OSI Reference Model

The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) Reference Model in 1978/1979 to facilitate the open interconnection of computer systems. An open interconnection is one that can be supported in a multivendor environment. This model established the global standard for defining the functional layers required for open communications between computers.

When the OSI Reference Model was developed almost 20 years ago, few commercial systems followed that model. Computer manufacturers at that time locked customers into proprietary, single-vendor architectures. Open communication was viewed as an invitation to competition. From the manufacturer’s perspective, competition was undesirable. Consequently, all functions were integrated as tightly as possible. The notion of functional
modularity, or layering, seemed antithetical to any manufacturer’s mission.
It is important to note that the model has been so successful at achieving its original goals that it almost renders itself moot. The previous proprietary, integrated approach has disappeared. Open communications, today, are requisite. Curiously, very few products are fully OSI-compliant. Instead, its basic layered framework is frequently adapted to new standards. Nevertheless, the OSI Reference Model remains a viable mechanism for demonstrating the functional mechanics of a network.

Despite its successes, numerous misperceptions about the OSI Reference Model persist. Consequently, it is necessary to provide yet another overview of this model in this section. The overview identifies and corrects these misperceptions.

The first misperception is that the OSI Reference Model was developed by the International Standards Organization (ISO). It was not. The OSI Reference Model was developed by the International Organization for Standardization. This organization prefers to use a mnemonic abbreviation rather than an acronym. The mnemonic abbreviation is based on the Greek word, isos, which means equal or standard.

The OSI model categorizes the various processes necessary in a communications session into seven distinct functional layers. The layers are organized based on the natural sequence of events that occurs during a communications session.

Figure below illustrates the OSI Reference Model. Layers 1–3 provide network access, and
Layers 4–7 are dedicated to the logistics of supporting end-to-end communications.

OSI Reference Model Layer      OSI Layer Number
Application                                                   7
Presentation                                                6
Session                                                         5
Transport                                                    4
Network                                                      3
Data Link                                                     2
Physical                                                        1