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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):
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.
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.