ned to operate in conjunction with voice across the same local loop. ADSL is the most commonly deployed service and offers a great deal to both business and residence subscribers. DSL Services DSL comes in a variety of flavors designed to provide flexible, efficient, high-speed service across the existing telephony infrastructure. From a consumer point-of-view, DSL, especially ADSL, offers a remarkable leap forward in terms of available bandwidth for broadband access to the Web. As content has steadily moved away from being largely text-based and has become more graphical, the demand for faster delivery services has grown for some time now. DSL may provide the solution at a rea- sonable cost to both the service provider and the consumer. Businesses will also benefit from DSL. Remote workers, for example, can rely on DSL for LAN and Internet access. Furthermore, DSL pro- vides a good solution for VPN access as well as for ISPs looking to grow the bandwidth available to their customers. It is available in a variety of both symmetric and asymmetric services and therefore offers a high- 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 Access Technologies bandwidth access solution for a variety of applications. The most com- mon DSL services are ADSL, ADSL2+, HDSL, HDSL-2, RADSL, and VDSL. The special case of G.lite, a form of ADSL, will also be discussed. Asymmetric Digital Subscriber Line (ADSL) When the World Wide Web and flat-rate access charges arrived, the typ- ical consumer phone call went from roughly four minutes in duration to several times that. All the engineering that led to the overall design of the network based on an average four-minute hold time went out the window as the switches staggered under the added load. Never was the expression, "In its success lies the seeds of its own destruction," more true. When AD