Multiple pairs of wires can prove troublesome when it ... - Index of

Contents
1. Introduction
1.1 The Fundamental Problem of Communications 5
1.2 The Transmission Medium-Attenuation Constraints 9
1.3 The Transmission Medium- Interference Constraints 12
1.4 The Transmission Medium- Bandwidth Constraints 13
1.5 DSL Keeps Unshielded Twisted Pair (UTP) Copper Cable
Attractive as a Premises Transmission Medium 18
1.6 A Brief History of DSL 21
1.7 Program 22
2. xDSL Modems: Fundamentals and Flavors
2.1 The Simple DSL Transceiver 24
2.2 The Many Flavors of DSL 28
2.2.1 IDSL 28
2.2.2 The HDSL Family: HDSL, SDSL, MSDSL and HDSL2 28
2.2.3 The ADSL Family: ADSL, MDSL, RADSL and
Splitterless DSL 33
2.2.4 VDSL 36
3. The Role of DSLAMsers 37
4. Virtual DSL: The Role of the DSL Simulator 39
5. Standards 46
6. Digital Subscriber Line - DSL Glossary 48
Bibliography 95





Index of Illustrations
Figure 1-1 Source, User pair with information 5
Figure 1-2 Representations of information 6
Figure 1-3 Examples of sources and users generating/desiring "data" 6
Figure 1-4 Source, transmission medium, user 7
Figure 1-5 Disturbance travelling in transmission medium 7
Figure 1-6 The model which represents the fundamental problem of
communications 8
Figure 1-7 Input data signal attenuating as it propagates down a
transmission medium 10
Figure 1-8 Regenerating and repeating an attenuated signal in order to
reach user 11
Figure 1-9 Example transfer function of a transmission medium 14
Figure 1-10 Binary data from source represented by impulse train put into
transmission medium by transmitter. Impulses are T seconds
apart 15
Figure 1-11 Input signal is positive impulse. Resulting output signal
shows
time dispersion 16
Figure 1-12 Cost trends of common transmission media 19
Figure 2-1 A typical DSL Transceiver block diagram 25
Figure 2-2 Transmitter of digital transmission system 26
Figure 2-3 Generic DSL Reference Model 27
Figure 2-4 T1 Components 29
Figure 2-5 The HDSL Architecture 30
Figure 2-6 Photo of Model 681/682 HDSL Modem 31
Figure 2-7 ADSL reference model 34
Figure 2-8 Conventional ADSL configuration with splitter 34
Figure 2-9 Photo of Model 684 MDSL Modem 35
Figure 2-10 The VDSL Architecture 37
Figure 3-1 DSL-based services reference diagram 39
Figure 4-1 Diagram of modem testing on local loop connection 41
Figure 4-2 Diagram of modem testing on coil of twisted pair cable 41
Figure 4-3 Diagram of modem testing on DSL Simulator 42
Figure 4-4 Photo of Model 454 - Local Loop Simulator 43
Figure 4-5 Photo of Model 455 - Local Loop Simulator 43
Figure 4-6 Photo of Model 457 - Automated Local Loop Simulator 44
Figure 4-7 Photo of Model 456 - Loop Interference Simulator 45
Figure 4-8 Diagram of Models 454 and 456 45

