (WSNs), because it provides users with the ability to interact with their environment and react to real-world events. It consists of a large number of tiny sensor nodes that are randomly deployed over a specified environment, to track a certain phenomenon and give the administrator the ability to instrument, observe, and react to it based on different types of routing protocols [4; 5]. These traditional WSNs are usually deployed to serve a certain application [6; 7; 8; 9; 10; 11; 12] or offer certain services, thus they are considered to be domain specific and task oriented networks. This leads to the redundant deployment of WSN infrastructure when we have to apply multiple applications over the same environment. A new emerging technology has been adopted to overcome this drawback; virtualization technology, it handles the abstraction of the deployed infrastructure from the offered services over it using different virtualization techniques to achieve different goals, as will be described later. By applying virtualization techniques over WSNs, it confers it the ability to be shared among multiple concurrent applications; this way, we will achieve better infrastructure utilization, these new networks are called Shared Sensor Networks (SSNs). There are many benefits for this emerging networks: First, as previously mentioned, it is a commercial solution that allows for multiple applications to share the same network. Second, the application developers will be isolated from the infrastructure technical details. Third, multiple vendors can work together under its umbrella. Fourth, a new type of electronic clouds will appear named cloud of sensors. Fifth, leasing infrastructure will be available. The idea of SSNs was surveyed in some previous papers, from different aspects, like in [13], where the authors considered the global meaning for each terminal of it without any actual technicalities. In [14], the authors provide readers a profound state of the art for SSNs, but they did not propose taxonomy for it. While, in [15], the authors provided a systematic review for it with a taxonomy based on the used mechanisms only, though they neglected the hybrid solutions for SSNs. This paper was decided to introduce SSNs from a basic perspective that is based on the used virtualization techniques, because, without virtualization, there will never be a SSN. Accompanying this taxonomy, we support readers by the terminologies that are presented and the appropriate diagrams to visually demonstrate architectures for SSNs. Thus, this article is concerned with describing these promising networks that will open a wide area for the tiny constrained devices to play a major rule in the fields of IoT, Cloud of Sensors, and Big Data. Therefore, it mainly targets the following:  
* Giving the reader a global vision of SSNs. * Clarifying the main differences between SSNs and traditional WSNs. * Designing a novel taxonomy for SSNs. * Designing a novel architecture to identify how the sensor node should be in the shared environment. * Designing a novel architecture to identify how the whole network should be to serve the shared environment. * Supporting readers with a comparative analysis between the available solutions along with the main differences, objectives and services supported by each type to help decision makers decide which type of SSNs is suitable for which situation. The rest of the article is organized, as follows. Section 2 provides a general overview of SSNs. Section 3 provides the novel taxonomy for SSNs that organize the types of SSNs that are based on the used virtualization techniques.