Appendix C -- The OSI Reference Model

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As with any emerging technology, the early days of networking were characterized by strictly proprietary hardware and software. No one agreed on any particular method of building and implementing networks. That meant your choices for suppliers were limited.

You could choose to buy your network from IBM, DEC, Burroughs, or a few others. This decision was momentous because after purchasing components from one manufacturer, you were locked in. No one else's equipment or software would work with yours. You had to buy everything from the original vendor and if they didn't happen to make a product that you really, truly needed, you were just out of luck.

To cut across this entirely proprietary universe, the International Organization of Standards developed a standard to make possible open systems that could communicate with one another no matter who manufactured them. Today, many proprietary systems still exist, and they don't always communicate with one another as well as we might hope, but they do have in common the Open Systems Interconnection (OSI) reference model. This model provides a framework for depicting the organization of network components and can give you a clearer picture of how these components relate to one another.

The OSI reference model is made up of seven layers. Each layer has its job to do and has to relate only to the layers on either side. The layers are shown in Figure C-1. From the bottom up, they are:

• Physical
• Data link
• Network
• Transport
• Session
• Presentation
• Application

Figure C-1. Layers of the OSI reference model.

Each layer in a network communication assumes that it's speaking to the same layer on another machine and uses a common protocol. The collection of protocols is called a protocol stack.