What is the OSI model?

The Open Systems Interconnection (OSI) model—also called the OSI reference model—is a conceptual model that divides network communication and interoperability into seven abstract layers. It provides a standardized model that enables different applications, computer systems and networks to communicate.

The OSI model emerged as a solution to communication incompatibilities between the diverse array of networking protocols in use around the turn of the century. The layers of OSI gave developers and engineers a framework for building interoperable hardware and software across networks by providing a categorical approach to networking.

At each layer of the stack—typically shown in reverse order to illustrate how data moves through a network—the OSI model provides guidelines and criteria for network components and their unique computing functions.

The layers are:

• Layer 7 The application layer initiates communication with the network, including the protocols and data manipulation processes that convert computer-readable network data into user-readable responses.

• Layer 6 The presentation layer prepares data for the application layer, including data translation, compression, and encryption.

• Layer 5 The session layer initiates and terminates connections between two devices interacting on the network, ensuring that resources are neither overused nor underutilized.

• Layer 4 The transport layer transmits end-to-end data between two devices interacting on the network, ensuring that data isn’t lost, misconfigured, or corrupted.

• Layer 3 The network layer handles data addressing, routing, and forwarding processes for devices interacting across different networks. If the devices are on the same network, they don’t need the network layer to interact.

• Layer 2, Unlike the network layer, the data link layer manages data routing between two interacting devices on the same network.

• Layer 1 The physical layer comprises the physical assets, like routers and USB cables, that convert data into strings of 1s and 0s for transmission to higher layers.

The OSI model focuses on providing a list of tasks for engineers to complete in building each layer of a network architecture, instead of specifying protocols for communication between layers. Its theoretical approach enables developers to visualize and build highly complex computing networks, even without prior knowledge of the networking system. It also helps teams understand how data traverses a network and tailor network functions with layer-specific coding.