The Open Systems Interconnection (OSI) Reference Model

The OSI (Open Systems Interconnect) Reference Model (ISO 7498) defines a seven-layer model of data communication with physical transport at the lower  layer and application protocols at the upper layers. This model, shown in Figure 1-5, is widely accepted as a basis for the understanding of how a network protocol stack should operate and as a reference tool for comparing network stack implementation 

Each layer provides a set of functions to the layer above and, in turn, relies on the functions provided by the layer below. Although messages can only pass vertically through the stack from layer to layer, from a logical point of view, each layer communicates directly with its peer layer on other nodes.The seven layers are: Application Network applications such as terminal emulation and file transfer Presentation Formatting of data and encryption Session Establishment and maintenance of sessions Transport Provision of reliable and unreliable end-to-end delivery Network Packet delivery, including routing Data Link Framing of units of information and error checking Physical Transmission of bits on the physical hardware In contrast to TCP/IP, the OSI approach started from a clean slate and defined  standards, adhering tightly to their own model, using a formal committee process  Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Chapter 1. Architecture, history, standards, and trends  21 without requiring implementations. Internet protocols use a less formal  engineering approach, where anybody can propose and comment on Request  for Comments, known as RFC, and implementations are required to verify  feasibility. The OSI protocols developed slowly, and because running the full  protocol stack is resource intensive, they have not been widely deployed,  especially in the desktop and small computer market. In the meantime, TCP/IP  and the Internet were developing rapidly, with deployment occurring at a very  high rate.