## 2 A General Overview on SSNs
### Fundamental Requirements for Building SSNs  
There are many solutions that are provided by virtualization to convert the application oriented WSNs into the multi-concurrent applications SSNs; these solutions must support the following requirements [14,16] to achieve this goal:  
* Coexistence: the physical infrastructure of SSNs should have the ability to understand the hypervisors, which gives it the ability to convert the actual resources to virtual ones. In SSNs there are two layers of coexistence. The first one is over the sensor node itself and it is called Low Level Virtualization (LLV); it ensures that the sensor nodes can support the concurrent execution of multiple applications. The second one is over the whole network and called High Level Virtualization (HLV); it is concerned with the ability of sensor nodes to dynamically form groups to perform the isolated and transparent execution of application tasks in such a way that each group belongs to a different application. * Heterogeneity: it should be platform-independent that allows for multiple independent sensor vendors to coexist in the same physical infrastructure. Additionally, it should be able to hold various protocols and algorithms implemented by different SVNSPs, and different Quality of Service (QoS) requirements, topologies, security levels, service types, and etc. * Resource Discovery: there must be a resource discovery mechanism to discover the available resources for allocating them to build a real shared environment. Thus, they could be managed and reserved for applications. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Criteria** & **WSN** & **SSN** \\ \hline Component & Sensor nodes & VSNs \\ \hline Ownership & Single Authority & Multiple Authorities \\ \hline Platform Dependency & Homogeneous platform & Heterogeneous platform \\ \hline Virtualization & Not support & Support \\ \hline Code Modularization & Single Code & Multiple Codes \\ \hline No. of Apps & Single & Multiple \\ \hline Connectivity & Physical & Logical \\ \hline Resource Sharing & No & Yes \\ \hline Redundancy & Yes & No \\ \hline Infrastructure/Service & One identity & Two identities \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main differences between Wireless Sensor Networks (WSNs) and Shared Sensor Networks (SSNs). Resource Sharing: it should be able to share all network resources (wireless radio, hardware, and software) over the network based on the rights given to the end user. * Isolation: it must ensure that the coexisting VSNs are isolated from each other to guarantee privacy, security, and fault- tolerance, thus any configuration changes, topology changes, customization, or faults over one VSN do not affect the others. * Flexibility: service providers should have enough authorities over the assigned region for them inside the SSN to change different network aspects, such as network topology, routing, control protocols, and forwarding functionalities, which are independent of the underlying physical network and also without affecting the other service providers. * Scalability: infrastructure providers should have a scalable physical network, which allows for modifications, such as adding VSNs to support the expected increment in coexisting VSNs, without affecting the given performance. * Manageability: due to the separation process between the infrastructure provider and the service provider, the manageability to the granted resources must be given to the service provider, to allow for him to manage, configure, and allocate VSNs, e.g., routing tables, virtual resource scheduling, admission, and even modifying protocols, etc. * Programmability: a service provider needs programmability to implement customized protocols and deploy diverse services, according to the end user application requirements under secure programming paradigms with a considerable level of flexibility. * Mobility: it should support the internal mobility between virtual resources in the same infrastructure and the external mobility between multiple service providers.