 to support Internet Protocol TV (IPTV) services, with an eye toward HDTV. However, despite improvements in data rates and reach, ADSL2 and ADSL2+ are still not sufficient to support applica- tions such as multiple HDTV channels; such uses require the use of VDSL, which is discussed later in this chapter. Another problem with ADSL2+ is that although the standard is approved and chipsets are plentiful, the equipment for multiple-vendor setups still requires interoperability testing. ADSL2-RE To expand carriers' addressable markets, the ITU has developed a reach-extended version of the ADSL2 specification-called ADSL2-RE-that allows DSL systems to reach up to 3.75 miles (6 km). This equates to more than a 20% increase in COV- erage and opens the door for carriers to sign up new subscribers. The ITU-T rati- fied ADSL2-RE under recommendation G.992.3 in 2003. While it can support up to 8Mbps downstream and 1Mbps upstream, when taking advantage of its main feature, ADSL2-RE extends a 768Kbps downstream service by approximately 0.5 mile (1 km), to 3.5 miles (5.5 km). RADSL RADSL, a nonstandard version of ADSL, adapts data rates dynamically, based on changes in line conditions. It can therefore operate over a wide range of loop Chapter 12 Broadband Access Alternatives lengths and conditions, up to a maximum of 3.5 miles (5.5 km). With RADSL, the modem adjusts the upstream bandwidth to maintain a certain speed on the downstream channel. If there is a large amount of line noise or signal degrada- tion, the upstream bandwidth may be significantly reduced, down to 64Kbps. RADSL can be configured to operate in either symmetrical or asymmetrical mode. The downstream rates range from 600Kbps to 7Mbps, and the upstream rates range from 64Kbps to 1Mbps. Most RADSL devices rely on Discrete Multitone (DMT) encoding. The ITU-T standardized VDSL, the highest-performing DSL option, under recom- mendation G.993.1 in 2004. Conventional VDSL relies on a single twisted copper pair, operating over very short distances. The loop length ran