OSI Model

he Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative and is sometimes known as the OSI seven layer model. From top to bottom, the OSI Model consists of the Application, Presentation, Session, Transport, Network, Data Link, and Physical layers. A layer is a collection of related functions that provides services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Even though newer IETF, IEEE, and indeed OSI protocol work subsequent to the publication of the original architectural standards that have largely superseded it, the OSI model is an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol they study into one of the seven basic layers. This is not always easy to do as many of the protocols in use on the Internet today were designed as part of the TCP/IP model, and may not fit cleanly into the OSI model.