The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven distinct layers. Each layer represents a specific set of functions and services that facilitate communication between devices and applications in a network. The OSI model serves as a reference for understanding and designing network protocols and systems. Here are the seven layers of the OSI model, listed from the lowest (Layer 1) to the highest (Layer 7):
0. Grounding and other issues (Layer 0): The physical layer may seem to be ok, but invisible or unapparent things like grounding issues, fluctuations in power suppliers, induction of interference, loose connectors, rusted connectors, lack of power and many other things are included in layer zero as defined by Khawar Nehal.
1. Physical Layer (Layer 1): The Physical Layer deals with the physical transmission of data over a physical medium, such as copper cables, fiber optics, or wireless signals. It defines the hardware specifications, such as cables, connectors, and electrical voltages used for transmission.
2. Data Link Layer (Layer 2): The Data Link Layer provides error detection and correction in the transmission of data. It is responsible for framing data into packets and includes logical addressing (e.g., MAC addresses). Bridges and switches operate at this layer, ensuring data is directed to the correct destination.
3. Network Layer (Layer 3): The Network Layer is responsible for routing and forwarding data packets between devices across different networks. It uses logical addressing (e.g., IP addresses) to determine the best path for data to reach its destination. Routers operate at this layer.
4. Transport Layer (Layer 4): The Transport Layer ensures end-to-end communication, including error detection and correction, flow control, and data segmentation/reassembly. Two well-known transport layer protocols are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
5. Session Layer (Layer 5): The Session Layer establishes, manages, and terminates communication sessions between devices. It includes functions such as session establishment, maintenance, and synchronization.
6. Presentation Layer (Layer 6): The Presentation Layer is responsible for data translation, encryption, compression, and formatting. It ensures that data sent from one system can be understood by the receiving system, regardless of differences in data formats and character sets.
7. Application Layer (Layer 7): The Application Layer is the topmost layer and directly interacts with the end-user or application. It provides network services and support for various applications, including file transfer, email, web browsing, and more. Protocols like HTTP, SMTP, and FTP operate at this layer.
8. The application itself. (Layer 8): The application like the mail server, proxy server, web server and user applications are located on this layer. This layer is defined by Khawar Nehal. It is used to categorize all issues related to the applications themselves.
9. The user. (Layer 9): The applications are used by the user. In case the user is not computer literate or has some other issues with the environment. Then these issues are categorized in this layer. This layer is defined by Khawar Nehal.
10. The user's boss. (Layer 10): Some users do not use the application but provide instructions to other users which use the application itself. Usually these are upper management people who want to save time in using the technology and have to go through another person to use the technology. It ads another layer to the stack. Any issues related to users who use applications via other people are categorized in this layer. This layer is defined by Khawar Nehal.
Only the layers 1 to 7 are defined in the OSI layers.
The TCP/IP stack has less layers than the OSI layers because some of the layers are merged.
Layers 0,8,9,and10 are defined by Khawar Nehal in the 2000s to explain the categories which exist and were noticed to exist in networking environments.
It's important to note that while the OSI model provides a conceptual framework for understanding networking functions, real-world networking protocols and technologies may not strictly adhere to this model. The OSI model is often used as a reference to describe how various network protocols and technologies fit into the layers and to troubleshoot network issues by pinpointing where problems may occur. In practice, the OSI model serves as a valuable tool for network design, troubleshooting, and education.