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OSI Model

The OSI (Open Systems Interconnection) model is a framework for understanding how communication occurs between different devices within a computer network. It is composed of seven different layers, each of which performs a specific function in the communication process. These layers are:

  1. Physical Layer: This layer deals with the physical connection between devices, including the transmission media (such as cables or wireless signals) and the hardware (such as network interface cards) used to transmit data. Protocols at this layer include Ethernet, WiFi, and Bluetooth.

  2. Data Link Layer: This layer is responsible for establishing a connection between two devices and ensuring that the data is transmitted accurately between them. Protocols at this layer include MAC (Media Access Control) addresses, which are unique identifiers assigned to each device on the network.

  3. Network Layer: This layer is responsible for routing data packets between devices, ensuring that they reach their intended destination even if the network topology changes. Protocols at this layer include IP (Internet Protocol), which provides a unique address for each device on the network, and routing protocols such as OSPF (Open Shortest Path First) and BGP (Border Gateway Protocol).

  4. Transport Layer: This layer is responsible for ensuring that data is delivered reliably between devices, including retransmitting any lost or corrupted packets. Protocols at this layer include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).  Some argue that SSL and TLS now reside on this layer.

  5. Session Layer: This layer is a framework for understanding how data is transmitted over networks. The session layer is responsible for establishing, maintaining, and terminating communication sessions between computers. Some of the protocols that operate at the session layer include:

    • NetBIOS (Network Basic Input/Output System)
    • RPC (Remote Procedure Call)
    • SIP (Session Initiation Protocol)
    • SS7 (Signaling System No. 7)

    NetBIOS is a protocol that provides services such as name resolution, datagram transmission, and session establishment for applications on a network. RPC is a protocol that allows a computer to request a service from a program located on another computer, and it is used to build distributed applications. SIP is a signaling protocol used for initiating, maintaining, modifying and terminating real-time sessions that involve video, voice, messaging and other communications applications and services between endpoints on the Internet. SS7 is a signaling system that is used to set up and tear down telephone calls, as well as to provide other services such as caller ID and call forwarding

  6. Presentation Layer: This layer is responsible for formatting and encoding data so that it can be transmitted between devices. Protocols at this layer include ASCII (American Standard Code for Information Interchange) and JPEG (Joint Photographic Experts Group).

  7. Application Layer: This layer is the highest layer in the OSI model and is responsible for providing services to the user, such as file transfer, email, and web browsing. Protocols at this layer include FTP (File Transfer Protocol), HTTP (Hypertext Transfer Protocol), and SMTP (Simple Mail Transfer Protocol).

In summary, the OSI model is a framework that helps to understand how communication occurs between devices on a computer network, with each layer performing a specific function in the process. Protocols at each layer provide the necessary standards and protocols to ensure that data is transmitted accurately and reliably between devices.

» The CSI Linux Knowledge Base

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