The Existing Copper Wire Infrastructure

DSL
products bring entirely new service capabilities to the existing copper wire
local access network. In order to understand the opportunities and challenges
relating to the deployment of DSL-based services, it is useful to review the
existing infrastructure of the telephone network.

Telephone Company Networks

The telephone networks currently in place within Incumbent Local Exchange
Carriers (ILECs) and Public Telephone Operators (PTOs) represent a huge capital
investment that has taken place over the last 120 years. This structure was
primarily designed for voice services. Over time, telephone networks have
undergone numerous modernization and infrastructure upgrades to take advantage
of advancements in transmission and switching technologies. In particular, high
capacity, fiber optic transmission facilities currently exist in nearly every
telephone company backbone network worldwide. The use of fiber optics improved
the quality of services, increased the capacity of traffic that can be supported
over the backbone network and reduced operational expenses for network
operators.

As a result, high capacity service capabilities exist between telephone
company offices. However, the situation is very different when you look at the
local loop access network–the last leg, which connects end-service users to
the telephone company backbone networks. Any discussion of the local loop and
high-speed data services must start with an examination of the topology of the
existing voice services physical network.

Figure
on next page represents a typical ILEC/PTO telephone network. Several Central
Offices (COs) are depicted as being outfitted with telephone switches and
transmission equipment, as well as digital loop carrier Remote Terminals (RTs).

From the home or office, twisted-pair copper wire local loops interconnect to
the telephone switch through a Main Distribution Frame (MDF). The MDF is the
central point at which all local loops terminate in the CO.

Central Offices are interconnected through an inter-CO network. This network
consists of Digital Access and Cross Connect Systems (DACS) and T/E-carrier
transmission equipment. Inter-CO networks have been upgraded to the latest in
fiber optic ring technology (SONET or SDH).

The Access Network

The
access network consists of the local loop and associated equipment that connects
the service user location to the central office. This network typically consists
of cable bundles carrying thousands of twisted-wire pairs to Feeder Distribution
Interfaces (FDIs). FDIs are points where dedicated cable is extended out to the
individual service users.

Some service users are located a long way from the central office and require
a very long local loop. One problem with a very long loop is that the electrical
signals dissipate energy as they traverse the loop, making the signals weak. In
a very simplistic way, it is like a radio signal. As you go farther away from
the transmitter, the signal gets weaker, resulting in lower signal-to-noise
levels.

Telephone companies found two primary ways to deal with long loops:

1. Use loading coils to modify the electrical characteristics of the local
   loop, allowing better quality voice-frequency transmission over extended
   distances (typically greater than 18,000 feet). In this extended-distance
   scenario, loading coils are placed every 6,000 feet on the line.

N.B.

We will learn later that loading coils are not compatible
with the higher frequency attributes of DSL transmissions and they must be
removed before DSL-based services can be provisioned. The use of loading coils
varies by telephone company and typically ranges from virtually none to as
high as 20 percent of the local loops within a given telephone company’s
access network.

2. Set up remote terminals where the signals can terminate
   at an intermediate point, aggregated and backhauled to the central office,
   which houses the switching equipment and high-capacity transmission
   equipment or, in other cases, to a Serving Wire Center (SWC) that does not
   have switching equipment but does have the transmission equipment that
   connects to other central offices. The backhaul to the CO or SWC via T1/E1
   circuits may be based on copper or fiber-based technologies.

N.B.

While initial telephone networks terminated the copper wire
loops directly in the CO, the combination of maintenance challenges associated
with long loops and issues associated with provisioning an increasing number
of loops created the need for architectural changes in the local access
network. Unfortunately, the same fiber optics that could be justified from a
CO connecting thousands of service users to other COs are not yet cost
justified for individual users. Therefore, a compromise solution was to
terminate loops at intermediate points using DLCs that are closer to the
service users. These intermediate points are referred to as remote terminals.

One advantage of terminating the loops at the DLC remote
terminal is that it reduces the effective length of the copper line, thus
improving the reliability of the service. An additional benefit is that Plain
Old Telephone Services (POTS) can be multiplexed into a higher-speed T1
(primarily a North American and Japanese standard supporting up to 24
digitized voice channels at 64 Kbps each) or E1 (an international standard
used primarily by the rest of the world, supporting up to 30 digitized voice
channels) format for transmission to a CO over a single fiber optic or four
wire circuit. While the RT architecture solves many problems for POTS, it
introduces complexities relative to the provisioning of DSL-based services.

DSL
transmissions can only be supported over contiguous copper wire loops.
Therefore, for a DSL-based service connected to an RT, the DSL portion must
terminate at the RT, where the DSL transport is then converted to a format
compatible with the DLC. The use of DLCs varies by telephone company and
typically ranges from almost none to as high as 30 percent of the local loops
within a given telephone company’s access network.

Current projections estimate that nearly 700 million copper
wire access lines connect homes and business customers to the Public Switched
Telephone Network (PSTN) worldwide. More than 95 percent of the local access
loops consist of a single-pair (two-wire circuit) twisted wire supporting POTS.

