#readwise # OSI Model - Wikipedia  ## Metadata - Author: [[wikipedia.org]] - Full Title: OSI Model - Wikipedia - URL: https://en.wikipedia.org/wiki/OSI_model ## Summary The OSI model is a framework that standardizes how devices communicate over a network. It consists of seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. This model helps in understanding and designing network systems by defining how data flows between layers. Although the Internet protocol suite is more commonly used today, the OSI model remains important for teaching and discussing networking concepts. ## Highlights The Open Systems Interconnection (OSI) model is a [reference model](https://en.wikipedia.org/wiki/Reference_model) from the [International Organization for Standardization](https://en.wikipedia.org/wiki/International_Organization_for_Standardization) (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection."[2](https://en.wikipedia.org/wiki/OSI_model#cite_note-ISO-7498-1-2) ([View Highlight](https://read.readwise.io/read/01jhpz0emtm6dgna9mjkv208qj)) --- **The model partitions the flow of data in a communication system into seven [abstraction layers](https://en.wikipedia.org/wiki/Abstraction_layer) to describe networked communication from the physical implementation of transmitting [bits](https://en.wikipedia.org/wiki/Bit) across a [communications medium](https://en.wikipedia.org/wiki/Transmission_medium) to the highest-level representation of data of a [distributed application](https://en.wikipedia.org/wiki/Distributed_application).** Each intermediate layer serves a class of functionality to the layer above it and is served by the layer below it. ([View Highlight](https://read.readwise.io/read/01jhpz17yeknfafw13ndm0401q)) ^robfxr --- **The [[Internet Protocol Suite - Wikipedia|Internet Protocol Suite]] as defined in [RFC 1122](https://datatracker.ietf.org/doc/html/rfc1122) and [RFC 1123](https://datatracker.ietf.org/doc/html/rfc1123) is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for the development of the [Internet](https://en.wikipedia.org/wiki/Internet). It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model.** ([View Highlight](https://read.readwise.io/read/01jhpz4efbf29ccspjyb4sww52)) --- ### Layer architecture #### Layer 1: Physical layer **The Physical Layer is responsible for the transmission and reception of unstructured raw data** between a device, such as a network interface controller, Ethernet hub, or network switch, and a physical transmission medium. **It converts the digital bits into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of pins, voltages, line impedance, cable specifications, signal timing and frequency for wireless devices.** Bit rate control is done at the physical layer and may define transmission mode as simplex, half duplex, and full duplex. The components of a physical layer can be described in terms of a network topology. Physical layer specifications are included in the specifications for the ubiquitous Bluetooth, Ethernet, and USB standards. **An example of a less well-known physical layer specification would be for the CAN standard.** ^ajrpl3 --- #### Layer 2: Data link layer **The data link layer provides node-to-node data transfer—a link between two directly connected nodes. It detects and possibly corrects errors that may occur in the physical layer.** It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for flow control between them. ^g9aayd --- **IEEE 802 divides the data link layer into two sublayers:** **Medium access control (MAC) layer – responsible for controlling how devices in a network gain access to a medium and permission to transmit data.** **Logical link control (LLC) layer – responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization.** ^emwq4v --- The MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet, 802.11 Wi-Fi, and 802.15.4 ZigBee operate at the data link layer. --- #### Layer 3: Network layer **The network layer provides the functional and procedural means of transferring packets from one node to another connected in "different networks".** A network is a medium to which many nodes can be connected, on which **every node has an address and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node, possibly routing it through intermediate nodes.** If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. ^2pxvur --- #### Layer 4: Transport layer **The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host** from one application to another across a network, while maintaining the quality-of-service functions. **Transport protocols may be connection-oriented or connectionless.** ^0y8i6x --- Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, **the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI**. ^6c16es --- #### Layer 5: Session layer **The Session Layer creates the setup, controls the connections, and ends the teardown, between two or more computers, which is called a "session".** ^lpimle --- #### Layer 6: Presentation layer **The Presentation Layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between OSs, and graphic commands.** The presentation layer transforms data into the form that the application layer accepts, to be sent across a network. Since the presentation layer converts data and graphics into a display format for the Application Layer, the Presentation Layer is sometimes called the syntax layer. ^eqiinb --- #### Layer 7: Application layer **The application layer is the layer of the OSI model that is closest to the end user, which means both the OSI Application Layer and the user interact directly with software application that implements a component of communication between the client and server, such as File Explorer and Microsoft Word.** Such application programs fall outside the scope of the OSI model unless they are directly integrated into the Application layer through the functions of communication, as is the case with applications such as Web Browsers and Email Programs. ^fs70q8 --- **Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP.** ^uia1dd ---