# OSI Model Introduction ## What is the OSI Model The OSI model, which stands for Open Systems Interconnection model, is a conceptual framework developed by the International Organization for Standardization in 1984. It describes how different network protocols and technologies work together to enable communication between devices on a network. The OSI model divides the complex task of network communication into seven distinct layers, each with a specific responsibility. This layered approach allows different technologies and protocols to be developed independently for each layer, as long as each layer provides the expected services to the layer above it and uses the services of the layer below it. ## Why the OSI Model Was Created Before the OSI model, different computer manufacturers developed their own proprietary networking solutions. A computer from one manufacturer could not easily communicate with a computer from another manufacturer because they used different, incompatible networking protocols and technologies. The OSI model was created to provide a universal standard framework so that products from different vendors could work together in the same network. Although the OSI model itself was not implemented directly as a protocol, it became the universal reference model for understanding and explaining network communication. ## The Seven Layers The OSI model consists of seven layers arranged in a hierarchy. From bottom to top, they are the Physical layer, the Data Link layer, the Network layer, the Transport layer, the Session layer, the Presentation layer, and the Application layer. The lower layers deal with hardware and the physical transmission of data. The middle layers handle addressing and routing. The upper layers deal with data formatting and application interfaces. A useful mnemonic for remembering the layers from bottom to top is Please Do Not Throw Sausage Pizza Away. ## How Layers Communicate Each layer in the OSI model communicates with the layer directly above it and the layer directly below it. When data is sent, it starts at the Application layer of the sending device and passes downward through each layer. Each layer adds its own header information to the data, a process called encapsulation. When the data reaches the Physical layer, it is transmitted as electrical signals, light pulses, or radio waves across the network medium. At the receiving device, the data passes upward through each layer. Each layer strips off and processes the header that was added by the corresponding layer on the sending side, a process called decapsulation. ## Protocol Data Units Each layer of the OSI model has a specific name for the unit of data it works with. At the Application, Presentation, and Session layers, the data unit is simply called data or a message. At the Transport layer, the data unit is called a segment for TCP or a datagram for UDP. At the Network layer, the data unit is called a packet. At the Data Link layer, the data unit is called a frame. At the Physical layer, the data unit is called a bit. ## Importance of the OSI Model Even though the internet is based on the TCP/IP model rather than the OSI model, the OSI model remains extremely valuable as a teaching and troubleshooting tool. When a network problem occurs, network engineers use the OSI model to systematically identify which layer the problem is at. For example, if a device cannot get an IP address, the problem is at the Network layer. If a cable is unplugged, the problem is at the Physical layer. By identifying the layer where the problem exists, technicians can focus their troubleshooting efforts appropriately.