xDSL Digital Subscriber Line technologies – ADSL, HDSL, VDSL

xDSL is a catchall name for a variety of DSL (Digital Subscriber Line) technologies developed to offer phone companies a way into the cable TV business. It isn’t a new idea; Bell Communications Research Inc. developed the first DSL back in 1987 to deliver video on demand and interactive TV over copper wires. That effort came to nothing and deployment since has been mostly limited to field trials, the technology being hampered by lack of industry-wide standards.

However, interest in xDSL received a major boost with the passage of the Telecommunications Reform Act of 1996. This legislation ended local service monopolies and allowed competition among local phone companies, long-distance carriers, cable companies, radio and TV broadcasters, and Internet Service Providers (ISPs). Suddenly, local exchange carriers needed a broadband service for the local loop to combat cable companies’ plans to offer cable modem and telephony services.

xDSL technologies are very fast, typically offering download speeds up to 52 Mbit/s and upload speeds ranging from 64 Kbit’s to over 2 Mbit’s, and come in a number of variants:

  • asymmetric (ADSL)
  • high-bit rate (HDSL)
  • single-line (SDSL)
  • very-high-data-rate (HDSL).

The different approaches have differing trade-offs between signal distance and speed and differences in symmetry of upstream and downstream traffic which, taken together, make them suited to different applications. ADSL (Asymmetric Digital Subscriber Line) is the most popular technology for broadband communications in home and business use.