## 2 A General Overview on SSNs
### SSNs Architecture
#### 2.4.2 SSNs Network Architecture  
Multiple ideas were previously presented in this issue to provide different solutions in different fields, which, infrastructure-wise, are based on the idea of virtualization, like Cloud of Sensors (CoS), IoT, and SSNs. One of the early proposals in this field was [17], where the main HLV idea handles the separation between the sensor service provider and the owner of the sensor infrastructure, it demonstrated challenges and opportunities without any technical details regarding how to build the that environment. The platform of SenShare [20] proposed an overlay based solution that allows for multiple concurrent applications to run over the shared WSN, which is by grouping sensors that are separately associated with each application in a virtual way. One of the best proposals in this filed was a multilayer WSN virtualization architecture that is presented in [21,22], where it proposed an architecture of four layers, with the support of two data and signaling paths, also with five interfaces to define data and signaling priorities. The first layer is the physical layer, which represents the physical deployed sensors, followed by a virtual sensor layer that provides a logical sensor representation, the third layer is a virtual sensor access layer that provides a unified interface for the next layer, finally the upper layer is the overlay layer that represents a separate overlay for each hosted application, this architecture was implemented over a fire monitoring scenario to obtain some measurements that were presented to study the overlay creation delay, sensor sending delay and the delay of fire alert notifications from the deployed sensors. The results show that most of the delays occurred due to LAN setting. This architecture was extended in [23] to give it the ability to interact with cloud systems, the authors also implement a prototype for this extension and they found that cloud computing eases the service addition and management over virtual networks, but many mechanisms are still needed. The concept of fog computing was introduced in [24] due to the non-scalability of the previous solution, where the proposed architecture is based on the master and slave layers, the first one being responsible for control functions and the second being responsible for flow and resource managements using virtual gateways, but there were not any maintained simulation or implementation for this work. In the field of CoS many solutions were presented to provide the sensor virtualization that is required as a base for building the sensor cloud as in [25], where a three layered architecture was presented: sensor centric, middleware, and client centric. The sensor centric layer represents the physical network deployment, the middleware layer represents the virtual sensors and the required management for them, and finally the client centric layer that represents the multiple user interface. Some studies are concerned with the network hypervisor itself, like in [26], it studied it as a corner stone for the technology of CoS, and proposed an architecture that describes its functionality in managing, abstracting, and virtualizing the resources of the deployed infrastructure, under three main units, which are: the virtualization unit, abstraction unit, and node manager unit. One of the most recent ideas for HLV was SoftWater architecture, which introduces the virtualization technology in the field of underwater communication [27]. It provided the network virtualization based on data plane and control plane. Data plane is an open, programmable, and virtualized network forwarding infrastructure, while the control plane aids the tools for network management and application arrangement. Virtualization SoftWater uses two hypervisors: the first one is the network hypervisor that manages resource allocation and management, while the second is wireless hypervisor that handles resource scheduling and policies management.