INTERNATIONAL JOURNAL OF NETWORK MANAGEMENT
Int. J. Network Mgmt 2001; 11: 265 – 276 (DOI: 10.1002/nem.423)
From voice-band modems to DSL technologies
By Mark Peden and Gavin Young
This paper provides an overview of the evolution of digital transmission in
the copper access network from voice-band modems to Digital Subscriber
Line (DSL) technologies. The various types of DSL technology are
described. Copyright  2001 John Wiley & Sons, Ltd.
Introduction
T
here are over 700 million copper pairs
in telephony access networks worldwide
which provide for extensive connectivity
of the world’s population. A combination of this
existing copper infrastructure and new transmission technologies mean that a new era of universal
broadband access can now begin at a fraction of
the cost, and in a fraction of the time required for
optical access networks.
Many network service providers (NSPs) now
have a broadband access strategy involving
deployment of ISDN, ADSL and VDSL transmission technologies for a new generation of services;
and the use of HDSL and HDSL2 for reducing
costs and connection time for existing services.
Additionally, the emergence of competitive carriers has helped fuel the race towards broadband
deployment. This paper provides an overview of
the most prevalent DSL technologies.
Technology Overview
Originally, due to the use of FDM channel
multiplexers in the PSTN network, customers
were constrained to using the 4 kHz voiceband
to convey data over the copper access twisted pair
and throughout the network. Voice band modems
were first introduced at sub-1000 bits per second,
before progressing through 2.4 and 4.8 kbit/s then
developed rapidly to pack more information into
the 4 kHz bandwidth. Analog modems operating at
9.6, 19.2, 28.8, 33 and 56 kbit/s have subsequently
appeared on the market.
With the advent of PCM digital transmission
in the backbone network and digital switching,
there was no longer a need to constrain the signals
transmitted in the copper access network to a 4 kHz
bandwidth. In principle, a copper transmission
system could now use any bandwidth as long as
the information transported could be conveyed
through the 64 kbit/s narrow-band digital PSTN.
Data-over-voice pair-gain systems and private
circuit systems were among the first to make
use of additional bandwidth on the copper pairs
by using frequencies above the voice band. Subsequently, basic-rate ISDN systems operating at
up to 160 kbit/s used more modern modulation schemes (2B1Q, 3B2T & 4B3T), together with
advanced digital signal processing, such as nonlinear echo cancellation, to more efficiently exploit
the available capacity by improving the spectral
efficiency. The transmitted signals for these ISDN
Mark Peden has a rich background in data networking and telecommunications. Prior to joining Simpler Networks, as Vice-President, Technology
Marketing, he was the Director of Technology at NorthPoint Communications. Mark is active in industry standards bodies such as the DSL
Forum, ATIS T1E1.4 and the ITU-T. In his current role with Simpler Networks, he is responsible for technology marketing, industry relations
and the company’s regulatory efforts.
Gavin Young recently moved to AdEvia as VP Engineering. Gavin was a founding Director of the DSL Forum serving on the board for 6 years.
He has previously led the DSL Forum Network Migration and VDSL working groups. He is currently overall technical chairman of the DSL
Forum.
Ł
Correspondence to: Allison Sokol, Voce Communications/DSL Forum PR Representative, 285 Hamilton Avenue, Suite 300, Palo Alto, CA
Ga301, USA
Copyright  2001 John Wiley & Sons, Ltd.
266
M. PEDEN AND G. YOUNG
DSL At-A-Glance
ISDN
144 kbit/s
ADSL— Asymmetric
Digital Subscriber
Line
HDSL— High Bit
Rate Digital
Subscriber Line
HDSL2 — High Bit
Rate Digital
Subscriber Line v.2
VDSL— Very high
bit rate Digital
Subscriber Line
2 wires for up to
6 Mbps downstream,
800 Kbps upstream
4 wires for 1.5 Mbps
channelized
symmetric service
2 wires and a single
pair version of HDSL
ISDN — Integrated
Services Digital
Network
2 wires up to
144 Kbps symmetric
service
2 wires for up to
53 Mbps asymmetric,
or 13 Mbps
symmetric
systems is mostly constrained to frequencies below
80 kHz.
Next in the evolution of digital transmission
systems was HDSL which was originally a ‘scaled
up’ version of basic rate ISDN. HDSL uses the same
line code as the ISDN standard (2B1Q) but operates
at 784 kbit/s to 1 Mbit/s on each pair.*. It uses
two or three copper pairs to deliver multi-megabit
services such as Primary-rate ISDN and T1 or E1
leased lines. A newer version of HDSL, HDSL2,
provides similar services with half the pairs of
those used for HDSL. HDSL2 will be discussed in
greater detail later.
ADSL is the most recent DSL system to enter
commercial deployment. ADSL employs asymmetric transmission in terms of both capacity and
bandwidth in order to improve the crosstalk environment, and hence improve the capacity in the
direction of exchange to customer compared to
HDSL. Unlike ISDN, HDSL, and HDSL2 which
use baseband frequencies for transmission, ADSL
uses passband modulation (between 25 kHz and
1.1 MHz) to keep the 4 kHz voice band free
for simultaneous analog telephony. Two international standards exist in ADSL today, G.dmt (ITU
G.992.1) and G.lite (ITU-T G.992.2) which will be
introduced later. Figure 1 illustrates these three
basic classes of modern copper transmission systems.
*The bulk of transmitted power being below 400 kHz.
