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What is DSL?
by Vix! July 12, 2000

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DSL (Digital Subscriber Line) describes a group of technologies that deliver high speed connections
over existing telephone copper wiring. The two-wire copper phone line from a telephone company's
central office to a customer's premise is often referred to as "the last mile." This "last mile" of copper
wiring has long been the bottleneck to providing fast data services to homes and businesses.

DSL utilizes a new line-coding scheme that allows very high-speed, affordable connections over this
last mile for the first time. 

In addition, DSL connections are "always on" or "always connected." This means that users do not need
to dial up each time they want a connection to the network.

Types of DSL

The different types of DSL provide different speeds and have different operating characteristics.
All serve as high-speed pipes for remote connections. Here is a brief summary of the current varieties of DSL:

HDSL (High-bit rate DSL):
The oldest of the DSL technologies, HDSL continues to be used by telephone companies deploying
T1 lines at 1.5 Mbps and requires two twisted pairs.

ADSL (Asymmetric DSL):
ADSL offers a wide variety of downstream and upstream rates. ADSL is often provisioned with greater downstream than upstream rates hence the "Asymmetric" name). These rates are dependent on the
distance a user is from the CO and may vary from up to 9 Mbps to as little as 384 Kbps.

RADSL (Rate Adaptive DSL):
Using modified ADSL software, RADSL makes it possible for modems automatically and dynamically
to adjust their transmission speeds. This often allows for good data rates for customers residing
long distances from the CO (Central Office).

SDSL (Symmetric DSL):
Developed as a modified HDSL software technology, SDSL is intended to provide 1.5 Mbps in both directions over a single twisted pair. However, the distance over which this can be achieved is less than 8,000 feet.

VDSL (Very-high rate DSL):
The newest of the DSL technologies, VDSL can offer speeds up to 25 Mbps downstream and 3 Mbps upstream. Similar to SDSL, the gain in speed can be achieved only at short distances. These maximum speeds can be achieved only up to 1,000 feet.

IDSL provides up to 144Kbps transfer rates in each direction and can be provisioned on any ISDN-capable phone line. Unlike ADSL and other DSL technologies, IDSL can be deployed regardless of the distance the
user is from the Central Office.

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