access to the physical cable. xDSL has the ability to meet the customer demand for high bandwidth right now, at costs that make sense. xDSL is a group of emerging Digital Subscriber Line (DSL) modem technologies for supporting high-rate traffic transmission over POTS lines. X stands for asymmetric in ADSL, rate adaptive in RADSL, high-speed in HDSL, and very high speed in VDSL. xDSL delivers Broadband over Copper the best thing about xDSL technologies is their ability to transport large amounts of information across existing copper telephone lines. This is possible because xDSL modems leverage signal processing techniques that insert and extract more digital data onto analog lines. The key is modulation, a process in which one signal modifies the property of another. ADSL Development and Deployment Progress Of all the emerging xDSL technologies, ADSL is receiving the most attention because there is a standard (DMT) for it, and its capabilities provide NSPs with a competitive offering to cable modems. But there is increasing interest in symmetrical xDSL offerings such as HDSL and SDSL. As a local access service, ADSL’s implementation has no critical drawbacks. It can be deployed as an overlay network where there is subscriber demand, eliminating the need for NSPs to risk building out their infrastructure unnecessarily in the hope that the technology will catch on. ADSL development and deployment is focused primarily in North America, followed by northern Europe and the Pacific Rim. In North America, US West, GTE, Ameritech, SBC, BellSouth, and Edmonton Tel (Canada) are the service providers leading the current wave of ADSL/xDSL deployment. Covad, Northpoint, and a handful of other CLECs are entering high-density metropolitan areas—typically offering a portfolio of xDSL offerings at different classes of service and price points, and competing with incumbent local exchange carriers. Chicago-based InterAccess was the first ISP ...
