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OSI Model (Open Systems Interconnection)

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to understand and design a network architecture that is robust, interoperable, and scalable.

Developed by the International Organization for Standardization (ISO), the OSI model divides the functions of a networking system into seven distinct layers. Each layer serves a specific function and communicates with the layers directly above and below it.

Layer 1 - Physical Layer:

  • Handles the physical connection between devices and the transmission of raw data bits over the physical medium (like cables, fiber optics, etc.).

  • Examples include Ethernet (cable) and Wi-Fi (wireless).

Layer 2 - Data Link Layer:

  • Responsible for node-to-node data transfer and error detection and correction in the physical layer.

  • Divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).

  • Examples include Ethernet, PPP (Point-to-Point Protocol).

Layer 3 - Network Layer:

  • Manages packet routing through logical addressing and path selection (IP addresses are part of this layer).

  • Examples include IP (Internet Protocol), ICMP (Internet Control Message Protocol).

Layer 4 - Transport Layer:

  • Ensures complete data transfer and data integrity between host-to-host (end-to-end) connections.

  • Manages flow control, error checking, and segmentation/reassembly of data.

  • Examples include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Layer 5 - Session Layer:

  • Manages sessions between network applications, establishing, managing, and terminating connections between end-user applications.

  • Examples include NetBIOS and RPC (Remote Procedure Call).

Layer 6 - Presentation Layer:

  • Translates data between the network and the application layer, ensuring the data is in a usable format.

  • Handles data encryption, compression, and translation.

  • Examples include SSL/TLS (Secure Sockets Layer/Transport Layer Security), MIME (Multipurpose Internet Mail Extensions).

Layer 7 - Application Layer:

  • The closest layer to the end-user, interacting with software applications that implement a communicating component.

  • Examples include HTTP (Hypertext Transfer Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol).

Understanding the OSI model is crucial for network professionals as it provides a universal language to describe networking functions and capabilities. It aids in diagnosing and resolving network problems and in designing and implementing new network solutions.

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