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Open Systems Interconnection (OSI) Model

The Open Systems Interconnection (OSI) model is a conceptual framework used to understand and standardize the functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Developed by the International Organization for Standardization (ISO), the OSI model is divided into seven layers, each specifying particular network functions. The layers, from top to bottom, are:

1. Application Layer (Layer 7):
   • This is the layer closest to the end user.
   • It provides network services to the applications of the user.
   • Examples include HTTP for web browsing, SMTP for email, and FTP for file transfer.
2. Presentation Layer (Layer 6):
   • Translates data between the application layer and the network.
   • Ensures that data sent from the application layer of one system is readable by the application layer of another system.
   • Handles data encryption, compression, and translation.
3. Session Layer (Layer 5):
   • Manages sessions in which data is transferred.
   • Responsible for setting up, managing, and terminating connections between applications.
   • Handles authentication and reconnection after interruptions.
4. Transport Layer (Layer 4):
   • Responsible for end-to-end communication and data flow control.
   • Segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system.
   • Protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) operate at this layer.
5. Network Layer (Layer 3):
   • Manages packet forwarding, including routing through intermediate routers.
   • Responsible for logical addressing, so that data can be routed across several networks (internetworking).
   • IP (Internet Protocol) is a key protocol that operates at this layer.
6. Data Link Layer (Layer 2):
   • Responsible for node-to-node data transfer.
   • Deals with the physical addressing and error correction.
   • Ethernet, Wi-Fi, and the Point-to-Point Protocol (PPP) operate at this layer.
7. Physical Layer (Layer 1):
   • Concerned with the transmission and reception of the unstructured raw bit stream over a physical medium.
   • Deals with the electrical, mechanical, and procedural interface to the physical medium.
   • Includes specifications for cables, connectors, and signaling options.

Understanding the OSI model is fundamental in the field of computer networking as it provides a standard for different computer systems to communicate on a network, enabling interoperability among various networking technologies and protocols.