Chapter 29: Layers of the OSI Model

Overview

OSI sounds like the name of a top-secret government agency you hear about only in Tom Clancy novels. What it really stands for, as far as this book is concerned, is Open System Interconnection, as in the Open System Interconnection Reference Model, also known as the OSI Reference Model or OSI Model (depending on how pressed for time you are).

The OSI Model breaks the various aspects of a computer network into seven distinct layers. These layers are kind of like the layers of an onion: Each successive layer envelops the layer beneath it, hiding its details from the levels above. (The OSI Model is also like an onion in that if you start to peel it apart to have a look inside, you're bound to shed a few tears.)

The OSI Model isn't itself a networking standard in the same sense that Ethernet and TCP/IP are. Rather, the OSI Model is a framework into which the various networking standards can fit. The OSI Model specifies what aspects of a network's operation can be addressed by various network standards. So, in a sense, the OSI Model is sort of a standard's standard.

The first three layers are sometimes called the lower layers. They deal with the mechanics of how information is sent from one computer to another over a network. Layers 4–7 are sometimes called the upper layers. They deal with how applications relate to the network through application programming interfaces.

REMEMBER 

Yes, I know the OSI Model has seven layers, not ten. The Part of Tens consists of chapters that present approximately ten topics worth knowing about. In this case, seven is close enough.