Copper Cable with HDSL:
A Technology Brief from PairGain Technologies, Inc.
Copper Cable: A Historical Perspective
Over a century ago, utility companies started to bury copper cable into the earth for the purpose of
telephone connectivity. Perhaps no other investment in the history of modern business has been so
nurtured, as this copper cable now represents a collective asset estimated at hundreds of billions of
dollars. It's a cable plant that today criss-crosses over virtually every mile in all developing
countries. And while much is made of the advanced communications information superhighway,
the truth of the matter is that most of that road is still paved with workhorse copper cable.
As time passed, individual houses and businesses were connected to their central telephone office
via individual cables at, by today's standard, very slow speeds. Central offices, on the other hand,
were connected to each other with what would become known as a T1 or E1 line -- comprised of
24 or 30 voice lines in one digital pipe. For clarification, the vernacular of T1 and E1 have come to
be synonymous with certain speeds and with phone company -- driven applications, however, in
their most basic forms, T1 is simply a 1.544 megabit per second (mbps) pipe and E1 is a 2.048
mbps pipe that can be used for a wide variety of public and private communications-both existing
Initially, these T1/E1 links functioned as the backbone for voice, data and video -- the most critical,
bandwidth-intensive arteries -- between central offices, with lower speed feeder lines tapping into
them. As it stands today, however, realities have shifted and T1/E1 lines have settled into a very
specific role. Though the use of T1/E1 is still heavy for inter-office applications in smaller central
offices, backbone status is now usually reserved for high-capacity media such as fiber, with T1/E1
being used as a building block or as an attachment to public and private backbones.
T1/E1 Today: An Essential Building Block
From its roots as a telco inter-office connectivity device, T1/E1 has settled in as a core building
block for voice, data and video delivery. In the early 1980s, T1/E1 migrated to the local loop-also
known as the last mile. The local loop is the distance between the local telco office and the end
user, usually about 2.5 miles or about 4 km, and it is in this capacity, as a customer access vehicle
for both public and private networks, that T1/E1 is most commonly used. At present, it is estimated
that in excess of 700,000 T1 lines have been installed in the U.S., and approximately 80 percent of
those lines have been implemented with copper. Outside the U.S. and Canada, except for Japan
and a few other countries, the standard for transmission of digital information, E1, is expected to
reach similar levels of usage as deregulation of public telephone companies continues around the
world and competition increases.
In large urban areas, heavy user concentration and sophisticated communications requirements
have mandated the use of fiber-though as a prerequisite in this setting, many customers must be
located within hundreds of feet from the end of a fiber optic line. In these instances-and only when
significant bandwidth (multiple T1/E1 lines) is an absolute necessity-fiber plants have been installed
and represent a cost-effective way to achieve local public and private network connectivity. In the
majority of cases, however, particularly in rural areas, or more typical cityscapes, the use of fiber
can't be cost justified. As a result, economics dictate the use of existing copper cable plants.
The Drawbacks of Repeatered T1/E1
The physical qualities of copper wire make high-speed signal transmission difficult. But as
necessity is the mother of invention, engineers designed a way to move signals at relatively high
speeds over copper. The system that was devised is based on repeaters, and today much of the
industry refers to local loop T1/E1 as repeatered T1/E1.
Inherently, copper wire distorts signal quality, so repeaters or amplifiers are used, tapping into
copper cable at prescribed intervals to restore signal quality. And therein lies the problem. In and
of themselves, repeaters are rather unsophisticated electronics devices, and in today's T1/E1
operating schemes, they need to be installed about every 3,000 to 4,000 feet or 1 km. It's a time-
and labor-intensive process.
Repeatered T1/E1 has a number of significant drawbacks:
The cost to install a repeatered T1/E1 line can reach several thousand dollars.
Installation is an arduous task, requiring a significant amount of labor; due to required circuit
design and line conditioning. The process can take several weeks for one line.
Typically, repeatered T1/E1 yields poor signal quality when compared to fiber optics and
Each segment of the line has to be checked between pairs of repeaters; wire pairs must be
selected carefully to avoid crosstalk; service records must be searched; bridged taps need
to be removed; if suitable pairs can't be found new cable routes need to be engineered and
Installation of repeaters may require re-engineering.
Repeaters can be numerous, and upon failure, troubleshooting and maintenance is costly.
To reconfigure repeatered T1/E1 lines for additional services, repeaters have to be
physically removed at an additional cost.
All of this labor comes at a time when public and private organizations are reducing
All of these drawbacks pointed to the need for a better way of delivering advanced digital services
to end users.
The Advent of HDSL
The telephone companies saw the realities. They had mile after mile of copper cable, they knew
the logistical and cost ramifications of fiber limited its deployment, and they recognized the flaws
of repeatered T1/E1. So they started looking for a better way, and in the late 1980s the collective
research arm of the Bell operating companies, Bellcore, devised a new method.
They called the concept High-bit-rate Digital Subscriber Line (HDSL). Its intent was
straightforward: to deliver a high-performance and cost-effective way of transmitting at speeds up
to 2 mbps over existing copper cable, a way that was adaptable, faster and precluded the need for
repeaters or special line conditioning. HDSL leverages the huge amount of existing copper cable,
delivering a streamlined alternative to the installation of competing solutions.
