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XDSL

Digital Subscriber Line (DSL)

 

A technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. XDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses. 

 

High Data-Rate Digital Subscriber Line (HDSL)

 

HDSL is simply a better way of transmitting T1/E1 over copper wires, using less bandwidth without repeaters. It uses more advanced modulation techniques to transmit 1.544 Mbps over lines up to 12,000 feet long.

 

Single-Line Digital Subscriber Line (SDSL)
 
SDSL is a single-line version of HDSL, transmitting T1/E1 signals over a single twisted pair, and able to operate over the plain old telephone service (POTS) so that a single line can support POTS and T-1/E-1 at the same time. It fits the market for residence connection which must often work over a single telephone line. However, SDSL will not reach much beyond 10,000 feet. At the same distance, ADSL reaches rates above 6 Mbps.
 

Asymmetric Digital Subscriber Line (ADSL)

 

ADSL is intended to complete the connection with the customer's premise. It transmits two separate data streams with much more bandwidth devoted to the downstream leg to the customer than returning. It is effective because symmetric signals in many pairs within a cable (as occurs in cables coming out of the central office) significantly limit the data rate and possible line length.

 

ADSL succeeds because it takes advantage of the fact that most of its target applications (video-on-demand, home shopping, Internet access, remote LAN access, multimedia, and PC services) function perfectly well with a relatively low upstream data rate. MPEG movies require 1.5 or 3.0 Mbps down stream but need only between 16 kbps and 64 kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth.

 

Very-High-Data-Rate Digital Subscriber Line (VDSL)

 

The most promising technology capable of delivering full service is very-high-data-rate DSL (VDSL). VDSL is both symmetric and asymmetric and provides up to 52 Mbps of bandwidth over voice on a single twisted-pair copper loop. VDSL technology provides the telco with the ability to create the type of business that is critical for success in the new millennium. Those service providers that can deliver full narrowband and broadband services consisting of voice, data, and video will be the dominating forces in the industry today and tomorrow.

 

 

 

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