Copyright  2001 John Wiley & Sons, Ltd.
784 kbit/s - 1 Mbit/s
Over 5 km
3.5 km
HDSL
Customer
Exchange
Up to 8 Mbit/s downstream
640 kbit/s upstream
ADSL
Variable Range
Figure 1. Classes of copper access transmission
systems
All of these systems were designed to operate between the customers’ premises and their
local exchange. In order to achieve any further
significant improvement in capacity, it is necessary to make the copper network shorter and/or
improve the modulation techniques of the technology used. One way to achieve this is by taking
fibre part-way into the access network and using
ADSL or VDSL for transmission over the remaining, shorter-distance, copper pairs to the customer
premises. Using these shorter loops, VDSL further
increased the bandwidth used on the copper pair,
with some systems using more than 10 MHz and
as high as 30 MHz — see Figure 2. The use of these
high frequencies for transmission over copper pairs
requires greater attention to EMC and RFI compatibility during the design because of the diminishing
cable balance as the frequency increases.1
VDSL
~1997
Time
ADSL
~1994
~1990
~1986
HDSL
ISDN
~0.1 MHz
~0.5 MHz
~1 MHz
10+ MHz
Bandwidth (MHz)
Figure 2. Copper access bandwidth
Int. J. Network Mgmt 2001; 11:265 – 276
FROM VOICE-BAND MODEMS TO DSL TECHNOLOGIES
Note that it is desirable to use the same line
for both the new high rate connections and the
original POTS service. This is expected to be
achieved by frequency division, using analog band
splitter/filters located at each end of the line as
shown in Figure 3.
With the multi-megabit access capability of
ADSL and VDSL comes the need to upgrade the
remainder of the network to avoid bottlenecks
arising elsewhere (e.g. by deploying ATM switches
together with SDH and WDM transmission in the
core).
Development of new types of DSL modem
technology has not halted improvement and development of the existing systems. For example, voice
band modems have evolved to deliver 56 kbit/s
to customers by exploiting the reduction in quantization noise when the analog/digital conversion
process at the POTS line-card is bypassed on the
service provider side. Also, narrowband ISDN line
cards with much improved range have become
available.
HDSL and HDSL2 are no longer seen as just an
expedient way of providing existing E1/T1 legacy
services using multiple wire-pairs. These systems
have become more flexible (e.g. rate-configurable
trading range for bit-rate). This potentially opens
up new markets for HDSL, such as campus LANs
using a single copper pair.
Today, ADSL is rate-adaptive, and the systems
are no longer simple ‘bit pumps’ with clock
and data interfaces. ADSL systems now exist
that have integrated ATM functionality, and may
integrate bridging or routing. Other types of DSL
Std UNI
• ATM-25
• Ethernet
Splitter
POTS
u/s
d/s
HPF
CAT5 data cable
ADSL
NT
Phone line carrying
POTS + ADSL
LPF
• Match impedance's
• Minimise losses
• Provide POTS <-> ADSL isolation
• Preserves POTS QoS
Normal
house
wiring
Provides stable MF data channel for ADSL
22/1/98
(c) BT 1998
4
Figure 3. Purpose of a splitter /filter. Normally
the High Pass Filter (HPF) resides in the same box
as the ADSL NT transceiver and is only shown
here collocated with the LPF to illustrate the signal
separation
Copyright  2001 John Wiley & Sons, Ltd.
267
Service
Provision
NTE5
NTE5
RC
RC
C
Wall of
Premises
Wall of
Premises
Existing Wiring
New
Wiring
LPF
ADSL
NTE
ADSL Splitter
Figure 4. Normal ADSL installation
systems may support Frame Relay or an IP-centric
model.
Conventionally the ADSL modem may have
been installed close to the master telephone socket,
just beyond a splitter as shown in Figure 4. A recent
development is to omit the splitter at the customer
end. The idea is to remove the need for new internal
wiring and the splitter/filter by allowing the
customer ADSL unit to be plugged directly into any
existing telephone socket as shown below. It would
also save the cost of an engineering visit. Originally
referred to as ‘Splitterless ADSL’ or ‘DSL Lite,’
this version of ADSL is now an ITU-T standard,
referred to as G.lite (or G.992.2). One other aspects
of G.lite is that it is often seen as a subset of
G.dmt in that it removes many of the options of
full rate ADSL (G.dmt). The net result of reducing
the complexity of the technology is enabling and
facilitating interoperability — a critical element to
mass market deployment. While G.lite is officially
referred to within the ITU-T as Splitterless ADSL,
it does not preclude the use of a distributed splitter
to optimize performance. This distributed filter is
often referred to as a microfilter.
For Splitterless ADSL to operate successfully,
modifications are needed to the ADSL modem
design to compensate for the increased mutual
interference between POTS and ADSL. This interference results in additional noise for the telephony, and reduced capacity for the ADSL. The
performance of Splitterless ADSL may be impacted
the type and number of telephones and other telephony CPE that are installed. It may be that, unlike
normal ADSL, simultaneous high-performance
Int. J. Network Mgmt 2001; 11:265 – 276
268
M. PEDEN AND G. YOUNG
NTE5
RC
Wall of
Premises
ADSL
NTE
Figure 5. ‘Splitterless installation’
operation of telephony and splitterless ADSL will
not be possible for some customers without the
addition of a second line or microfilter. The latter option can be achieved whilst maintaining the
objectives of a cost-effective and user-installable
solution by the use of small, inexpensive low-pass
filters (microfilters) that may be plugged into some
phone sockets to provide the necessary isolation
between the telephones and DSL system.
Recent VDSL developments include increased
interest in symmetrical transmission, and the use
of VDSL from the exchange and in campus environments. The US standards body, T1E1.4, has
recently introduced two ‘trial balloon’ specifications for VDSL— a multi-carrier approach and a
single-carrier approach. It is expected that there
will be some industry consolidation on a unified
approach within two years.
T
he creation of new multimedia-rich
content and applications will ultimately
require significantly faster data rates. VDSL
to the rescue.
The creation of new multimedia-rich content
and applications will ultimately require significantly faster data rates. VDSL to the rescue.
However, with the emergence of VDSL, there is
significant additional strain on the network backbone. New models are being developed that allow
Copyright  2001 John Wiley & Sons, Ltd.
the injection of last-mile content, and re-define
much of today’s internet, through the introduction
of hybrid networks. This may include the combination of satellite distribution models as well
as introducing fiber connections to localized content (outside of the traditional DS-3/OC-3/STM-1
backhauls).
All DSL technologies have benefited from the
continuing advances in electronics. Apart from
improvements in functionality, there is modest scope for DSL modems to access more of
the intrinsic information capacity of the copper
pairs. This relies on more sophisticated modulation and coding techniques, and making use
of improvements in silicon integration to generate additional improvements by reducing cost,
and reducing power consumption. Table 1 (reproduced from reference 2) compares the capacity actually obtained by some existing systems
against the theoretical capacity of their channels.*
The signal processing used in voiceband
modems operates at relatively slow rates, enabling
sophisticated modulation and coding algorithms,
such as multi-dimensional trellis coding, to be
implemented at low cost. Voice band modems are
close to the limit in realizing the capacity available
in the voiceband.
Attribute
V.34
Modem
ISDN
HDSL
ADSL
Channel
capacity
(kbit/s)
Throughput
(kbit/s)
Transmission
efficiency
(%)
34.88
700
1700
10,000
28.8a
160
800
7000
23
47
70
83
a
On the basis of equivalent assumptions, a 33 kbit/s
modem operates at 95% transmission efficiency.
56 kbit/s modems effectively operate over a different
channel with higher capacity due to the removal of a
source of quantization noise.
Table 1. Practical capacity as a percentage of
theoretical
*For system details and assumed channel characteristics see [G4].
Int. J. Network Mgmt 2001; 11:265 – 276
FROM VOICE-BAND MODEMS TO DSL TECHNOLOGIES
Similar techniques were not practical when
ISDN and HDSL were originally conceived and
adopted as standards, so these systems are less
efficient. By the time ADSL appeared, signal
processing power and integration had improved
and ADSL began to make better use of available
capacity than its DSL predecessors by exploiting
techniques such as multi-tone modulation, ReedSolomon error correction coding and trellis coding.
The increased availability of fast signal processing power is now being used for new improved
variants of the earlier DSL systems. HDSL2 is
a new variant of HDSL that seeks to deliver
1.5 Mbit/s T1 services over a single copper pair
with similar range to that currently achieved with
2-pair T1 HDSL systems. To achieve this increased
transmission efficiency, standards-based HDSL2
examined use of modulation techniques such as
POET3 and OPTIS which are a much more sophisticated approach than the existing 2B1Q line code.
HDSL2 also employs coding techniques.
In addition to improvements in DSL technology,
any further improvement in capacity depends on
maintaining the available network capacity by
judicious control of the crosstalk environment, and
spectrum management. For example, HDSL2 has
been defined as a standard in the T1 HDSL market
(primarily North America). However, HDSL2 may
not be able to deliver 2 Mbit/s E1 services over
a single copper pair without causing interference
to ADSL systems operating in the same cable.
Recent increased interest in flexible symmetric
transmission over a single copper pair at a range
of rates has resulted in standards bodies such as
ETSI and the ITU exploring modulation schemes
for another type of DSL known as SHDSL where
‘S’ denotes Symmetric. This promises to provide
a symmetric DSL system that is more flexible and
‘crosstalk friendly’ to ADSL than either HDSL or
HDSL2.
The Roles of IP and ATM in ADSL
and VDSL Access Systems
The history of several years silicon and systems
development of today’s broadband access technologies has contributed to the IP versus ATM
debate. In some parts of an end-to-end network,
bandwidth can be used to solve problems associated with quality of service. For example, DWDM
Copyright  2001 John Wiley & Sons, Ltd.
269
can be used in the core and Gigabit Ethernet in the
customer’s building. However, apart from using
fibre or coax bearers, most access delivery media
don’t have the luxury of excess bandwidth. When
today’s broadband access systems and silicon for
ADSL, LMDS and satellite were first being developed several years ago, ATM was the only way of
managing and policing traffic and offering absolute QoS mechanisms that could underpin service
level guarantees.
All this led to the initial use of ATM and its
subsequently being embedded in ADSL silicon.
IP was seen more like ‘just another application’
to be transported rather than a comprehensive
networking technology. The LMDS air interface
is based on ATM to enable statistical multiplexing and assignment of bandwidth on demand.
It is only recently that IP developments such
as RSVP, DIFFserv and MPLS have come along
to move IP-centric networks towards equivalent functionality. The use of ATM in much of
today’s DSL silicon is, of course, not to the exclusion of IP. In the DSL Forum’s system guidelines for ATM-centric architectures (as opposed
to its packet-based recommendations) IP is carried over ATM (as described in the previous
section).
