and extend my hand in greeting, you would quite naturally reach your hand out, grab mine, and shake it. There is a commonly accepted set of social rules that we agree to abide by, one of which is shaking hands as a form of greeting. It doesn't work everywhere: In Tibet, it is customary to extend one's tongue as far as it can be extended as a form of greeting (clearly a sign of a great culture!). In China, unless you are already friends with the person you are greeting, it is not cus- tomary to touch in any fashion. You, of course, had a choice when I extended my hand. You could have hit it, licked it, spit in it. But, because of the accepted rules that govern Western society, you took my hand in yours and shook it. These rules that govern communication-any form of communication-are called protocols. And the process of using protocols to convey information is called data communications. It's no accident, incidentally, that the obnoxious racket that analog modems make when they are attempting to connect to each other is called a handshake. The noise they make is their attempt to negotiate a common set of rules that works for both of them for that particular session. The Science of Communications Data communications is the procedure required to collect, package, and transmit data from one computing device to another, typically (but not always) over a wide area network. It is a complex process with many lay- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols ers of functionality. To understand data communications, we must break it into its component parts and examine each part individually, relying on the old adage that "the only way to eat an elephant is one bite at a time." Like a Russian Matreshka doll, data communications are made up of layer upon layer of operational functionality that work together to accomplish the task a