single twisted-pair. It adapts the data rate dynamically, based on any changes occurring in the line conditions and on the loop length. With RADSL, the rates can vary widely, from 600Kbps to 7Mbps downstream and from 128Kbps to 1Mbps upstream. RADSL can be configured to be a symmetrical or an asymmetrical service. VDSL VDSL provides a maximum span of about 1 mile (1.5 km) over a single twisted-pair. Over this distance, you can get a rate of up to 13Mbps downstream. But if you shorten the distance to 1,000 feet (300 m), you can get up to 55Mbps downstream and up to 15Mbps upstream, which is enough capacity to facilitate delivery of several HDTV channels as well as Internet access and VoIP. With VDSL2 you can get up to 100Mbps both downstream and upstream, albeit over very short distances. Advantages and Disadvantages of Twisted-Pair Twisted-pair has several key advantages: High availability-More than 1 billion telephone subscriber lines based on twisted-pair have been deployed, and because it's already in the ground, the telcos will use it. Some say that the telcos are trapped in their copper cages; rather than build an infrastructure truly designed for tomorrow's applica- tions, they hang on to protecting their existing investment. It is a huge investment: More than US$250 billion in terms of book value is associated with the twisted-pair deployed worldwide. This can be construed as both an advantage and a disadvantage. Coaxial Cable Low cost of installation on premises-The cost of installing twisted-pair on premises is very low. Low cost for local moves, adds, and changes in places-An individual can simply pull out the twisted-pair terminating on a modular plug and replace it in another jack in the enterprise, without requiring the intervention of a technician. Of course, this assumes that the wiring is already in place; oth- erwise, there is the additional cost of a new installation. Twisted-pair has the following disadvantages: Limited frequency spectrum-The total usable frequency spectrum of twisted-pair copp