oop). The subscriber
line is a metallic twisted-pair network link between a customer and a telephone Central Office (CO).
While some of the existing DSL analysis equipment is already capable of assessing the performance
rate, it requires two-point operation (sending test signals from one end of the loop and measuring the
signals at the other end) involving the dispatch of a service vehicle. This leads to expensive testing
processes and it is therefore an undesirable solution for DSL access providers. A more cost-effective
solution would be to estimate the expected DSL performance on the basis of single-point
measurements, preferably from the provider end. However, such test equipment is not currently on
the market [1], [2].
In the single-point measurement procedure proposed by Wong and Aboulnasr [3] the
performance, in terms of the insertion loss (IL) (see Section 1.4 for definition), of a local loop is
estimated from a single-point input impedance measurement from the CO end. The measured input
impedance is modeled with an RC network regardless of the complexity of the loop topology. Their
method is designed to approximate the IL at one frequency point. To assess DSL connectivity, the
performance over the entire DSL bandwidth needs to be considered.
This work poses the single-point subscriber line measurement as a model-based system
identification problem. A local loop model consists of the network topology and the characteristics
namely type and length of each twisted-pair segment. Simulation of the model is based on
transmission line theory [4], [5] and the electrical characteristics of twisted-pairs. While the local loop
1
Chapter 1: Introduction
is a linear time-invariant system, its behavior is complex and nonlinear in its parameters. Hence,
common identification techniques (e.g., least squares and instrumental variable methods) are not
applicable, and we therefore investigate an iterative modeling approach to search for the best-fitting
loop model. Upon successful identification of the loop, the theoretical DSL access performance rate
can then be readily computed.
The remainder of this chapter will solidify the research foundation by briefly introducing DSL
technology, telephony (DSL) local loop history and expected structures, and the effect of the local
loop structure on DSL performance. These materials are extracted from [6] [17]. This chapter
concludes with the thesis overview .
1.2 DSL
Technology
Since the mid-1990s, we have experienced a rapid data-rate-hungry evolution of the Internet,
especially with the emergence of the World Wide Web (WWW), its multimedia-intensive contents, and
the booming growth of WWW hosts and users. In addition, the number of small offices/home
offices (SOHO), which often require fast access to a corporate local area network (LAN), has been
growing steadily in recent years. Such high data-rate demands surpass the capability of now mature
voiceband modems (< 56 kbps). The first higher-speed alternative, the Integrated Services Digital
Network (ISDN), was introduced in the mid-1990s to accommodate high data-rate demand.
However, despite superior access speed, at 128 kbps, it did not gain wide popularity due to its high
cost, incompatibility with the existing telephone service (POTS plain old telephone service), and
discrepancies in the ISDN standards. To search for even faster speeds without repeating ISDNs
oversights, several broadband access technologies (operating > 1.5 Mbps) one of which is DSL
technology emerged, and these new services have been gaining popularity rapidly in data
communication in recent years.
A broadband communication network, or simply a broadband network, is capable of transporting
voice, data, and video in a single converged communication network. While such a unified network is
not yet realized, largely due to the already existing independent infrastructure for each service, several
broadband access technologies capable of broadband communication are already available today.
They are classified into three basic categories: copper-loop, cable (both fiber and coaxial), and wireless
access technologies. The DSL technology, equivalent to the copper-loop access technology, has been
developed by the telecommunications industry to utilize its largest asset, millions of miles of installed
2
Chapter 1: Introduction
twisted-pairs, beyond the existing POTS. At the present time, DSL technology has shown many
advantages over other alternative broadband access solutions.
DSL technology gains much of its advantage over its peer technologies by utilizing, and sometimes
even sharing with POTS, telephony subscriber lines to provide an affordable, ubiquitous data access
solution. However, because of the narrowband nature (04 kHz) of POTS, the local loops are not
always conditioned for broadband DSL access. In other words, high-speed access is not guaranteed
for all local loops. To compound the issue, the local loops are generally laid out without sufficient
documentation, preventing DSL providers from theoretically calculating the expected data rate for a
local loop. This is the primary driving force for the development of local-loop DSL performance
prediction capability.
