# End-to-End Service Availability in Heterogeneous Multi-Tier Cloud-Fog-Edge Networks
## 3 Materials and Methods
### Multi-Tier Systems
* Air. Aircraft and high-altitude drones can have onboard fog computing systems to process data in real time before sending essential data to the ground or the cloud. * Ground. Infrastructure like cellular towers or regional data hubs. For the marine context, surface vessels or buoy systems can have fog systems to handle localized oceanic data. 3. Edge Level. This level is closer to the data source and often responsible for more immediate, time-sensitive computations: * Space. Satellites, especially those in low Earth orbits, are edge devices gathering and sometimes processing data before sending data to the ground or across the satellite network. * Air. Individual drones or low-altitude aircraft can act as edge devices, making real-time decisions based on immediate data, like adjusting flight paths for obstacles. * Ground. Roadside units (RSUs) in intelligent transportation systems, regional communication hubs, and even individual buildings in smart cities operate at the edge level. * Ocean. Submarines or deep-sea drones equipped with sensors and communication tools act as edge devices, processing underwater data in real time. 4. IoT Level. This is the level at which data are primarily gathered. * Space. There is not a direct IoT equivalent in space in the traditional sense. However, individual sensors on satellites that pick up specific types of data can be seen as IoT devices. Air. Sensors on drones or aircraft, which gather data like atmospheric conditions, camera feeds, etc., are the IoT devices of the air segment. * Ground. In a smart city, for example, traffic cameras, environmental sensors, or even devices in our homes are all IoT devices. In intelligent transportation systems, vehicular onboard units (OBU) operate at this level. * Ocean. Primary sensors placed on the ocean floor, those attached to marine animals for tracking, or sensors on floating buoys fall under the IoT level. The SAGIN system's intricacies and complexities make it a fascinating model for multi-level digital networking. The alignment of its physical systems with the cloud-fog-edge-IoT hierarchy ensures optimal data processing, transmission, and storage, regardless of where the data originate, be it space, air, ground, or ocean. While SAGIN provides a comprehensive framework integrating multiple domains, its true strength lies in its adaptability. Depending on the application, this network can be molded, emphasizing certain layers and domains over others, ensuring optimal performance, responsiveness, and efficiency. Depending on the specific application, different SAGIN domains (space, aviation, terrestrial, oceanic) can form different configurations of the network architecture, which in general can be represented by a hierarchical structure with a different number of sublevels in the classical three-level cloud-fog-edge system. Without differentiating between main levels and sublevels, and simply viewing the architecture as a hierarchy of \(n\) distinct levels, the network configuration from an application operation point of view can be presented by the model shown in Figure 3.  
In such a model, you only need to keep track of each level and its associated nodes. Let us represent a node in the hierarchy using two indices:  
--level in the hierarchy  
--index within that level  
So, node \(N_{ij}\) would be represented as \(j\) node in the \(i\) level. Without differentiating between main levels and sublevels, and simply viewing the architecture as a hierarchy of \(n\) distinct levels, the network configuration from an application operation point of view can be presented by the model shown in Figure 3. In such a model, you only need to keep track of each level and its associated nodes. Figure 3: The multi-level cloud–fog–edge architecture. The hierarchy of the architecture can be represented by a set of levels \(L=\left\{L_{i}|i=\overline{1,n}\right\}\) and set of nodes \(N=\left\{N_{ij}\middle|i=\overline{1,n};j=\overline{1,k}\right\}\), where \(i\) represents the level in the hierarchy and \(j\) represents an index within that level.