Several network operators are taking advantage
of this IP over ATM approach by using an endend architecture enabling product offerings for
either layer 3 IP or layer 2 ATM services over
a common platform. In fact in some parts of the
world the regulators give the operators little choice
but to offer a wholesale layer 2 service enabling
service providers to add their own IP layer. Hence
their service provider customers then have the
choice of which network product best suits their
services. For example some companies procure the
IP product to construct IP VPNs and others procure
the ATM product e.g. for VoD. The ATM products
are not restricted to larger traditional operators. In
the USA, new network operators (CLECs) which
have only been in business a couple of years are
providing their own DSL equipment on unbundled
copper loops rented from the incumbent telco.
These companies were starting with a blank sheet
of paper when it came to designing their network.
Some of them are offering only layer 2 ATM
network transport as a wholesale product because
that is what their ISP and corporate customers
require.
Int. J. Network Mgmt 2001; 11:265 – 276
270
When the Universal ADSL Working Group
(UAWG) was formed, its focus was on enabling
a mass market consumerisation of ADSL. One
key deliverable was producing the international
standard specification for G.lite (G.992.2). This
initiative was driven by the PC industry and
the world’s leading telephone companies. The
existing ADSL specifications were used as the
starting point for for G.lite work within the UAWG
and accelerated the ITU standards process. Given
that ADSL silicon had taken several years for the
leading vendors to produce their most integrated
chips, the UAWG also followed the IP over ATM
approach in order to expedite time to market.
G.lite being bundled into PCs via mass market
distribution channels also enables both ATM and
IP services to be transported.
One of the most recent broadband access developments is that of Voice over DSL (VoDSL). The
two more common ways that the voice can be
carried across the ADSL transport are VoIP or
VToA (or VoATM). The industry is pursuing both
approaches since there are perceived to be two very
different markets for VoDSL. Broadly speaking one
market is addressed by completely replicating the
quality and feature functionality of today’s PSTN
by using Voice over ATM. The other is to dispense
with the legacy baggage of today’s PSTN and to
focus on new value added services and integration
with data (e.g. click to talk web sites using VoIP)
without replicating the exact quality levels of the
PSTN. The development of VoDSL enables voice
and data services to be simultaneously delivered
down a single copper phone line. This bundling
of digitally derived voice together with data will
be equally applicable to other broadband access
technologies where VoIP and VToA could be utilized. Many view VToA as a stepping stone to
implementing an end-to-end VoIP model.
There are a number of reasons why carriers are
excited about introducing VoDSL— regardless of
the model. The most fundamental reason is that
it has the ability to significantly lower the cost
of deploying voice services. No longer does an
additional pair need to be run for another voice
line. A call to the carrier (or a visit to a website),
a few keystrokes by an administrator, and a new
line becomes active. Additionally, it provides the
ability to address the copper exhaust problem.
With the explosion of the Internet, people are
getting second and third phone lines to keep
Copyright  2001 John Wiley & Sons, Ltd.
M. PEDEN AND G. YOUNG
their voice line and, in some cases, fax line,
free. When much of today’s copper plant was
deployed, few had forecasted the high demand
for additional phone lines in the residential, SOHO
and small business space. The lack of available
pairs has created a copper exhaust issue that can
be effectively addressed by introducing packetized
voice. One final benefit is the potential to give
customer control — not only setting up another
phone line but adding or subtracting services as
well as customizing those services (i.e., limiting
call times, specifying incoming callers, etc.) at the
click of a button.
T
he more important issue is what will the
markets require tomorrow (e.g. multicast)
and how should the technology evolve to best
serve those markets?
There is logic (technical and marketing) in the
history behind why ATM functionality has ended
up in broadband access silicon and systems and
there appears to be very real markets for both ATM
and IP delivery over such systems today. However,
the more important issue is what will the markets
require tomorrow (e.g. multicast) and how should
the technology evolve to best serve those markets?
The world is moving towards an increasingly IPcentric future. It seems less likely that the ATM
layer will be removed from say ADSL since the
impact on silicon and interoperability of mass
market ADSL/G.lite products would slow down
broadband access to the masses. What is possible
is that as the new developments for IP QoS and
connection oriented capability etc. are developed
in the Internet Engineering Task Force (IETF), then
the ATM layer within broadband access systems
may become ‘dumbed down’ so as not to duplicate
addressing and signalling mechanisms at two
layers.
The relative merits of ATM-or IP-centric broadband access systems will continue to be debated
with vendors pursuing and further developing
their own preferred approaches. Already DSL
access muxes (DSLAMs) have been evolved from
simple VC cross-connects. Some vendors have
developed DSLAMs that are SVC-capable ATM
Int. J. Network Mgmt 2001; 11:265 – 276
FROM VOICE-BAND MODEMS TO DSL TECHNOLOGIES
edge switches and others have developed them
with integrated IP routing and multi-cast capabilities. Proprietary LMDS systems are also now
starting to appear with more inherent IP capability. Multi-Protocol Label Switching (MPLS) is seen
by some vendors as the way to integrate IP and
ATM capability to get the best from each. Its role
in broadband access systems is not yet defined but
some vendors are pursuing it. As always, interoperable standards-based products are preferred by
most operators and progress in this area could dictate the speed of adoption and ultimate success in
the market of the ATM, IP and MPLS approaches
to broadband access.
Concluding Remarks
The DSL technologies outlined in this paper
are still evolving. Each new generation brings
improvements in functionality, performance and
levels of integration. This trend of technology
development and innovation will continue, in the
same way that voice band modems evolved to
exploit the capacity of the voice channel more
efficiently.
In summary, the installed copper pair network
presents a challenging environment for high-speed
transmission. Operators can leverage this asset to
release the potential for competitive broadband
services using DSL technologies. DSL enables
service providers to maximize the existing copper
plant to transform ordinary telephone lines into
high-speed broadband networks.
If you are interested in learning more about the
benefits of DSL or putting it to work for you,
check out the centre of activity — DSL Forum.
Meetings are held quarterly in various places
around the world. Just point your browser to
http://www.dslforum.org for additional information. For general information on the technology, a consumer site has also been set up at
http://www.dsllife.com.
References
1. Foster KT. The radio frequency environment for high
speed metallic access systems. IEEE Globecom ‘96,
VDSL Workshop (invited paper), November 1996,
London.
Copyright  2001 John Wiley & Sons, Ltd.
271
2. Chen W. A proposal for ADSL Issue 2 to include
the low complexity ATU-R. ANSI contribution
T1E1.4/96-199r1, July 1996.
3. Scneider K. Simulated performance of HDSL
transceivers. ANSI T1E1.4/97-444, December 1997.
Glossary of Terms
2B1Q Two Binary, one Quaternary. A line coding
technique that compresses two binary bits of data
into one time state as a four-level code.
ADSL (Asymmetric Digital Subscriber Line)
BellCore term for delivery of digital information
over ordinary copper phone lines. ADSL uses
a system of frequency division whereby lower
frequency POTS signals are delivered to the home
unaltered while digital signals traverse the phone
line at higher frequencies for delivery to end
stations such as a video CODEC or PC. Asymmetric
refers to the fact that the downstream (to the
user) channels can outweigh the upstream (to the
network) channels by a ratio as high as 20 : 1. This
asymmetry is a good fit for video on demand and
Internet access applications where the paradigm
is a small request up to the network and a large
delivery to the user.
American National Standards Institute (ANSI)
The US standards organization that establishes
procedures for the development and coordination
of voluntary American National Standards.
ATM: asynchronous transfer mode A highspeed multiplexing and switching method utilizing
fixed-length cells of 53 octets to support multiple
types of traffic. Note: ATM, specified in international
standards, is asynchronous in the sense that cells
carrying user data need not be periodic.
bit rate (BR) 1. In a bit stream, the number of bits
occurring per unit time, usually expressed in bits
per second. Note: For n-ary operation, the bit rate is
equal to log2 n times the rate (in bauds), where n is
the number of significant conditions in the signal.
2. The rate of transmission of information in binary
(two-state) form in bits per unit time.
channel capacity The maximum possible information transfer rate through a channel, subject to
specified constraints.
CLEC Abbreviation for competitive local exchange
carrier. The new local exchange carrier that is
attempting to compete outside its traditional
operating territory. [After FCC]
Int. J. Network Mgmt 2001; 11:265 – 276
272
DS: digital signal A signal in which discrete
steps are used to represent information. Note 1:
In a digital signal, the discrete steps may be
further characterized by signal elements, such
as significant conditions, significant instants, and
transitions. Note 2: Digital signals contain m-ary
significant conditions.
DS3: digital signal 1. A digital signal rate of
44.736 Mb/s, corresponding to the North American T3 designator. 2. A digital signaling rate of
32.064 Mb/s, corresponding to the Japanese T3
designator.
DWDM (Dense Wave Division Multiplexing)
A SONET term. High-speed versions of WDM,
which is a means of increasing the capacity of
SONET fiber optic transmission systems through
the multiplexing of multiple wavelengths of light.
Each wavelength channel typically supports OC48 transmission at 2.5 GBPS. A 32-channel system
will support an aggregate 80 GBPS.
ETSI (European Telecommunications Standardization Institute) An organization that produces
technical standards in the area of telecommunications.
frame relay An interface protocol for statistically
multiplexed packet-switched data communications
in which (a) variable-sized packets (frames) are
used that completely enclose the user packets they
transport, and (b) transmission rates are usually
between 56 kb/s and 1.544 Mb/s (the T-1 rate).
Note 1: In frame relay, (a) there is neither flowcontrol nor an error -correction capability, (b) there
is information -content independence, (c) there is
a correspondence only to the ISO Open systems
Interconnection — Reference Model Layers 1 and 2,
(d) variable-sized user packets are enclosed in
larger packets (frames) that add addressing and
verification information, (e) frames may vary in
length up to a design limit, usually 1 kilobyte
or more, (f) one frame relay packet transports
one user packet, (g) implementation of fast-packet
technology is used for connection -oriented frame
relay services, and (h) there is a capability to
handle time -delay insensitive traffic, such as LAN
interworking and image transfer. Note 2: Frame
relay is referred to as the local management
interface (LMI) standard and is specified in ANSI
T1.617.
frequency 1. For a periodic function, the number
of cycles or events per unit time. 2. The number
of cycles occurring per second of an electrical
Copyright  2001 John Wiley & Sons, Ltd.
M. PEDEN AND G. YOUNG
or electromagnetic wave; a number representing a
specific point in the electromagnetic spectrum.
G.dmt ADSL G.dmt ADSL (also known as Fullrate ADSL) is one standard for home DSL service.
The G.dmt variety can download data at -up to
8 megabits per second, and send data upstream
at up to 1.5 megabits per second, if the modem
is located within 10,000 – 12,000 feet of the phone
company’s CO (central office). Up to 18,000 feet
away from the CO, G.dmt ADSL can reach up to
1.5 megabits per second downstream. This type
of DSL may require the telephone company to
install a device called a ‘‘splitter’’ on the phone
line, requiring an installation visit to your home.
G.dmt Asymmetric Digital Subscriber Line full
rate, which allows the ADSL line to support up
to 8 Mbps downstream and 1 Mbps upstream and
requires that a device called a POTS splitter be
installed at the subscriber or business premise.
G.lite ADSL (or simply G.lite) Building on
the momentum of DSL deployments, G.lite is a
medium bandwidth version of ADSL developed
for the consumer market segment that allows
Internet access at up to 30 times the speed
of the fastest 56 K analog modems. It is an
ITU (International Telecommunications Union)
standard ADSL service for the delivery of speeds
of up to 1.5 megabits downstream and up to
500 kilobits upstream. In most cases it will operate
over existing home telephone wiring (reduces
the need for phone companies to send out a
installer to complete an onsite installation, i.e.
simpler installation process) and can be installed
by the familiar ’plug and play’ process on most
home computers. G.lite is a globally standardized
interoperable ADSL system per ITU Rec. G.992.2.
and is currently primarily in use in the USA.
IP: Internet protocol A DOD standard protocol designed for use in interconnected systems
of packet-switched computer communication networks. Note: The internet protocol provides for
transmitting blocks of data called datagrams from
sources to destinations, where sources and destinations are hosts identified by fixed-length addresses.
The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through small-packet networks.
ISDN: integrated services digital network An integrated digital network in which the same timedivision switches and digital transmission paths are
Int. J. Network Mgmt 2001; 11:265 – 276
FROM VOICE-BAND MODEMS TO DSL TECHNOLOGIES
used to establish connections for different services.
Note 1: ISDN services include telephone, data, electronic mail, and facsimile. Note 2: The method used to
accomplish a connection is often specified: for example, switched connection, non-switched connection,
exchange connection, ISDN connection.
ITU: International Telecommunication Union
A civil international organization established to
promote standardized telecommunications on a
worldwide basis. Note: The ITU-R and ITU-T are
committees under the ITU. The ITU headquarters
is located in Geneva, Switzerland. While older than
the United Nations, it is recognized by the U.N. as
the specialized agency for telecommunications.
LMDS: local multipoint distribution system.
multiplexing (MUXing) The combining of two
or more information channels onto a common transmission medium. Note: In electrical communications,
the two basic forms of multiplexing are timedivision multiplexing (TDM) and frequency-division
multiplexing (FDM). In optical communications, the
analog of FDM is referred to as wavelength-division
multiplexing (WDM).
OC Abbreviation for optical carrier. The nomenclature for the line rate of the optical transmission
signal. [T1.106-1988]
PCM: pulse-code modulation Modulation in
which a signal is sampled, and the magnitude (with
respect to a fixed reference) of each sample is quantized and digitized for transmission over a common
transmission medium. Note 1: In conventional PCM,
before being digitized, the analog data may be processed (e.g. compressed), but once digitized, the
PCM signal is not subjected to further processing
(e.g. digital compaction) before being multiplexed
into the aggregate data stream. Note 2: PCM pulse
trains may be interleaved with pulse trains from
other channels.
POTS: plain old telephone service A call that
requires nothing more than basic call handling.
[T1.667-1999]
PSTN: public switched telephone network A
domestic telecommunications network usually
accessed by telephones, key telephone systems,
private branch exchange trunks, and data
arrangements. Note: Completion of the circuit
between the call originator and call receiver in a
PSTN requires network signaling in the form of
dial pulses or multifrequency tones.
QoS: quality of service 1. The performance specification of a communications channel or system. Note:
Copyright  2001 John Wiley & Sons, Ltd.
273
QOS may be quantitatively indicated by channel
or system performance parameters, such as signalto-noise ratio (S/N), bit error ratio (BER), message
throughput rate, and call blocking probability. 2.
A subjective rating of telephone communications
quality in which listeners judge transmissions by
qualifiers, such as excellent, good, fair, poor, or
unsatisfactory.
RSVP (Resource Reservation Setup Protocol)
Provides priority data transmissions based on
reservation protocol.
STM Signal traffic management.
T1 First ANSI Telecommunications Standards
Committee tip conductor.
T1 line A full-duplex digital transmission facility
that is composed of transmission media (optical
or metallic) and regenerators that carry one DS1
signal. [After T1.408-1990]
T1 (carrier) See T-carrier.
T-carrier The generic designator for any of several digitally multiplexed telecommunications carrier systems. Note 1: The designators for T-carrier
in the North American digital hierarchy correspond to the designators for the digital signal (DS)
level hierarchy. See the associated table below. Note
2: T-carrier systems were originally designed to
transmit digitized voice signals. Current applications also include digital data transmission. Note 3:
If an ‘F’ precedes the ‘‘T’’, a fiber optic cable system
is indicated at the same rates. Note 4: The table
below lists the designators and rates for current
T-Carrier systems. Note 5: The North American
and Japanese hierarchies are based on multiplexing
24 voice-frequency channels and multiples thereof,
whereas the European hierarchy is based on multiplexing 30 voice-frequency channels and multiples
thereof. See table below.
