Internet access is one of the demands for the new era and
incumbent local exchange carriers want to have better transmission speed and
larger bandwidth using copper wire installed in the local loop. With new
technology arises, Digital Subscriber line was born and saved the ILEC on
providing high-speed internet for users using its traditional copper wires. A
figure below shows how DSL is configured and connected using local loop
networks.
Here are the families of DSL and their technical
descriptions:
Asymmetrical DSL
There are a family of DSL with different speeds and
capability. One of the most famous xDSL is the ADSL or the Asymmetrical DSL.
According to Regis Bates, ADSL is the new modem technology to converge the existing twisted
pair telephone lines into the high−speed communications access capability for
various services. Most people consider ADSL as a transmission system instead of
a modification to the existing transmission facilities. In reality, ADSL is a
modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current
technology.
The
"asymmetric" in ADSL refers
to the fact that the downstream data rate, or the data coming to your computer
from the Internet, is traveling faster than upstream data, or the data
traveling from your computer to the Internet. Upstream data rates are slower
because Web page requests are fairly miniscule data strings that do not require
much bandwidth to handle efficiently.
ISDN DSL
ISDN Digital Subscriber Line (IDSL) uses ISDN-based
technology to provide a data communication channel across existing copper
telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual
channel ISDN connection at 128kbit/s. The digital transmission bypasses the
telephone company's central office equipment that handles analogue signals.
IDSL uses the ISDN grade loop without Basic
Rate Interface in ISDN transmission mode. The benefits of IDSL over ISDN are
that IDSL provides always-on connections and transmits data via a data network
rather than the carrier's voice network. IDSL also avoids per-call fees by
being generally billed at a flat-rate.
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 a T1 line (up to
1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in
Europe over a somewhat longer range and is considered an alternative to a T1 or
E1 connection. It was developed to overcome the problem of sending data
on copper wires using T1 lines. HDSL does not also require repeaters over
longer wires and also bridge taps is not a primary problem.
SDSL
Symmetric Digital Subscriber Line
is a technology that allows more data to be sent over existing
copper telephone lines (POTS). SDSL works by sending digital pulses in the high-frequency area
of telephone wires and cannot operate simultaneously with voice connections
over the same wires. SDSL requires a special SDSL modem. SDSL is called sysmmetric because it supports the same data
rates for upstream and downstream traffic.
RADSL
Rate Adaptive
Digital Subscriber Loop was developed by Westell and has a potential of 2.2
Mbps downstream and 1.0 Mbps upstream. What makes RADSL more flexible is that
the sync rate can be dynamically adjusted up or down as line conditions change.
This makes it more of a viable alternative where line conditions are marginal
due to distance or other factors. In many respects, RADSL is an enhanced ADSL
to meet a more diverse set of line conditions. Like ADSL, RADSL can piggyback
on the POTS line. RADSL does not require any splitters or filters.
VDSL
Very high rate
Digital Subscriber Loop is a DSL still in development with a current downstream
capacity of 52.8 Mbps, and upstream of 2.3 Mbps. At this time, VDSL is limited
to very short loop lengths, and is not yet a viable alternative. It may find
application where there is fiber to the neighborhood, and thus the copper loop segment
is relatively short.
G.SHDSL
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 192 kbps and 2.31 Mbps.
A table below is shown to summarize the capabilities of
each DSL network and its transmission rate.
Sources:
