The OSI Reference Model is the seven-layer framework for organizing communication between computer systems, standardized by the International Organization for Standardization. Its authoritative form is ISO/IEC 7498, “Information technology — Open Systems Interconnection — Basic Reference Model,” first published as ISO 7498 in 1984 and revised as ISO/IEC 7498-1 in 1994. The model gives a common vocabulary so that standards from different vendors and countries could be developed to interconnect “open” systems rather than locking customers into a single manufacturer’s proprietary networking.
The model divides the problem of networking into seven layers, each building on the services of the one below it. From the bottom up they are: the physical layer (transmission of raw bits over a medium), the data link layer (reliable transfer across a single link), the network layer (routing and relaying across an interconnected set of networks), the transport layer (reliable end-to-end transfer between processes), the session layer (organizing and synchronizing dialogue), the presentation layer (data representation and syntax), and the application layer (the interface to the application processes themselves).
The intellectual architecture was set out in detail by Hubert Zimmermann, whose 1980 paper in IEEE Transactions on Communications, “OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection,” remains the canonical firsthand account. Zimmermann describes the work of ISO subcommittee TC97/SC16, created in 1977, which adopted the layered architecture and identified the initial protocols expected to populate each layer. The paper explains the layering principles — decomposition by function, well-defined service interfaces between adjacent layers, and the goal of minimizing the information that must cross each interface.
OSI emerged from a specific historical moment. Through the late 1970s and 1980s, OSI was promoted by national governments and standards bodies as the official, vendor-neutral future of networking, in deliberate contrast to the running TCP/IP protocols of the ARPANET research community. The competition between the two architectures became known as the protocol wars. In practice the seven-layer OSI protocol stack was largely never deployed at scale, while TCP/IP became the basis of the global Internet.
Despite that outcome, the OSI Reference Model endures as the dominant teaching framework and shared vocabulary for networking. Engineers still speak of a “layer 2 switch,” a “layer 3 router,” or a “layer 7 firewall” by reference to the OSI numbering. The model’s lasting contribution was less its specific protocols than its disciplined demonstration that network functionality can be cleanly decomposed into layered abstractions.