By definition, POTS is designed to carry a voice
conversation, which for adequate fidelity requires the line to handle
frequencies from 0 Hz up to about 3,400 Hz (1 Hz = 1 cycle per second). This
narrowband service has historically supported only voice calls or analog modem
transmissions at speeds commonly ranging from 9.6 to 33.6 kilobits per seconds
(Kbps), and more recently approaching the 56 Kbps range.

On a global scale, a very small percentage of the PSTN
connections are provisioned with Basic Rate Interface (BRI) Integrated Services
Digital Network (ISDN) services. With Basic Rate ISDN, customers have the option
of either two B-channels (Bearer channels) for one voice and one data, two
voice, or two data (64 Kbps each); or 128 Kbps by combining both B-channels for
data service. Basic Rate ISDN also provides a 16 Kbps D-channel (Data channel)
that supports signaling for the B-channel and is capable of carrying packet
data.

N.B.

Basic Rate ISDN is a baseband service that is implemented
using the lower 80,000 Hz of the frequency spectrum. As with newer DSL-based
services, ISDN’s use of frequencies above 34,000 Hz prevents its use over
loops with loading coils, and special ISDN-compatible interface cards must be
installed in the DLCs to pass ISDN service through to an ISDN-compatible switch.

Dedicated T1/E1 Access Using the Local Loop Network

A common surprise for many users is the realization that the
same physical copper wire lines that are used to provision POTS and ISDN can be
engineered and conditioned to provision T1/E1 services today as they have been
for decades.

In some markets, the condition of the copper wire loops
cannot be reliably engineered to support T1/E1 services. In these cases, T1/E1
services are provisioned using fiber optic cables.

Telephone companies have historically charged substantially
higher recurring monthly service charges for a T1 or E1 access service, commonly
ranging from $600 per month to $2,000 per month, compared to a more traditional
$15 to $50 per month for an analog phone line. The perception is that special T1
or E1 circuits have to be deployed to support the high-speed services. In
reality, the same copper wires are used but special engineering design rules
have to be followed.

Dedicated T1 or E1 access is higher priced, in part, due to
the time and expense for the initial circuit engineering required to tune up the
service and the cost to maintain the service. One reason for the strict
engineering design guidelines and ongoing maintenance expense is that
traditional T1 and E1 transmission equipment use very simple modulation
techniques, such as Alternate Mark Inversion (AMI) for T1 and High Density
Bipolar 3 (HDB3) for E1, which were based on electronic circuitry, developed
over three decades ago.

Traditional T1 and E1 modulation techniques can only be
supported over relatively short distances. As a result, the implementation of
T1/E1 over longer loops requires that the loop be broken down into multiple
concatenated stages with electronic repeaters at intermediate points to detect
and regenerate the signal for transmission down to the next stage. The resulting
special circuit engineering includes placing repeater equipment with 2,000 to
3,000 feet of the end points and not more than 3,000 to 6,000 feet between
repeaters, depending on wire gauge.

N.B.

T1/E1 is a digital service that receives digital information
in the form of "1s" or "0s" from the adjoining system
elements. As a function of the T1 AMI/E1 HDB3 coding scheme, each bit of digital
information is transmitted over the copper wire loop using an analog waveform
that is modulated to represent a corresponding 1 or 0. That is, for example, AMI
coding schemes support 1 bit per baud, where a baud is one cycle of a sinusoidal
waveform and the waveform is modulated to represent either a 1 or 0. The number
of cycles per second is referred to as the frequency in hertz. Therefore, the
transmission of the 1,536,000 bits of information payload, in the case of T1,
plus the associated framing and overhead information (which equals a total of
1,544,000 bits per second) requires use of the frequency spectrum from 0 to
1,544,000 Hz. A by-product of using these high frequencies is the distance
limitation of less than 6,000 feet of 22-gauge wire between repeaters.
Increasing the bits per baud results in the use of fewer frequencies and avoids
some of the high-frequency attenuation, which in turn provides longer loop
reach.

Traditional T1 and E1 transmission equipment cannot operate
on loops that have bridged taps. As a result, all bridged taps have to be
removed before traditional T1/E1 transmission equipment can be provisioned.
While this may sound like a trivial exercise, the lack of proper documentation
and opening and closing cable splices often makes the process of locating and
removing bridged taps a time-consuming and therefore costly challenge.

N.B.

A bridged tap is any portion of a loop that is not in the
direct talking path between the CO and the service user’s terminating
equipment. A bridged tap may be an unused cable pair connected at an
intermediate point or an extension of the circuit beyond the service user’s
location.

Private/Campus Networks

The sheer scale of 700 million local access lines has driven
an industry focus on the telephone company access network. However, there is a
significant market based on private copper wire networks that are, in some
cases, isolated to a self-contained campus environment. Private/campus networks
can often be visualized as a carrier-like architecture, where a single building
or locations connect to this site using embedded copper wire on the campus. DSL
technologies can dramatically improve operations within this environment, and
this market segment is proving to be a potential early adopter of DSL-based
solutions.

The DSL Series is brought to you in association with Paradyne
Corp.