Note 1: The DS designations are used in connection
with the North American hierarchy only. Note 2:
There are other data rates in use, e.g. military
systems that operate at six and eight times
the DS1 rate. At least one manufacturer has
a commercial system that operates at 90 Mb/s,
twice the DS3 rate. New systems, which take
advantage of the high data rates offered by optical
communications links, are also deployed or are
under development.
throughput 1. The number of bits, characters, or
blocks passing through a data communication system,
or portion of that system. Note 1: Throughput may
vary greatly from its theoretical maximum. Note 2:
Int. J. Network Mgmt 2001; 11:265 – 276
274
M. PEDEN AND G. YOUNG
T-Carrier Systems
North American
Japanese
European (CEPT)
Level zero
(Channel data
rate)
64 kb/s (DS 0)
64 kb/s
64 kb/s
First level
1.544 Mb/s (DS1)
(24 user channels)
1.544 Mb/s (24 user
channels)
2.048 Mb/s (30 user
channels)
(Intermediate level,
North American
Hierarchy only)
3.152 Mb/s (DS1C)
(48 Ch.)
Second level
6.312 Mb/s (DS2)
(96 Ch.)
6.312 Mb/s (96 Ch.),
or 7.786 Mb/s (120
Ch.)
8.448 Mb/s (120
Ch.)
Third level
44.736 Mb/s (DS3)
(672 Ch.)
32.064 Mb/s (480
Ch.)
34.368 Mb/s (480
Ch.)
Fourth level
274.176 Mb/s (DS4)
(4032 Ch.)
97.728 Mb/s (1440
Ch.)
139.268 Mb/s (1920
Ch.)
Fifth level
400.352 Mb/s (5760
Ch.)
565.148 Mb/s (7680
Ch.)
565.148 Mb/s (7680
Ch.)
Throughput is expressed in data units per period
of time; e.g. in the DDN, as blocks per second. 2.
The maximum capacity of a communications channel
or system. 3. A measure of the amount of work
performed by a system over a period of time, e.g.
the number of jobs per day.
TIP: terminal A device capable of sending,
receiving, or sending and receiving information
over a communications channel.
VoD Video on Demand.
VoIP Voice over IP; Internet Telephony; using
the Internet to transmit voice.
VoDSL Voice over DSL.
VPN (Virtual Private Network) A network that
is constructed by using public wires to connect
nodes. For example, a number of systems enable
creation of networks using the Internet as the
medium for transporting data. These systems
use encryption and other security mechanisms
to ensure that only authorized users can access
the network and that the data cannot be intercepted.
wavelength The distance between points of corresponding phase of two consecutive cycles of a
wave. Note: The wavelength, , is related to the
propagation velocity, v, and the frequency, f , by
D v/f .
Copyright  2001 John Wiley & Sons, Ltd.
WDM: wavelength-division multiplexing In
optical fiber communications, any technique by which
two or more optical signals having different
wavelengths may be simultaneously transmitted
in the same direction over one fiber, and then be
separated by wavelength at the distant end.
DSL Flavors
ž DSL (digital subscriber line) A technology
that exploits unused frequencies on copper
telephone lines to transmit traffic typically
at multi-megabit speeds. DSL can allow voice
and high-speed data to be sent simultaneously
over the same line. Because the service is
‘always on’, end users don’t need to dial in
or wait for call set-up. With DSL you are
wired for speed.
—Asymmetric flavors—
Asymmetrical variations include: ADSL, G.lite
ADSL (or simply G.lite), RADSL and VDSL. The
standard forms of ADSL (ITU G.992.1, G.992.2, and
ANSI T1.413-Issue 2) are all built upon the same
technical foundation, Discrete Multi Tone (DMT).
Int. J. Network Mgmt 2001; 11:265 – 276
FROM VOICE-BAND MODEMS TO DSL TECHNOLOGIES
The suite of ADSL standards facilitates interoperation between all standard forms of ADSL.
ž ADSL (Full Rate asymmetrical DSL) ADSL
offers differing upload and download speeds
and is usually configured to deliver up to six
megabits (Mbps) of data per second (6000K)
from the network to the customer — that is up
to 120 times faster than dialup service and
100 times faster than ISDN. ADSL enables
voice and high-speed data to be sent simultaneously over the existing telephone line. This
type of DSL most predominantly in commercial use for business and residential customers
around the world. Good for general Internet
access and for applications where downstream
speed is most important, such as video-ondemand. ITU-T (International Telecommunications Union) Recommendation. G.992.1
and ANSI Standard T1.413-1998 specify fullrate ADSL.
ž G.lite ADSL (or simply G.lite) The G.lite
standard was ratified in 1999 and was specifically developed to meet the plug-and-play
requirements of the consumer market segment. G.lite is a medium bandwidth version
of ADSL developed for the consumer market
segment that allows Internet access at up to
30 times the speed of the fastest 56 k analog
modems. It is an ITU standard ADSL service
for the delivery of speeds of up to 1.5 megabits
downstream and up to 500 kilobits (kbps)
upstream. In most cases it will operate over
existing home telephone wiring and thereby
reduce the need for phone companies to send
out an installer to complete an onsite installation, (i.e. simpler installation process) and can
be installed by the familiar ‘‘plug and play’’
process on most home computers. G.lite is
an International Telecommunications Union
(ITU) standard globally standardized interoperable ADSL system per ITU G.992.2.