While there are many varieties of DSL technology, collectively referred to as xDSL, their overall
systems are very similar. As illustrated in Figure 1-1, the DSL system consists of the subscriber
premise, central office (CO) premise, and the local loop, which connects two premises. Primary
components of the CO are the DSL Access Multiplexer (DSLAM) and the CO-end modem (xTU-C).
The DSLAM links the DSL traffic to a higher-speed backbone network, such as OC (Optical Carrier)-
3 and OC-12, further connected to a Network Service Provider (NSP), and the xTU-C interfaces the
local loop and the CO. The xTU-R is the subscriber-side counterpart of the xTU-C. For those DSL
technologies capable of coexisting with POTS e.g., Asynchronous DSL (ADSL) and Very High
Speed DSL (VDSL) technologies POTS splitters, which combine POTS and DSL service over a
subscriber line, exist on the local loop side of both modems. The POTS connections are made to
telephone devices on the subscriber side and to the public switched telephone network (PSTN) on the
CO side.
3
Chapter 1: Introduction
Subscriber
CO
Local Loop
(Existing Telephone Line)
Voice
Switch
DSLAM
PSTN
PSTN
Internet
Internet

Figure 1-1: Simplified DSL Architecture.
Standardization of xDSL technologies has been underway since the late 1980s. For instance,
Committee T1 of the American National Standard Institute (ANSI) initiated the ADSL effort, which is
the most prominent xDSL technology to date. Also, the International Telecommunications Union
(ITU) and European Technical Standard Institute (ETSI) have joined the effort, and thus their
standards for ADSL are largely based on the ANSI standard.
1.3 Subscriber Loop Environment
As discussed in the previous section, the primary motivation behind the development of DSL
technology is the exploitation of existing local telephone loops. The invention of the twisted-pair local
loop in the late 19
th
century was an innovative solution to reduce crosstalk between wires in the
telephone cable, which encloses many wires in its sheath. Some of the first loops are still in operation
today in some parts of the country. This section introduces the expected features of the twisted pairs
and local loops.
1.3.1 Twisted-Pair
Types
One of the aforementioned twisted-pair characteristics is the type, which is mainly categorized by
the twisted-pairs physical characteristics. The wire gauge size and cable insulation type are two
characteristics that are widely documented twisted-pair classifications. Other classifications
encountered are those based on bit-rate (5 classes based on maximum supported bit rate), ambient
temperature, cable location (e.g., aerial, buried, or buried in a conduit), and cable manufacturer.
4
Chapter 1: Introduction
However, the wire conductor material, which makes a large contribution to the cable electrical
characteristics, is not often used for discrimination since it is almost always copper.
In the U.S., the gauges of twisted pairs are specified using American Wire Gauge (AWG)
designations. The typical gauges found in U.S. loops are 19, 22, 24, and 26 AWG, with a larger AWG
number corresponding to a smaller wire diameter. Thinner twisted pairs (larger AWG number) are
often used underground close to the CO while thicker wires are used as an aerial drop line to the
customer premise.
Figure 1-2 illustrates the variation in the twisted-pair characteristics with respect to varying gauge
sizes namely, 22, 24, and 26 AWG based on the tabulated twisted-pair characteristics in the
ANSI T1.601-1999 standard. Figure 1-2(a) and (b) are the attenuation function
and phase
function
(the real and imaginary parts of the propagation function
) respectively. Figure
1-2(c) and (d) show the characteristic impedance Z
(the magnitude and phase, respectively). The
insulation type of these twisted-pairs is plastic or PIC (polyolefin-insulated cable) and is measured at
21 °C ambient temperature.
( )
f
( )
f ( )
f
( )
f
0
Unlike the gauge size, which appears in a different part of the local loop, the variation in cable
insulation material has a more historical background. The earliest twisted-pair cables were insulated
with paper (often referred to as pulp) and were installed until the 1970s. One of the disadvantages
of pulp insulation is its sensitivity to the surrounding hu