Overview of OSI

The ISO (International Standards Organization) has created a layered model, called the OSI (Open Systems Interconnect) model, to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions that can improve Internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.

The OSI Reference model defines seven layers that describe how applications running upon network-aware devices may communicate with each other. The model is generic and applies to all network types, not just TCP/IP and all media types, not just Ethernet. It is for this reason that any network technician will glibly throw around the term "Layer 4" and expect to be understood.

It should be noted however, that most protocols in day-to-day use work on a slightly modified layer system. TCP/IP, for example, uses a 6-rather that a 7-layer model. Nevertheless, in order to ease the exchange of ideas, even those who only ever use TCP/IP will refer to the 7-layer model when discussing networking principles with peers from a different networking background.

Confusingly, the OSI was a working group within the ISO (International Standards Organisation) and therefore,many people refer to the OSI Reference model as the ISO Reference model. They are referring to the same thing.

Traditionally, layer diagrams are drawn with Layer 1 at the bottom and Layer 7 at the top. The remainder of this article describes each layer, starting from the bottom and explains some of the devices and protocols that may be found in the data centre operating at this layer.