HDSL2 T
ECHNICAL
B
RIEF
T
HE
S
INGLE
L
OOP
A
DVANTAGE
O
VER
SDSL
Overview
Demand for high-speed data services and Internet access is on a dramatic
upswing in the business world. As a result, telecommunications service
providers have widely adopted T1 lines as the preferred vehicle to provide
the speed and bandwidth to deliver these vital services. In 1997, there were
approximately 1.7 million T1s installed in the United States. Conservative
estimates show T1 deployment growing at a healthy 30 to 40 percent a
year.
High bit-rate digital subscriber line (HDSL) technology is the dominant
choice among carriers for delivering T1s. In fact, more than 50 percent of
the T1s being installed today on copper employ HDSL. A new standard has
emerged in delivering T1s over unbundled copper called HDSL2.
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491
HDSL2 Technical Brief
HDSL technology has proven to be highly reliable and has played a major role in easing

many time and cost issues associated with T1 line provisioning and installation. For
example, an HDSL T1 solution can reach 12,000 feet. For the same distance, the old
repeatered T1 method would require labor-intensive installation of a repeater module
every 3,000 feet.
In the telecom world, carriers are constantly under pressure to deliver services more
cost effectively, while reducing installation intervals. Another pressure point is the
carriers’ need to conserve their preciously limited but highly coveted copper resources.
To meet these requirements of change, the industry has begun a migration from two
copper pair solutions such as HDSL, to single-pair solutions. The theory is that these
single-pair offerings will provide the same data rates as two-pair technologies while
using half the copper resources.
Equipment vendors are currently working together in the American National Standards
Institute (ANSI) T1E1.4 subcommittee to develop HDSL2, a next-generation, single-pair
version of HDSL. HDSL2 provides full duplex 1.544 Mbps transmission over a single pair
to an unrepeatered carrier serving area (CSA) distance of 12,000 feet. HDSL2 will be
spectrally compatible with other services in the same cable, including HDSL, ADSL, T1
and ISDN. It will also be able to meet a 5 dB noise margin in worst-case crosstalk and
have a latency of less than 500 microseconds. Another goal in the ANSI committee is for
HDSL2 to be vendor-interoperable.
The plan is to bring HDSL2 standards-compliant systems to market for testing and
commercial rollout by the end of 1998.
Single-pair digital subscriber line (SDSL) is another technology that delivers data over
fewer copper pairs. Several different vendors have developed solutions claiming to fit
this technological slot. There is confusion in the industry, however, about what the true
definition of SDSL is. None of these solutions offer the performance quality or standards
committee cooperation inherent with HDSL2.
A “Single” Solution
Single-Pair Confusion
Technology
Data Rate Mode

Distance Limit
from Central
Office
Number of
Copper Pairs
HDSL2
SDSL
1.544 Mbps
1.544 Mbps
Duplex
Duplex
12,000 feet
10,000 feet
1
1
The first definition of SDSL could be described as “HDSL divided by two.” It essentially
takes one loop from the existing two-loop HDSL approach. As a result, it provides 784
kbps data rate, or half of the full 1.544 Mbps of HDSL. With this 50 percent data rate,
this definition clearly does not stack up to HDSL2.
The second SDSL definition includes a single HDSL loop on which the clock has been
sped up at least two times. This allows transport of 1.544 Mbps over the span. Another
approach is the proprietary CAP over a single loop, with which a carrier can also achieve
full DS-1 transport at 1.544 Mbps.
The HDSL Standby
3
4/98
491
HDSL2 Technical Brief
These latter two approaches also do not meet HDSL2 specifications. First, neither is able
to achieve full 12,000 foot CSA loop reach without worst-case crosstalk. Additionally,

they are not spectrally compatible with other services. As a result of not meeting these
criteria, these SDSL approaches are not being considered by ANSI standards committees.
Equipment vendors understand the importance of maximizing existing copper reserves
to carriers while optimizing equipment and installation costs. This is why most of the
leading vendors – usually fierce competitors – are working hand-in-hand in the T1E1.4
committee to develop a well-defined, interoperable standard for single-pair copper
service delivery. That standard is HDSL2. Because of its rigid performance guidelines, it
will ensure carriers the most optimal network performance.
ADC Telecommunications is taking a lead role in the development of HDSL2. In fact the
lead editor of the HDSL2 standard is an ADC engineer. When the technology is ready for
the commercial market, vendors will integrate HDSL2 into existing platforms for
carriers to deploy T1-based service. ADC’s Soneplex
®

and Cellworx
™
product platforms
will accommodate HDSL2, allowing carriers to maximize the existing infrastructure
they’ve grown comfortable with and relied upon.
The applications of both HDSL2 and its predecessor are the same. HDSL will still have a
role as well in T1 deployment. It will be required for circuits of longer distances, where
there’s a requirement to power a significant amount of components in the outside plant.
HDSL2 is a natural for short distances or unbundled copper T1 deliveries. Both will
accommodate frame relay for T1, Internet interconnection, LAN extension, remote LAN
access and ISDN.
Despite concerns that there is a lot of confusion and misconception in the market about
the various “single-loop” service delivery technologies, the emerging HDSL2 technology
offers clear-cut advantages for carriers when compared with the several divergent SDSL
offerings:
• HDSL2 is an emerging ANSI standard. This offers interoperability between vendors

and the thorough technical review of the ANSI T1E1.4 subcommittee. Conversely, SDSL
solutions are proprietary. When all is said and done, standard solutions are always more
cost-effective for carriers.
• HDSL2 has full CSA loop reach, even in the presence of worst-case crosstalk. SDSL
has unacceptably lower performance under the same conditions.
• Some SDSL solutions are not spectrally compatible with other services, whereas
HDSL2 is engineered to be compatible.
HDSL2 is the natural evolution of and improvement upon the well-established HDSL
technology, which carriers use for a majority of today’s T1 deployment. The multi-
vendor cooperation and painstaking testing during the standardization process will
ensure that HDSL2 brings the same quality and reliability as its predecessor plus many
more copper-saving enhancements. For carriers considering single-loop T1 technolo-
gies, HDSL2 is the clear choice.
One Pair, One Real
Choice
491 4/98 Original © 1998 ADC Telecommunications, Inc. All Rights Reserved An Equal Opportunity Employer
Specifications published here are current as of the date of publication of this document. Because we are continuously improving our products,
ADC reserves the right to change specifications without prior notice. At any time, you may verify product specifications by contacting our
headquarters office in Minneapolis. ADC Telecommunications, Inc. views its patent portfolio as an important corporate asset and vigorously
enforces its patents. Products or features contained herein may be covered by one or more U.S. or foreign patents.
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