Initially, deployment of HDSL was slow, and it was used by engineers to provide T1/E1 links on
the more difficult and potentially most complex routes. As prices improved, HDSL implementation
has become the fastest and most cost competitive method of establishing T1/E1 links on copper
wire for public and private networks. Just as importantly, HDSL's adaptive capabilities provide a
level of signal quality comparable to a fiber optic link, and services can be turned up in hours
instead of weeks or months.
How HDSL Works
HDSL allows public carriers and private organizations to make optimal use of one of their largest
corporate assets, the embedded copper loop plant. HDSL transforms that copper wire -- the wire
that heretofore was viewed as being single-voice telephone line -- into high-speed digital channels
capable of providing advanced service options, not only to corporations and small business, but to
the home. Today, HDSL's foremost application is to provide advanced digital services to local loop
customers and corporate end users.
HDSL applies advanced electronics, allowing telecommunications carriers and private
organizations to use existing copper transmission lines to carry fiber quality traffic, without having
to manipulate the wire itself. Ultimately, HDSL yields more productive service for end users and
at a reduced cost, because it quadruples the distance a digital signal can travel without the need for
Here's how it works. A single HDSL-compatible card is attached at the central office and another
card is installed at the customer premise in a telco environment or at two separate sites in a private
campus area network. These systems use advanced integrated circuit designs, employing complex
digital signal processing (DSP) techniques and software-based algorithms.
HDSL sends signals at normal power levels. The differentiation is HDSL's singular, efficient
ability to maintain or restore signal integrity in the face of copper's imperfections. HDSL creates a
mathematical model of copper wire, allowing the transmission device to adroitly and precisely
compensate for copper-based distortion. This adjustment happens continuously, so the transmission
signal will not degrade as wire or environmental conditions change.
When HDSL Makes Sense
HDSL is an ideal solution in any situation where dispersed sites need to be connected, and when
time, budget and performance are significant factors. It is being used today to provide telephone
companies and organizations deploying campus area networks with a way to offer advanced
high-speed digital services to end users that previously would have required the installation of other
costlier and time-intensive alternatives.
The installation of fiber optic or coaxial cable is a long-term and costly proposition. For example, it
has been estimated that it would take up to 20 years to reach the majority of customers of a large
U.S. regional Bell operating company through the use of fiber optic or coaxial cable. HDSL on the
other hand, can reach most of a telephone company's customers almost immediately since the
copper infrastructure is already in place.
The Benefits of HDSL
Significant benefits can be derived from the use of HDSL for the delivery of advanced digital
HDSL decreases the cost of installing T1/E1 lines, and significantly decreases the amount
of time required to install them.
HDSL allows you to extend connectivity using standard 24-gauge (0.5 mm) copper cable
over 2.3 miles or 3.6 km. With certain hardware enhancements or heavier gauge copper,
you can facilitate connectivity over distances up to 5 miles or 7 km.
The adaptive digital signal processing algorithms employed by HDSL result in much higher
transmission quality than repeatered T1/E1.
On the remote end of the line, HDSL uses a minimal amount of power, making remote
powering from the central office feasible.
HDSL solutions are deployable in over 99 percent of the local loop plant.
No cable conditioning, bridged tap removal or cable plant reinforcement is required with
The elimination of repeaters increases overall system reliability and transmission
No separate monitoring equipment is required with HDSL.
Through today's advanced electronics, HDSL is immune to crosstalk, while yielding signal
quality comparable to fiber optics, bit error rate 10-10 (BER 10-10).
The Applications of HDSL
There are several major applications for HDSL, and certainly there are infinite possibilities for the
technology's use going forward.
The first such application involves services provided by the phone companies, generically labeled
public network access or data network access. This involves harnessing the power of a T1/E1 line,
for business or residential uses, to move voice, data or video traffic. The local phone company
provides that service and charges you for it, based on applicable local tariffs.
HDSL in the Private Sector
In addition to the so-called public applications of HDSL, there is a burgeoning opportunity for
HDSL to add value in the private sector, mainly, as an augmentation to emerging enterprise
networks. In this setting there is a natural role for HDSL to play.
Outside of the telco setting, HDSL is becoming increasingly effective in campus area networks. In
this context, a campus is defined as a setting where multiple locations or buildings are located a
few miles or kilometers apart, a single building with multiple floors or a single building spread over
an expanse of real estate (e.g.: a large manufacturing facility). The implied commonality in any of
these campus situations is an embedded copper cable plant.
Traditionally, campus area networks are prevalent in a number of industries, including local, state
and federal governments and utility companies. In addition, HDSL has been particularly well
accepted within hospital, university/college and military base settings. HDSL is being used, and is
especially well-suited, for a number of specific campus applications, including:
LAN connectivity/extension-allowing networks and users in different locations to easily
PBX networks/channel extensions-to distribute voice traffic from a centralized PBX to
usersthroughout the campus.
