il a technological solution was proposed. That solution was DSL. It is commonly believed that the local loop is incapable of carrying more than the frequencies required to support the voice band. This is a misconception. ISDN, for example, requires significantly high bandwidth to support its digital traffic. When ISDN is deployed, the loop must be modified in certain ways to eliminate its designed-in bandwidth limita- tions. For example, some long loops are deployed with load coils that "tune" the loop to the voice band. They make the transmission of fre- quencies above the voice band impossible but allow the relatively low- frequency voice band components to be carried across a long local loop. High-frequency signals tend to deteriorate faster than low-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 Terms of Use as given at the website. Access Technologies Chapter 6 signal components, SO the elimination of the high frequencies extends the transmission distance and reduces transmission impairments that would result from the uneven deterioration of a rich, multifrequency sig- nal. These load coils, therefore, must be removed if digital services are to be deployed. A local loop is only incapable of transporting high-frequency signal components if it is designed not to carry them. The capacity is still there; the network design, through load coil deployment, simply makes that additional bandwidth unavailable. DSL services, especially ADSL, take advantage of this "disguised" bandwidth. DSL Technology In spite of the name, DSL-Digital Subscriber Line-is an analog tech- nology. The devices installed on each end of the circuit are sophisticated high-speed modems that rely on complex encoding schemes to achieve the high bit rates that DSL offers. Furthermore, several of the DSL ser- vices, specifically ADSL, G.lite, VDSL, and RADSL-all discussed later in this section-are desig