discussion on CSA guidelines later in this section.) A number of companies have deployed T1 access over HDSL2 at rates 40 percent lower than typical T-Carrier prices. Furthermore, a number of vendors including 3Com, Lucent, Nortel Networks, and Alcatel have announced their intent to work together to achieve interoperability among DSL modems. Rate-Adaptive Digital Subscriber Line (RADSL) Rate-Adaptive Digital Subscriber Line, or RADSL (pronounced "Rad- zel"), is a variation of ADSL designed to accommodate changing line con- ditions that can affect the overall performance of the circuit. Like ADSL, it relies on DMT encoding, which selectively "populates" subcarriers with transported data, thus allowing for granular rate-setting. Very High-Speed Digital Subscriber Line (VDSL) Very High-Speed Digital Subscriber Line (VDSL) is the newest DSL entrant in the bandwidth game, and shows promise as a provider of extremely high levels of access bandwidth-as much as 52 Mbps over a short local loop. VDSL requires fiber-to-the-curb (FTTC) architecture and recommends ATM as a switching protocol; from a fiber hub, copper tail circuits deliver the signal to the business or residential premises. Band- width available through VDSL ranges from 1.5 to 6 Mbps on the upstream side, and from 13 to 52 Mbps on the downstream side. Obvi- ously, the service is distance sensitive, and actual achievable bandwidth drops as a function of distance. Nevertheless, even a short loop is respectable when such high-bandwidth levels can be achieved. With VDSL, 52 Mbps can be reached over a loop length of up to 1,000 feet-a not unreasonable distance by any means. 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 G.lite Because the installation of splitters has proven to be a contentious and problematic issue, the need arose for a version of ADSL that did 