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xDSL is a technology that allows high speed data
transmission over existing copper cables that are already in place with
most telephone companies, businesses and school campus grounds. The
technology has much the same look and feel of standard leased line or
short haul modems.
The basic differences between the xDSL technologies
are a matter of
speed, operating distance, the ratio between upstream and downstream
speeds and suitable applications.
HDSL
The most mature xDSL technology is high-bit-rate DSL (HDSL). Two
HDSL pairs can replace a T1 line, achieving speeds of 1.544 Mbps over
distances as far as 12,000 feet — without repeaters. Telephone carriers
have been using HDSL for years to provision T1 lines, enabling them to
turn up services in hours instead of weeks or months. Almost every T1
line installed during the past few years has relied on HDSL modems, and
more than a quarter of a million HDSL lines already are in place.
Because HDSL offers signal quality comparable to fiber-optic links,
telephone companies and network administrators also are using it to
deploy campus area networks with connected LANs to provide
advanced high-speed digital services for their end users.
HDSL's robust transmission scheme also enables data to be sent over a
single twisted-pair copper wire at speeds up to 768 kbps in both
directions. With a throughput rate six times higher than that of ISDN,
this
mature, stable technology is ideal for applications that require greater
symmetry, such as access to corporate intranets, real-time
videoconferencing and telemedicine. And because HDSL is far less
complex, more integrated and less power hungry than ADSL, HDSL
modems are priced at roughly half that of equivalent ADSL solutions. In
addition, by scaling down the bandwidth of HDSL to 386 kbps, loop
reaches in excess of 15,000 feet can be achieved.
Because HDSL is a proven, mature technology with far less expensive
modems, this broadband technology promises to continue playing a vital
role as telcos and private organizations look for more efficient, timely
and
cost-effective ways to connect users and networks in business
applications.
"In its current incarnation, HDSL is much less expensive than ADSL
and
it holds tremendous promise to fulfill high-bandwidth demand for the near
term," said Kieran Taylor, a broadband consultant for TeleChoice. He
expects HDSL to dominate the market through 1999.
ADSL
Asymmetric DSL (ADSL) is the xDSL family member grabbing the most
headlines. As its name implies, ADSL provides asymmetric throughput.
Because it transmits a much higher downstream rate than upstream, it is
suitable for Internet access applications where far more information is
downloaded than uploaded.
This modem technology has been touted to support data rates as high as
9 Mbps downstream to the user and 640 kbps upstream from the user.
ADSL faces distance limitations, however. Those maximum rates can be
supported for distances as far as 9,000 feet from the central office. At
12,000 feet, ADSL can support downstream rates of 6.3 Mbps; at
16,000 feet, it can support 2.048 Mbps, or E-1 rates. At 18,000 feet, it
supports downstream rates of 1.544 Mbps, the same as a T1. The
upstream rate can be as low as 64 kbps for these distances. In the
United States, close to 80 percent of customers are less than 18,000 feet
from a central office.
During video-on-demand trials in 1993 and 1994, carriers found out the
hard way that ADSL itself is not profitable unless users are sold on the
applications it enables. But when the Internet and its ensuing bandwidth
problems became applications in search of an emerging technology,
ADSL seemed to be the perfect fit.
HDSL, however, currently has several advantages over its ADSL cousin.
Foremost among these is that HDSL is a proven, less expensive
technology with substantially less complexity. With ADSL transceivers in
their infancy, digital signal processing algorithms are complex and can
take up to five to eight times the silicon gate count as an HDSL
implementation, making ADSL more expensive. ADSL also takes about
three times more additional power than HDSL, as well as requiring more
processing memory.
What's more, given the asymmetric nature of ADSL, it is not optimized to
support many important business applications which require roughly
equal amounts of bandwidth upstream and downstream, such as LAN
interconnection, videoconferencing and multimedia.
However, with its expected performance, ADSL will represent an
excellent residential transmission option. The POTS overlay capability
will also help carriers preserve their copper infrastructure, as only one
line will be required to offer residences telephone and data access.
Two other xDSL cousins, RADSL and SDSL, can be considered
variants of ADSL. They use similar line codes, but software is modified
so that the speed of the channels can be enhanced to support certain
applications.
RADSL
Rate Adaptive DSL (RADSL) allows individual modem pairs to
automatically and dynamically match their transmission speeds depending
on line quality. The primary difference between ADSL and RADSL is
that RADSL can work at different speeds. This overcomes the distance
limitations for xDSL, enabling carriers to serve customers who reside a
long way from the nearest central office. Although a remote customer
may not get the fastest RADSL speeds, due to the distance of the copper
wire needed to be traveled, they'll get the best possible rate given the
circumstances.
SDSL
Single-line DSL (SDSL) has yet to be introduced to the market. Its
proponents put its performance at 1.5 Mbps in both directions, meaning
that a single twisted-pair line can take the place of a T1 as well as
provide POTS capabilities. However, the distance scales back to less than
8,000 feet. The combination of support for a symmetrical data flow and T1
throughput on a single line makes SDSL useful for applications like
videoconferencing and some kinds of servers, where this distance reduction
is not an issue.
VDSL
The youngest and fastest member of the xDSL is very-high-rate DSL
(VDSL). Technical specifications are still being finalized for VDSL, and
it
will take time before it is commercially available. It is, however, a
promising option for future high-definition television and high-speed
residential access when fiber to the curb becomes more commonly
available.
What VDSL gains in speed, it loses in distance. Throughput rates will
depend on actual line length between the customer premises and the
central office switching equipment. Maximum speeds for VDSL are 25
Mbps downstream and 3 Mbps upstream over lines up to 1,000 feet in
length.
Whatever xDSL option you and your customers may
choose, digital
subscriber line technologies are making low-cost, high-speed remote
access a reality. They are breaking the speed barriers in the local loop
by
using the existing copper infrastructure to deliver broadband services.
Together these technologies are satisfying today's bandwidth needs while
paving the way for tomorrow's information superhighway.
Email
us with any questions or
how to order.
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