The OSI Model

In 1984, the International Organization for Standardization (ISO) defined a standard, or set of rules, for manufacturers of networking components that would allow these networking components to communicate in dissimilar environments.This standard is known as the Open Systems Interconnect (OSI) model and is a model made up of seven layers.Each layer of the OSI model is responsible for a specific function or task within the stages of network communication. The seven layers of the OSI model, from highest to lowest, are application, presentation, session, transport, network, data link, and physical. Network communication starts at the application layer of the OSI model (on the sending system) and works its way down through the layers to the physical layer. The information then passes along the communication medium to the receiving computer, which works its way back up the layers starting at the physical layer. Figure 2-4 shows an example of packets being transmitted down through the OSI layers of the sending computer, across the medium, and back up the OSI layers on the receiving computer. Be sure to refer to this figure frequently when going through this section.

Each layer of the OSI model is responsible for certain functions within the process of sending data from one system to another. Each layer is responsible for communicating with the layers immediately above it and below it. For example, the presentation layer will receive information from the application layer, format
it appropriately, and then pass it to the session layer. As another example, thepresentation layer will never deal directly with the network or data link layers.

Let’s look at the layers from the point of view of two computers that will send data between each other: COMPUTER1 and SERVER1 are going to exchange data on the network. COMPUTER1 will be known as the sending computer, and SERVER1 will be known as the receiving computer, as shown in Figure 2-5.

The data exchange starts with COMPUTER1 sending a request to SERVER1. It is important to notice as you progress through the layers that whatever function is performed at a layer on the sending system must be undone at the exact layer on the receiving system. For example, if the presentation layer compresses the data on the sending system, the presentation layer will decompress the data on the receiving system before passing the data up to the application layer.