andwidth. Distance is an important criterion because it places certain design limitations on the network, making it more expensive as the circuit length increases. Over distance, signals tend to weaken and become noisy, and specialized devices (amplifiers, repeaters, regenerators) are required to periodically clean up the signal quality and maintain the proper level of "loudness" to ensure intelligibil- ity and recognizability at the receiving end. Quality is related to distance in the sense that they share many of the same affecting factors. Signal level is clearly important, as is noise, both of which were just discussed. Distortion is a slightly different beast and must be dealt with equally as carefully. Noise is a random event in net- works caused by lightning, fluorescent lights, electric motors, sunspot activity, and squirrels chewing on wires; it is unpredictable and largely random. Noise, therefore, cannot be anticipated with any degree of accu- racy; its effects can only be recovered from. Distortion, on the other hand, is a measurable, predictable character- istic of a transmission channel and is usually frequency-dependent. For example, certain frequencies transmitted over a particular channel will be weakened, or attenuated, more than other frequencies. If we can mea- sure this, then we can condition the channel to equalize the treatment that all frequencies receive as they are transmitted down that channel. This process is indeed known as conditioning, and is part of the higher cost involved in buying a dedicated circuit for data transmission. For example, think about the last time you attended a parade. How did you know the band was approaching? You heard the low-frequency tones of the bass drums and tubas long before you saw the band-or you heard the higher frequencies of the trumpets and trombones. Higher frequency 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