 first described by Bell Labs researcher Claude Shannon, who is widely known as the father of information the- ory. In 1948, Shannon published A Mathematical Theory of Communi- cation, which is now universally accepted as the framework for modern communications. We won't delve into the complex (but fascinating) mathematics that underlie Shannon's Theorem, but simply know that Figure 1-25 QAM, this time with noise added to the constellation 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. First Things First Chapter 1 Figure 1-26 Oscilloscope trace his conclusions are seminal: The higher the noise level in a circuit, the lower the achievable bandwidth. The bottom line? Noise matters. It mat- ters so much, in fact, that network designers and engineers make its elimination the first order of business in their overall strategies for cre- ating high-bandwidth networks. This is one of the reasons that optical fiber-based networks have become so critically important in modern transport systems-they are far less subject (but not immune!) to noise, and absolutely immune to the electromagnetic interference that plagues copper-based networks. Cable companies that now offer data services have the same issues and concerns. Every time a customer decides to play installer by adding a cable spur for a new television set in his or her home and crimping the connector on the end of the cable with a pair of pliers instead of a tool specifically designed for the purpose, he or she cre- ates a point where noise can leak into the system, causing problems for everyone. And it gets even more melodramatic than that: According to John Judson, a cable systems maintenance manager in the Midwest, unauthorized connection to the cable network can cause problems that go way beyond unauthorized access to service. He observes: Cable networks are high-frequency systems. Some of 