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 and emerging.
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.
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 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:
All of these drawbacks pointed to the need for a better way of delivering advanced digital services to end users.
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.
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 amplification.
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.
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.
Significant benefits can be derived from the use of HDSL for the delivery of advanced digital services.
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.
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:
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.
Fiber access
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.
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 copper wiring.
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 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.