## 2 A General Overview on SSNs
### Challenges behind Design of SSNs  
Despite the numerous opportunities for SSNs, its spreading is still constrained by several challenges [16; 17]; this section will briefly describe some of these challenges, as follows:  
* Isolation: is a basic issue in SSNs as it allows the logical separation of VSNs, while they coexist in the same physical infrastructure. It ensures that the technical changes occurring in one VSN (configuration, customization or topology change) do not affect other VSNs. * Interfacing: is the connectivity description among different entities overall the SSN as the interfaces between different infrastructure providers under the same SSN, which is necessary in information sharing among them and interfaces between infrastructure provider and service provider required before creating the VSNs and to express their requirements of resources to serve end users. Additionally, the interface between the service provider and the end users is needed to explain the communication between them. Thus, the interfaces should consider communication delays and reliability among diverse entities and should also be carefully designed to become compatible with different kinds of requirements. * Constrained sensor resources: as known sensor nodes are resource constrained devices, this problem affects the design of virtualization solutions. * Embedded VSN: refers to the static and dynamic allocation of virtual sensors and it links the physical sensors and paths between them. This issue is a big challenge due to the constrained sensors resources. Add it to the previous point (it not related to it)
* Resource discovery: due to the large number of resources over SSNs, which are distributed over multiple infrastructure providers, there will be a real need from infrastructure providers to discover the active and passive resources in their physical network, so that may reserve some resources for its own use and to be able to share reachability information among them to establish links (interfaces) between them. * Resource Allocation: the SSNs resource allocation schemes are responsible for deciding how to get an optimal placement of a VSN over the physical sensors' network. The users may require resources that are placed in multiple VSNs, in this case, the service provider must assign them to that user while hiding the details from him, and this issue needs a strong management mechanism. * Resource Scheduling: due to the large number of shared resources over the SSN, the infrastructure providers need a strong mechanism to schedule them for the reservation process of the hosted applications. * Naming and Addressing: as described above, the SSN is a heterogeneous system, which hosts different technologies, shares different resources and offers different services which are concurrently served by it; thus, the physical and logical network representation should adopt a strong addressing scheme that identifies these diverse system identities. * Security: a SSN is based on abstracting the real network infrastructure into multiple virtual sensor networks to provide concurrent services over them to a large number of users, thus SSNs must provide strong authenticity mechanisms to give each network entity (IP, SP, app. End user, etc.) enough privileges to execute its dedicated tasks without affecting the privacy of other entities. * SSNs Marketing: since SSN is an emerging paradigm of sensor technology, its economic model is still an open issue, because it contains multiple entities that share it, starting with the infrastructure provider, which will sell the physical network until the service provider will sell the service to the end user, each entity will share the market model in its own responsibility. * Controlling Shared Resources usage: infrastructure providers should provide an accessing mechanism to control access to its physical network, that is to improve the security and guarantee that the reserved resources to the service provider will not exceed the physical structure.