Video conferencing/distance learning-enabling multiple video hook-ups, allowing a subject at
one site to be seen and to interact with people at various distributed sites.
Fiber backbone extension-using HDSL and traditional copper circuits to connect with
high-speed fiber optic backbones across acampus network.
There are several opportunities within a campus setting in which HDSL implementation is
extremely effective-where it can be established quickly, cost-effectively and where it can serve a
specialized, valuable function.
In many larger metropolitan areas, cities have gone to great lengths, and spent millions of dollars,
to install vital fiber backbones using SONET (Synchronous Optical Network) in the U.S., or SDH
(Synchronous Digital Hierarchy) in countries where E1 is the standard. SONET/SDH provides
gigabytes of available bandwidth for mission-critical applications. Due to cost and logistical factors,
the scope of these backbones is limited, with only a relatively small percentage of sites being able
to tap directly into the backbone. There often remains a great many associated city buildings and
end users that could benefit from access to this fiber, but direct fiber connection is prohibitive. This
situation is a perfect fit for HDSL.
With HDSL, satellite locations can be linked quickly, transparently and inexpensively, using existing
copper to produce fiber optic-quality transmission (BER 10-10).
Traditionally, fiber is employed within a campus area network on a more conservative basis,
usually being used to connect a small number of buildings or to connect critical users at just one
site. In this environment, FDDI (Fiber Distributed Data Interface, a network backbone technology
that uses a fiber ring to connect critical resources and transmitting at 100 mbps) has gained
significant acceptance. The most convenient and versatile way to give users outside the scope of
the FDDI ring access to fiber is through HDSL. Again, users can use existing copper cable plants
to link buildings and connect to the central routes that enable access to the fiber ring. The cost to
accomplish this is small, roughly 80 percent less than that of fiber, and can virtually happen in
under one hour.
Other campus area network HDSL applications
HDSL provides a unique capability for campus area connectivity in several other scenarios. First,
it can provide instant line-speed enhancement-from 56 kbps to 2.048 mbps, 30 times faster-for
building-to-building connection. Second, HDSL can significantly extend the potential operating
distance between sites. Connectivity using DSU/CSU connection or T1/E1 line drivers is limited to
4,000 feet (1.2 km) and 2,000 feet (.6 km) respectively. With HDSL, you can facilitate T1/E1
connections over four times the distance with no repeaters. And finally, HDSL is ideal for system
redundancy and disaster recovery. In mission-critical situations using fiber or another medium,
copper-based HDSL provides a natural back-up, with the capability to instantly get networks up
and running when fiber links fail.
The Future of HDSL
As telcos and private organizations continue to look for more efficient, timely and cost-effective
ways to connect users and networks, HDSL promises to play a vital role. As corporations
increasingly rely on highly-available switching based backbone networks for their business needs, it
becomes essential to extend the performance and bandwidth benefits of the backbone to all parts
of the enterprise. HDSL is a key enabling technology that allows a wide range of high-speed
digital connectivity options from the backbone network to other corporate sites over existing
On the horizon, is an extension to HDSL, asymmetrical digital subscriber line (ADSL), capable of
transmitting signals over copper cable at 6 Mbps. ADSL uses an advanced signal processing
technique that significantly transforms traditional copper transmission speeds. It sends signals in
one direction at extreme speeds (up to 6 mbps), while the return channel operates at a more
modest rate of 100 to 600 kbps. The ADSL channel is transparent to normaltelephone activity, and
can be accomplished using the same copper pair. Going forward, ADSL will have widespread
impact and application. The power of the home computer -- on-line shopping, banking, video
phones and video on demand -- can be made reality with ADSL, which uses the copper cable
resources on hand in every residence. ADSL technology will make these advanced consumer
applications economically feasible.
HDSL and CopperOptics
HDSL is an innovation that bridges the gap between copper cable and fiber optics, via a process
that PairGain Technologies calls CopperOptics. Essentially, it leverages the huge investment in
copper cable plants worldwide, by enabling pristine signal transmission over existing copper cable
at speeds of up to 2 mbps. Used for public network access and in private campus area networks,
HDSL employs sophisticated electronics at either end of the copper cable to elegantly send
information over copper wire. By using existing cable and inexpensive electronics, HDSL can be
implemented quickly and without the excessive labor charges required to run fiber optic cable or
implement repeatered T1/E1. With the abundance of copper cable throughout the world, the
application of HDSL is virtually limitless.
PairGain stands as the dominant supplier in the HDSL market, with over 75 percent market share.
In an era of computing/communications jargon, PairGain Technologies has created a more concise,
memorable way to think of HDSL...CopperOptics, referring to the overriding benefit of HDSL
using PairGain technology, namely, fiber optic quality transmission over copper. PairGain's HDSL
products allow telcos and corporate end users to provision advanced digital services at lower costs
and in less time than with traditional repeatered T1/E1 or fiber optic deployment.
PairGain has become synonymous with the term CopperOptics, and essentially, it has come to
define our business: delivering high-speed, fiber optic-quality digital transmission over the last mile
of ordinary copper wire, allowing both public and private telecommunications providers to
seamlessly link networks and end users.