Access technologies : DSL and cable

114 Pages·2002·0.345 MB·English

by  James H Green

Access Technologies:
DSL and Cable
Executive Briefings in
Key Technologies
James Harry Green
McGraw-Hill
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DOI 10.1036/007138247X
Contents
1 Introduction to Access Technologies 1
2 Data Transmission Fundamentals 15
3 Telephone Local Loop Characteristics 37
4 Digital Subscriber Line Technology 49
5 Cable Access Technology 61
6 Wireless Access Technology 75
7 Fiber-optic Access Technology 91
8 Application Considerations 99
Appendix 1 Glossary 109
iii
1
Introduction to Access Technologies
T
elecommunications networks have made some remark-
able strides in the last few decades. Fiber-optic tech-
nology has revolutionized information transport, providing
enormous amounts of high-quality bandwidth that the older
transcontinental microwave network could not support. High-
speed routers and switches and new protocols have brought the
Internet to the masses, making it possible to obtain information
and communicate anywhere in the world for a flat monthlyfee.
Despite these advances, a major bugbear remains. The high-
speed backbone network is accessed over a copper cable local
loop that was designed for a nineteenth-century network. To be
sure, copper wire quality increased significantly during the
twentieth century, and local loop technology improved, but
local access remains the chokepoint for the vast majority of
Internet users.
1
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2 Access Technologies: DSL and Cable
From the start, data networks were designed around the
voice-frequency circuit because nothing else was available.
Older telecommunications systems had a high error rate
because of noise that came from a variety of sources such as
clattering switches and relays, atmospheric conditions, and
technician activity. Since noise was an analog phenomenon
induced into analog circuits, the longer the path, the higher
the noise level, and the greater the error rate. Although a high
noise level is annoying to voice sessions, it is fatal to data, so
when fiber optics came along to replace the analog micro-
wave network that laced the country, it brought a revolution in
transmission quality. No longer was bandwidth constrained
by the channelized voice circuits in the wide area. Common
carriers and large users served by fiber optics could now
obtain an ample supply of digital capacity in whatever band-
width they chose.
The irony of the fiber-optic revolution is that, except for
users large enough to justify the expense of bringing fiber
directly to their premises, bandwidth is still constrained by the
capacity of a twisted pair of copper wires. The reason is that
the local architecture was designed long before any potential
for broadband data communications existed and the cost of
changing it is higher than the revenue potential can support.
The wide area network is concentrated into a backbone of
shared switches and circuits, fanning out to millions of dedi-
cated connections to every business and household in the
developed world. Not only are the multipliers huge, but also
these cables must pass through some expensive real estate.
Ideally, service providers would bring fiber optics to every
business and residence, but the cost of digging up the streets
is enormous and the only way it can be justified is by bring-
ing services for which subscribers will pay enough to justify
the cost. Telephone service is a given—a lifeline service that
few can do without. Beyond that are entertainment services,
most of which ride on coaxial cable today. The community
antenna television (CATV) providers have already invested
the capital to bring a broadband channel to more than 80 per-
cent of the households. Were either telephone or CATV to
start from scratch today, the billions that were invested in
Introduction to Access Technologies 3
twisted-pair wire and coax would instead be invested in fiber
optics, but replacement is an expensive proposition.
Only in the past decade has a third service emerged for
which subscribers are willing to pay: information. Before the
advent of the Internet, it was difficult to conceive that infor-
mation would become such an important commodity, but
today access to the Internet is a third revenue stream that has
important implications for the future. The issues are clear to
the service providers. Large business users require fiber optics
because no other medium can support their bandwidth require-
ments. Therefore, not only the incumbent local exchange car-
riers (ILECs, i.e., the traditional telephone companies), but
also competitive local exchange carriers (CLECs) and com-
petitive access providers (CAPs), are bringing fiber to those
users with enough bandwidth demand to justify it.
That leaves the small businesses and residences, which is
where the real multipliers are. The fact is that the copper wire
and coaxial cable plant is already in place and the providers
don’t expect enough revenue to justify replacing it. There-
fore, they are deploying methods of extending the life of their
existing plant. The ILECs are applying digital subscriber line
(DSL) technologies to their twisted-pair wire. CATV pro-
viders are converting their one-way systems, which were orig-
inally designed to deliver entertainment services, to two-way
systems. Wireless providers plan to bypass wired alternatives
altogether by using radio frequency devices. They have
already made inroads into the CATV market with direct
broadcast satellite services. Wireless providers such as
Teligent and Wavestar have made broad forays into the voice
and data market, but have encountered financial problems
that tend to dampen the enthusiasm of future investors. These
three classes of service providers are eyeing each other’s mar-
kets hungrily. Satellite providers are including local channels
to woo CATV customers. Cable companies are preparing to
offer telephone service, and local exchange carriers (LECs)
are positioning themselves to deliver video on demand
(VOD). All three offer Internet access, either by providing
their own service or as common carriers for Internet service
providers (ISPs).
4 Access Technologies: DSL and Cable
For larger businesses, Internet access has become a neces-
sity that falls just behind telephone service in importance.
These companies and agencies require full-time connections
to the Internet. In addition, most multilocation organizations
have a wide area network to tie all sites together for access to
corporate databases and e-mail. Residential subscribers and
small businesses, on the other hand, cannot justify the cost of
broadband access that most corporations enjoy, so they are
left with dial-up access. Once users become accustomed to
high-speed access at the office, however, the comparatively
interminable waiting time of dial-up access becomes so
painful that they are willing to pay for a better way.
Better alternatives are becoming available, but it is difficult
to know what to believe. Service providers’ advertisements
often make extravagant claims of access speed based on ide-
alized conditions that most users will not experience. Horror
stories of lengthy delays and inept technicians abound, many
of which are unfortunately true. The objective of this book is
to provide users with information about the various access
technologies, what to expect from them, and where they fit.
Let’s begin by looking at the default method that most users
employ, the public switched telephone network (PSTN).
The Public Switched Telephone
Network
From its inception in the nineteenth century, the PSTN has
had a simple and straightforward architecture. Subscribers
are connected by copper wire to a central switching system
located in a building known as the central office (CO). The
CO is also known as a serving wire center (SWC), so called
because all of the cables from the subscribers route to this
center. The CO houses one or more switching systems. In the
early days of the telephone these were manually operated, but
today’s switching systems use computer-driven electronic
switches. Older systems are analog, but analog technology is
obsolete and is being replaced by digital switches as eco-
nomics permit. Large metropolitan areas have multiple wire
Introduction to Access Technologies 5
centers that are connected by groups of local trunks, as Fig.
1.1 shows. A trunk is any circuit between switches. All users
share trunks and the central switching fabric within the CO.
These shared pathways are engineered to a low probability of
blockage, but calls may be blocked during unusual calling
peaks such as disasters. If a call cannot be completed because
of blockage, the network returns a fast busy signal.
This point is key to understanding access technologies: the
bandwidth restrictions in the PSTN are not in the local loop.
The switching and trunking networks limit the bandwidth to
that needed for a voice session. Except for longer telephone
loops, the local loop has far more bandwidth than a voice
session requires.
The local network connects to the long-haul network over
access trunks. The interexchange carriers (IXCs) provide
trunks from their switching systems to the LECs and inter-
connect their switches with intertoll trunks. Today, virtually
all of these trunks ride over fiber-optic facilities. For more
information on how the optical network functions, refer to the
I-book Optical Networking, available at http://shop.mcgraw-
hill.com/cgi-bin/pbg/indexebooks.html. The optical backbone
provides a high-quality facility with low error rates that is
used by voice and data alike.
Before optical networking became prevalent, those con-
structing data networks had little choice but to obtain analog
circuits from the public telephone network. These circuits
were identical to telephone circuits except that they were not
switched. Instead, they were connected directly to form dedi-
cated or private line circuits. In prefiber days the circuits were
derived over analog microwave or coaxial cable. In either
case, for data the error rate was high and the bandwidth was
constrained by the voice circuit, which is nominally 0 to 4
kHz—actually about 300 to 3300 Hz. Into this network the
Bell System began to deploy digital toll switches, converting
the analog circuits to digital circuits through a device known
as a channel bank. Gradually, as fiber optics replaced the
microwave and as competitive carriers entered the picture,
channel banks gave way to direct digital connections.
For voice connections, the all-digital network resulted in a
dramatic increase in quality. With analog circuits, noise is
Fax Local Switch

