DSL
(Digital Subscriber Line) is 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.
Types of DSL
ADSL
The variation called ADSL (Asymmetric Digital Subscriber Line) is
the form of DSL that will become most familiar to home and small
business users. ADSL is called "asymmetric" because most
of its two-way or duplex bandwidth is devoted to the downstream
direction, sending data to the user. Only a small portion of bandwidth
is available for upstream or user-interaction messages. However,
most Internet and especially graphics- or multi-media intensive
Web data need lots of downstream bandwidth, but user requests and
responses are small and require little upstream bandwidth. Using
ADSL, up to 6.1 megabits per second of data can be sent downstream
and up to 640 Kbps upstream. The high downstream bandwidth means
that your telephone line will be able to bring motion video, audio,
and 3-D images to your computer or hooked-in TV set. In addition,
a small portion of the downstream bandwidth can be devoted to voice
rather data, and you can hold phone conversations without requiring
a separate line. Unlike a similar service over your cable TV line,
using ADSL, you won't be competing for bandwidth with neighbors
in your area. In many cases, your existing telephone lines will
work with ADSL. In some areas, they may need upgrading.
G.SHDSL
Also known as G.991.2, G.SHDSL is an international standard for
symmetric DSL developed by the ITU. G.SHDSL provides for sending
and receiving high-speed symmetrical data streams over a single
pair of copper wires at rates between 192Kbps and 2.31 Mbps.
HDSL
HDSL (High bit-rate Digital Subscriber Line), one of the earliest
forms of DSL, is used for wideband digital transmission within a
corporate site and between the telephone company and a customer.
The main characteristic of HDSL is that it is symmetrical: an equal
amount of bandwidth is available in both directions. HDSL can carry
as much on a single wire of twisted-pair cable as can be carried
on an E1 line (up to 2.048 Mbps) in Pakistan.
SDSL
SDSL (Symmetric DSL) uses a single twisted-pair line carrying 2.048
Mbps each direction on a duplex line. It's symmetric because the
data rate is the same in both directions.
VDSL
VDSL (Very high data rate DSL) is a developing technology that promises
much higher data rates over relatively short distances (between
51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length).
It's envisioned that VDSL may emerge somewhat after ADSL is widely
deployed and co-exist with it.