1. Introduction
1.1 The Fundamental Problem of Communications
The subject of interest in this book is the use of Digital Subscriber Line
(DSL) technology to increase the rate and improve the quality of data
communications over copper cable. It is an important topic both within the
context of data communications today and into the future. All, or almost
all, aspects of this subject will be explored. However, it seems rather
forbidding just to jump into this topic. Rather, it is more appropriate to
take a step back and talk about the nature of communications first, in
order to introduce some needed terminology. Such a step back will also
provide us with a broader perspective on the subject of DSL technology as a
transmission facilitator. In short, it will help us to answer the
question, "Why should we be interested in DSL?"
The reader well-versed in data communications may, of course, choose to
skip this introduction and suffer no real penalty.
The subject of communications really begins with the situation shown in
Figure 1-1. Here is an entity called the Source and one called the User -
located remotely from the Source. The Source generates Information, and
the User desires to learn what this Information is.
Figure 1-1: Source, User pair with information
Examples of this situation abound. However, let us focus our attention on
the case illustrated in Figure 1-2. Here, the Information is a sequence of
binary digits - 0s and 1s, commonly called "bits." Information in this
case is termed "data." Information of this type is generally associated
with computers, computing-type devices, and peripherals - equipment shown
in Figure 1-3. Limiting Information to data presents no real limitation.
Voices, images, indeed most other types of Information can be processed to
look like data by sampling and Analog-to-Digital conversion.
Figure 1-2: Representations of information
Figure 1-3: Examples of sources and users generating/desiring "data"
In practice, it is impossible for the User to obtain the Information
without the chance of error. Such errors may spring from a variety of
deleterious effects, which we will examine, in greater detail later in this
chapter.
The possibility of error means that the User seeking the Information - that
is, the binary sequence - must be content in learning it to within a given
fidelity. The fidelity measure usually employed is the Bit Error Rate
(BER). The BER is the probability that a specific generated binary digit
at the Source, a bit, is received in error, opposite to what it is, at the
User.
There are some real questions as to how appropriate this fidelity measure
is in certain applications. Nonetheless, it is so widely employed in
practice that further discussion is not warranted.
The question then arises as to how to send the binary data stream from the
Source to the User. We refer to any physical entity used for this purpose
as a Transmission Medium.
As shown in Figure 1-4, the Transmission Medium is located between the
Source and the User, accessible to both. The Transmission medium has a set
of properties described by physical parameters. This set of properties
exists in a quiescent state; however, at least one of these properties can
be stressed or disturbed at the Source end. This is accomplished by
imparting energy in order to stress the property. The disturbance affects
the parts of the Transmission Medium around it, then travels from the
Source end to the User end. Once the disturbance or stressed property
reaches the User end, it can be sensed and measured. This propagation of a
disturbance by the Transmission Medium is illustrated in Figure 1-5.
Figure 1-4: Source, transmission medium, user
Figure 1-5: Disturbance travelling in transmission medium
There are many types of transmission media. The Transmission Medium could
be air, with the stressed property being the air pressure put on sound
waves. It could be an electromagnetic field set up in space by the current
put on an antenna - a radio or wireless system. It could be a pair of
electrical conductors, with the stressed property being the potential
difference (the voltage) between the conductors - an electrical
transmission line. It could be a cylindrical glass tube with the stressed
property being the intensity of light in the tube - a fiber optic cable.
Even written communication can be interpreted in this fashion: a sheet of
writing paper provides the Transmission Medium, with the stressed property
being the light-dark pattern on the paper.
The Source can have a disturbance to the Transmission Medium generated in
sympathy to the Information - that is, it can generate a disturbance which
varies in time exactly as the Information. This encoded disturbance will
propagate to the User. The User can then sense the disturbance and decide
the identity of the Information that it represents. The process of the
Source generating a disturbance in sympathy with the Information and
launching it into the Transmission Medium is referred to as "modulation and
transmission." The process of the User sensing the received disturbance
and deciding what Information it represents is referred to as "reception
and demodulation." In this work, we will refer to the device that carries
out modulation and transmission as the Transmitter. We will refer to the
device that carries out reception and demodulation as the Receiver.
The whole of data communications then devolves to the model illustrated in
Figure 1.6. Here, the Source generates bits as Information. The User
wants to learn the identity of this Information, these bits. The entities
used to get the Information from the Source to the User are the
Transmitter, the Transmission Medium and the Receiver. The fundamental
problem of communications is to choose the terminal equipment - the
Transmitter and Receiver - and to choose the Transmission Medium so as to
satisfy the requirements for a given Source-User pair.
Figure 1-6: The model which represents the fundamental problem of
communications
The fundamental problem of communications is one of design. Collectively,
the combination of Transmitter, Transmission Medium and Receiver is known
as the "communication link" or "data link" - the latter term deriving from
the limitation placed on the Information to the form of a sequence of bits.
The disturbance launched into the Transmission Medium by the Transmitter
is usually referred to as the "input data signal." The resulting
disturbance at the Receiver is termed the "output data signal." In the
context of our discussion, the fundamental problem is to design a data link
appropriate for connecting a given Source-User pair.
There is no cookbook method to solve this design problem and come up with
the best unique solution. While there is science here, there is also art.
There are always alternative solutions. Each solution has its own
particular twist, which in turn provides some additional attractive feature
to the solution. However, the feature is peripheral to Sourc