# End-to-End Service Availability in Heterogeneous Multi-Tier Cloud-Fog-Edge Networks
## 3 Materials and Methods
### Multi-Tier Systems  
In the realm of geographically distributed networks, as the demand for real-time processing and localized decision making has surged, the classical three-level architecture (cloud-fog-edge) has sometimes proven inadequate. This has led to the evolution of more nuanced, adaptable architectures known as multi-tier systems. Multi-tier systems introduce additional processing layers between traditional levels, thus enhancing the granularity of data management and processing. Here are some case study examples of multi-level cloud-fog-edge architectures:  
* Smart City. A metropolitan area can have multiple tiers of fog infrastructure. A neighborhood fog layer of gateways and micro data centers connects into a wider city-level fog system for broader connectivity and storage, managed by a municipal IT department. Both feed into a regional cloud data center. * Private 5G Network. For a large corporate campus or industrial site, Wi-Fi can provide a lower fog tier for indoor/short-range wireless access. 5G small cells overlay this to cover the extended campus. A central on-premises private 5G core integrates the radio access network with internal IT systems and an optional public cloud service. * Retail Chain. Point-of-sale devices and inventory sensors in stores form the edge layer. In-store servers and networking comprise the fog layer. Regional distribution centers provide a second tier of fog resources. The central enterprise cloud contains core business systems and analytics. * Environmental Monitoring. A first tier of simple sensors monitors local conditions like temperature. More advanced gateways aggregate data from clusters of these devices over a wider area. Periodic drone flights act as temporary fog hotspots to backhaul data. The cloud provides centralized data historian capabilities. In the modern age, as digital communication infiltrates nearly every aspect of our lives, the need for a unified, seamlessly integrated network becomes more pressing. While traditional network structures focused on specific domains, be it terrestrial, aerial, or marine, the future is seen in the convergence of these domains, culminating in a robust, all-encompassing communication paradigm known as the Space-Air-Ground Integrated Network (SAGIN) [35; 36; 37]. At its core, SAGIN represents the zenith of multi-level networking, bringing together the expansive reaches of space with the immediacy of terrestrial networks, the mobility of aerial systems, and the depth of oceanic communication. It is a vision that extends beyond the classic three-tier architecture of cloud, fog, and edge, encapsulating the vastness and dynamism of our planet's communication needs. This AI-enabled architecture represents not just an upgrade but a transformative shift in how we envision and implement network infrastructure. The multi-level architecture of SAGIN (Figure 2) is not merely a stratification of platforms but a meticulously crafted interplay of functionalities, in which each level brings its unique strengths. The space domain in SAGIN refers to satellites and other space-based assets. These satellites bridge gaps between remote areas, providing global coverage. They act as high-speed data relays, ensuring that no corner of the Earth remains disconnected. In this expansive void, they are the guardians, ensuring that the internet's reach is truly worldwide, unimpeded by terrestrial challenges such as mountains or dense urban construction. In the context of studying a multi-tier or multi-layer satellite communication system, satellites in low Earth orbit (LEO), medium Earth orbit (MEO), and geostationary orbit (GEO) can be considered as three distinct layers. Each of these layers serves specific functions and possesses unique characteristics that influence their operational purposes and the types of services they provide (Table 2). As with previous discussions, the information provided is a synthesis of commonly recognized knowledge in the realm of satellite communications.