Local trunks Y

L
Access trunks
Telephone F
M SONET/SDH Backbone
Local Switch Toll Switch

Telephone Local trunks
6 A

E

T
Local Switch

PBX
Local
Customer Premise Local Local Toll Transmission
Access
Equipment Loop Switching Switch Equipment
Trunks

Figure 1-1 Architecture of the Public Switched Telephone Network

Team-Fly®
Introduction to Access Technologies 7
cumulative. As amplifiers along the way boost the signal,
they also increase the noise. Digital circuits are periodically
regenerated, which keeps noise at a low level. Coupled with
the immunity to interference that is inherent in fiber optics,
digital circuits ensure voice signal quality. They also lower
the error rate for data to the point that errors may occur only
once in a trillion bits. Digital circuits also increase the band-
width by a factor of three or four compared to the maximum
data capacity of an analog circuit, but that is not enough for
broadband applications such as the Internet. The PSTN is
built on a time-division multiplexing (TDM) model. The
lowest level in this model, a DS-0, operates at 64 kbps, which
is the widest bandwidth the PSTN is designed to switch. If
more bandwidth than that is needed, say for a conference-
quality videoconference, it is necessary to bond multiple
channels together through inverse multiplexing.
The problem is further aggravated by the characteristics of
the local loop. The cable plant in the telephone network was
designed to support analog telephones. Most of the intelligi-
bility in the human voice is contained within the narrow pass
band of 0 to 4 kHz. The local telephone networks in the
world were designed at a time when either a private entity
such as the Bell System and independent telephone compa-
nies in North America or the postal telephone and telegraph
(PTT) agencies in the rest of the world owned everything
including the telephone set. These entities had the objectives
of minimizing investment and controlling maintenance cost.
Therefore, until recently with the advent of speed dial and
analog display telephones, all of the intelligence in the net-
work resided in the CO.
Even as the world has converted to digital, additional
shortcomings of the PSTN remain. For one thing, the digital
circuit hierarchy does not scale well. Voice circuit bandwidth
is 64 kbps. The next step up from a single digital channel is
T1/E1, which has 24 channels in North America and Japan
and 30 channels in most of the rest of the world, with nomi-
nal transmission speeds of 1.5 and 2.0 Mbps, respectively.
The next step up the hierarchy, T3/E3, has transmission
speeds of 45 and 34 Mbps. In the wide area, users can obtain