# End-to-End Service Availability in Heterogeneous Multi-Tier Cloud-Fog-Edge Networks
## 3 Materials and Methods
### Traditional Three-Level Cloud-Fog-Edge Architecture
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Level** & **Location** & **Functionality** & **Communication** \\ \hline Cloud & Data centers that can be & & \begin{tabular}{c} Houses vast computational resources, \\ data storage, and advanced analytics \\ tools. Ideal for complex operations and \\ long-term data storage. \\ \end{tabular} & \begin{tabular}{c} Primarily communicates with the fog \\ layer, receiving aggregated or processed \\ data, and sending back processed \\ insights or commands. \\ \end{tabular} \\ \hline Fog Level & Typically within the local & & \begin{tabular}{c} Offers intermediate processing \\ capabilities, often handling tasks like \\ data aggregation, preliminary analytics, \\ and local storage. It can offload some of \\ the immediate processing needs from the \\ edge while filtering and reducing the \\ amount of data sent to the cloud. \\ \end{tabular} & \begin{tabular}{c} Acts as a two-way bridge, \\ communicating with the cloud for more \\ advanced processing or updates and \\ with the edge to receive raw data or to \\ send immediate commands. \\ \end{tabular} \\ \hline Edge Level & Typically located close to & \begin{tabular}{c} Immediate processing of data: because of \\ the proximity to the data source, edge \\ devices can process data almost instantly, \\ making them crucial for time-sensitive \\ applications. \\ \end{tabular} & \begin{tabular}{c} Primarily communicates with nearby \\ devices or systems. \\ Can send summarized or processed data \\ back to the fog or cloud layers for further \\ processing or storage. \\ Suitable for intermittent \\ connectivity—not always required to be \\ online. \\ \end{tabular} \\ \hline IoT Level & \begin{tabular}{c} Integrated into our daily \\ environment, from our \\ homes and workplaces to \\ public spaces. These are \\ the endpoints of data \\ collection. \\ \end{tabular} & \
\begin{tabular}{c} Raw data collection: IoT devices \\ primarily function as data collectors, \\ sensing changes in their environment \\ and reporting them. \\ Limited processing: Some IoT devices \\ have the ability to minimally process \\ data, deciding what is worth sending \\ forward in the network hierarchy. \\ \end{tabular} &
\begin{tabular}{c} IoT devices usually communicate data to \\ the edge level, which might be a local \\ gateway or processing unit. This \\ communication might be continuous or \\ event-driven (i.e., only when a change \\ is detected). \\ Uses low-power communication \\ protocols like Zigbee, LoRaWAN, or BLE \\ Bluetooth Low Energy) for short-range \\ transmissions. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Assignment of levels in three-level cloud–fog–edge architecture. There are many examples of case studies illustrating applications of four-tier architecture with cloud, fog, edge, and IoT layers. Smart Transportation. In an intelligent transportation system, vehicles act as edge nodes with onboard sensors and computers. Roadside units like traffic signals and message signs provide fog-level computing and connectivity. The cloud tier collects and analyzes regional traffic data to coordinate signals, dispatch emergency services, and relay information to vehicles. Smart Grid. Smart meters at homes and buildings work as edge devices to send electricity usage data. Local transformers, substations, and routers comprise the fog layer to handle neighborhood-level processing and communications. Utility providers leverage the cloud for system-wide monitoring, control, analytics, and automation. Video Surveillance. Networked security cameras are edge nodes that can pre-process visual data before sending video feeds to the fog layer for further analysis, compression, and storage. Additional analytics and long-term archives reside in the cloud. Augmented Reality. AR headsets or glasses function as edge devices that integrate real-time views with overlaid computer-generated content. Local gateways and micro data centers provide fog-level processing such as occlusion handling and multi-user synchronization.