Broadband DSL

The demand for inexpensive and dependable communications services that resolve local loop problems and enable access to bandwidth-intensive multimedia applications contributes to the development of residential broadband access networks. This chapter examines the capabilities of the DSL (Digital Subscriber Line) technology suite and powerline network solutions for enabling geographically separated subscribers to reliably access basic and sophisticated teleservices from dispersed residential and workplace venues.

DSL is also known as xDSL. The letter “x” stands for any one of the various DSL technologies that are grouped under the DSL umbrella. DSL technologies include ADSL (Asymmetric DSL), RADSL (Rate-Adaptive DSL), VDSL (Very High-Speed DSL), and ADSL.Lite (also known as Universal ADSL and G.Lite). In addition, HDSL1 (High Bit-Rate DSL, Phase 1), HDSL2 (High Bit-Rate DSL, Phase 2), SHDSL (Symmetric High-Bit Rate DSL or G.shdsl), SDSL (Symmetric or Single-Line DSL), IDSL (ISDN or Integrated Services Digital Network DSL), and CDSL (Consumer DSL) are also part of the DSL technology suite. DSL technologies support links to a broad spectrum of Web portals and Internet services and applications such as e-mail exchange, IP telephony, and television programming. In addition, DSL is an enabler of tele-education, teleshopping, E-banking, teletraining, VPNs (Virtual Private Networks), video gaming, teleworking, telementoring, tele-entertainment, and telemedicine.

DSL technologies transform copper twisted pair lines into high-speed links for provisioning connections to basic and advanced communications networks via the local loop. By fostering fast information transmission over the PSTN (Public Switched Telephone Network), DSL eliminates the need for costly optical fiber installations and infrastructure upgrades.

Bandwidth bottlenecks and user frustrations drive the search for last-mile technologies that cost-effectively deliver voice, video, and data services. Developed by Bellcore in 1989, Digital Subscriber Line (DSL) technology operates on telephone wires originally intended to provision voiceband communications. The term “line” refers to the PSTN link or local loop between a user location and the local telephone exchange, also called the telephone company Central Office (CO). Bell Atlantic and British Telecommunications launched initial DSL trials in 1993.

At the outset, DSL technology was regarded as an interim solution for transporting interactive television programming and video-on-demand (VOD) to single family homes and high-rise apartment buildings. Fiber-to-the-home (FTTH) and fiber-to-the-curb (FTTC) configurations were slated to replace DSL solutions. However, as a consequence of the expense and time involved in installing fiber optic cabling directly to the subscriber location, FTTH and FTTC implementations were not widely implemented.

In addition to FTTH and FTTC solutions, cable networks based on optical fiber or a hybrid optical fiber and coaxial cable (HFC) infrastructure were also expected to replace DSL as a residential broadband access solution. With the accelerating popularity of the Web and the demand for fast access to broadband services from the home and the workplace, cable modem technology is now a major DSL competitor solution.

In recent years, the tremendous growth in the utilization of computer networks has contributed to a re-assessment of the distinctive features of the various DSL technologies and their capabilities in cost-effectively delivering bandwidth-intensive services to the customer premise. The endorsement of local loop unbundling in the United States and the European Union has also contributed to the evolvement of symmetric and asymmetric DSL solutions that vary in channel capacity, interactive capabilities, rates enabled, and maturity.

DSL technologies operate over the last mile or local loop. Local loop refers to that length of the copper phone line that interconnects the local telephone exchange and the customer premise. Loop length or the distance between a customer site and the local telephone exchange, line gauge or thickness, and line quality are among the variables that directly impact the availability and reliability of DSL service.

DSL technologies provide an unprecedented amount of affordable bandwidth over ordinary copper telephone lines. Standards groups in the DSL arena define specifications, procedures, methods, and approaches for supporting DSL equipment compatibility and interoperability, seamless communications over the local loop, internetworking operations, and economical and reliable multimedia delivery to the desktop.