Chapter 8. Channels and Coding

Communications is all about language and, unfortunately , understanding computer communications requires learning another language, one that explains the technology that allows us to send messages, music, and even pictures electronically .

We're going to talk about channels , which are nothing more than the paths your electronic messages take. A communication channel is like a television channel. More than that, a television channel is a communication channel ”a designated pathway for the television signal to get from the broadcasting station to your antenna and television set.

Different channels require technologies for moving information. Paper mail is a communication channel that requires you to use ink and paper (maybe a pencil in a pinch ). Electronic communication requires that you translate your message into electronic form.

You cannot directly read an electronic message, no more than you could read it were it written in a secret code. In fact, when your message becomes electronic, it is written in a nonsecret code. Although human beings cannot read it, computers can, because some aspects of the signal correspond to the information in your message, exactly as code letters correspond to the text of a secret message.

As background for understanding how computer communications work, we're first going to look at the characteristics and limitations of the channels that your computer might use. Then we'll take a look at some of the ways that signals get coded so they can be passed through those channels ”first as expansion buses inside your computer, then as serial and parallel communication channels between computers. Note that this coding of data is an important technology that has wide applications. It's useful not only in communications but also data storage.