# End-to-End Service Availability in Heterogeneous Multi-Tier Cloud-Fog-Edge Networks
## 3 Materials and Methods
### Multi-Tier Systems
The air domain consists of drones, aircraft, and other aerial vehicles. These airborne assets provide a dynamic layer of connectivity capable of rapidly adjusting to changing network demands. For areas where terrestrial networks might be momentarily compromised, perhaps due to natural calamities, these aerial entities can swoop in, providing immediate, albeit temporary, connectivity. They act as bridges, ensuring that communication remains uninterrupted during transitions or disturbances. Terrestrial or the ground domain in SAGIN remains the primary backbone for most communication needs. From massive data centers to intricate networks of fiber-optic cables, the ground-based infrastructure supports the bulk of data processing, storage, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer of Space** & **Proximity** & **Characteristics and Functions** & **Applications** \\ \hline \multirow{4}{*}{LEO Layer} & Closest to the Earth, typically at altitudes between 160 and 2000 km. & Quick data transmission due to low latency, high-resolution Earth imaging. Often involves constellations for continuous global coverage. & Internet service in remote areas, Earth observation, space research, and scientific studies. \\ \hline \multirow{4}{*}{MEO Layer} & Occupying a mid-range & Acts as a bridge between the & Primarily home to global navigation satellite systems (GNSS) like GPS, GLONAS, and Galileo. Also utilized 2000 and 35,786 km. & Accurate of GEO, balancing latency and coverage. & for certain communication purposes, especially in areas lacking infrastructure. \\ \hline \multirow{4}{*}{GEO Layer} & Positioned at an altitude of & Provides a fixed view of a particular & Ideal for stable communication links, region of the Earth, facilitating & TV broadcasting, weather monitoring, surveillance, and defense systems. \\ \cline{1-1}  & exactly 35,786 km above the equator. & & \\ \cline{1-1} \cline{2-4}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Layers of the satellite communication system. Figure 2: Space–Air–Ground Integrated Network. and transmission. With the rise of smart cities and intelligent transportation systems, the ground becomes a bustling hub of IoT devices, all communicating, processing, and making decisions in real time. The ocean domain dives deep, quite literally. With underwater sensors, submarines, and aquatic drones, this layer seeks to harness the vastness of the oceans. Whether for marine research, tracking underwater pipelines, or ensuring the safety of shipping routes, the ocean level provides a crucial link, allowing for real-time data collection and communication even in the most remote marine environments. The multi-level design of SAGIN is not merely about its spatial organization but also about its functionality symbiosis. Each level, while distinct in its operations, complements the others in one common network. In the SAGIN architecture, the integration of physical systems with digital network hierarchies like cloud-fog-edge-IoT presents a fascinating yet intricate challenge. Each segment of SAGIN has specific functionalities and plays a vital role in ensuring seamless communication. 1. Cloud Level. The cloud represents the most abstract and vast digital storage and computational capacities. In the context of SAGIN, it includes: * Space. Ground-based control centers for space objects can interface with cloud platforms to handle vast amounts of data from satellites, ensuring global coverage and communication with remote satellites. * Air. Major control centers for managing large-scale aerial operations, such as fleets of drones or aircraft, rely on cloud infrastructure for coordination and data analysis. * Ground and Ocean. Large data centers, tasked with handling terrestrial and marine data, connect directly to the cloud. 2. Fog Level. Fog computing acts as an intermediate processing layer, residing closer to data sources: * Space. Nearby satellite clusters can communicate and process data in a local fog network before sending data down to Earth or to another satellite cluster.