Cisco Networking: OSI Network Model Overview
The International Organization for Standardization (ISO), defines how network devices should be designed to communicate with each other. The ISO has proposed a network model that allows for this communication to take place, and although this is good from a theoretical level, it is not always followed, especially since it was published after many networking protocols and methods had been created.
The Open System Interconnection (OSI) model, which has seven layers and defines what types of activities should be conducted at each layer. You can use two mnemonic phases to remember the order of these layers. From Layer 1 up, it would be Please Do Not Throw Sausage Pizza Away; from Layer 7 down, it would be All People Seem To Need Data Processing.
When thinking about the OSI model, picture it on both the sending and receiving computers. Imagine that a piece of data moves through the process from one computer to another, and as that data passes each layer, a header, or a modification, is performed on the information at each layer.
As the data moves through the system, only the application layer needs to know anything about the data’s actual content, but each layer needs to know how to deal with the layer before and after it (for example, the session layer needs to know how to communicate with the presentation and transport layers).
ISO figured that if a group wanted to write a new session interface, they could substitute it at the session layer, without knowing anything about layers any further away than directly before or after it.
The following illustrates the process of data movement by showing the flow of a piece of data — in this case, a get request from a web browser — through the layers and traveling between two computers. The first thing that happens is that the application layer applies a header to the data with relevant information related to the application that is used with this data.
This application layer header is used by both the presentation layer to identify where the data came from as well as by the application layer on the receiving computer to send the data to the correct application. The application layer hands off the data — with its application header — to the presentation layer, which considers everything that it has received from the application layer as its data.
The presentation layer also applies a header, possibly identifying the character formatting (ASCII or Unicode) and hands off the data — now data and two headers — to the session layer. This process continues with each layer applying a header with relevant information for the receiving computer to use to move the data to the correct application.
After the web server has retrieved the requested data — the contents of default.htm — the data would have an application header attached to it, and the whole process of sending the information back to the other computer would proceed down through the layers, over the network, and back up through the layers on the receiving computer.
The final result would be that the web page, default.htm, would be displayed in the computer’s web browser.