Introduction to Data Communications
32. Introduction to the ISO - OSI Model

32. Introduction to the ISO - OSI Model

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 to improve internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.

Understanding the function of each layer is instrumental in understanding data communication within networks whether Local, Metropolitan or Wide.

32a. OSI Model Explained

This is a top-down explanation of the OSI Model, starting with the user's PC and what happens to the user's file as it passes though the different OSI Model layers. The top-down approach was selected specifically (as opposed to starting at the Physical Layer and working up to the Application Layer) for ease of understanding of how the user's files are transformed through the layers into a bit stream for transmission on the network.

There are 7 Layers of the OSI model:

• 7. Application Layer (Top Layer)

• 6. Presentation Layer

• 5. Session Layer

• 4. Transport Layer

• 3. Network Layer

• 2. Data Link Layer

• 1. Physical Layer (Bottom Layer)