ž RADSL (rate adaptive DSL) Another version
of ADSL. Transmission technology that supports both asymmetric and symmetric applications on a single twisted pair telephone
line. Allows adaptive data rates up to up to
6.0 Mbps of data per second (6000K) downstream from the network to the customer
Mbps and up to 1.0 Mbps upstream. This predecessor to the international standard, G.dmt,
Copyright  2001 John Wiley & Sons, Ltd.
275
has a US technical recommendation (TR-59)
defined within ATIS T1 that specifies CAP, or
Carrierless Amplitude Phase modulation — an
alternative line code to DMT or discrete multitone.
ž VDSL (very high bit rate DSL) Up to 26 Mbps
on very short lines. In most cases, VDSL lines
will be served from neighborhood cabinets
that link to a Central Office (CO) via optical
fiber. It is particularly useful for ‘campus’ environments— universities and business parks,
for example. VDSL is currently being introduced in market trials to deliver video services
over existing phone lines.
—Symmetric flavors—
Symmetrical variations include: SDSL, HDSL,
HDSL-2, SHDSL and IDSL
ž SDSL (symmetric DSL) SDSL is a version of
symmetric DSL that may include bit-rates toand-from the customer ranging of 128 kbps
to 2.32 Mbps. It is symmetric because it supports the same data rates for upstream and
downstream traffic. The equal speeds make
SDSL useful for LAN (local area network)
access, video-conferencing, and for locations
hosting their own Web sites. Some applications (especially for businesses) require more
upstream capacity than ADSL offers. Other
applications, such as Web browsing and reading email are better served by ADSL. SDSL is
an umbrella term for a number of supplierspecific implementations over a single copper
pair providing variable rates of symmetric
service. SDSL employs the widely-used 2B1Q
modulation, but the industry is expected to
quickly move towards the higher performing
and standardized G.shdsl technology developed by the ITU with support from T1E1.4
(USA) and ETSI (European Telecommunications Standards Institute).
ž SHDSL is state-of-the-art, industry standard
symmetric DSL. Symmetric DSL means that
the same rate is sent to and from the customer via a telephone line. SHDSL equipment conforms to the ITU Recommendation
G.991.2, also known as G.shdsl, expected to
be approved by the ITU-T February 2001.
Int. J. Network Mgmt 2001; 11:265 – 276
276
SHDSL achieves 20% better loop-reach than
older versions of symmetric DSL, it causes
much less crosstalk into other transmission
systems in the same cable, and multi-vendor
interoperability is facilitated by the standardization of this technology. SHDSL systems
may operate at many bit-rates, from 192 kbps
to 2.3 Mbps, thereby maximizing the bit-rate
for each customer. G.shdsl specifies operation
via one pair of wires, or for operation on
longer loops two pairs of wire may be used.
For example, with two pairs of wire, 1.2 Mbps
can be sent over 20,000 feet of 26 AWG wire.
Whereas ADSL is best suited for applications
using asymmetric bit rates and traditional
voice service on the same wire pair, SHDSL
is best suited by data-only applications that
need high upstream bit-rates. Though SHDSL
does not carry voice like ADSL, new voiceover-DSL techniques may be used to convey
digitized voice and data via SHDSL. SHDSL
is expected to be well suited to many business
customers.
ž HDSL (high data rate DSL) This variety
created in the late 1980s delivers symmetric
service at speeds up to 2.3 Mbps in both
directions. Available at 1.5 or 2.3 Mbps,
this symmetric fixed rate application does
not provide standard telephone service over
the same line and is already standardized
through ETSI and ITU. Seen as an economical
replacement for T1 or E1, it uses one, two or
three twisted copper pairs.
ž HDSL-2 (Second-generation HDSL) This variant delivers 1.5 Mbps service each way, supporting voice, data, and video using either
ATM (asynchronous transfer mode), privateline service or frame relay over a single copper
pair. This ANSI (American national Standards Institute) standard for this symmetric
Copyright  2001 John Wiley & Sons, Ltd.
M. PEDEN AND G. YOUNG
service that gives a fixed 1.5 Mbps rate both
up and downstream. HDSL2 does not provide standard voice telephone service on the
same wire pair. HSDL2 differs from HDSL in
that HDSL2 uses one pair of wires to convey
1.5 Mbps whereas ANSI HDSL uses two wire
pairs.
ž IDSL (integrated services digital network DSL)
This is a form of DSL that supports symmetric data rates of up to 144 kbps using
existing phone lines. It is unique in that it
has the ability to deliver services through a
DLC (Digital Loop Carrier: a remote device
often placed in newer neighborhoods to simplify the distribution of cable and wiring from
the phone company). While DLCs provide a
means of simplifying the delivery of traditional voice services to newer neighborhoods,
they also provide a unique challenge in delivering DSL into those same neighborhoods.
IDSL addresses this market today and in the
near future, ADSL and G.lite will as they
are implemented directly into those DLCs.
IDSL differs from its relative ISDN (integrated services digital network) in that it is
an ‘‘always-on’’ service, but capable of using
the same terminal adapter, or modem, used
for ISDN.
For more DSL information:
www.dslforum.org: in-depth technical DSL overview
www.dsllife.com: consumer-friendly DSL information
If you wish to order reprints for this or any
other articles in the International Journal of
Network Management, please see the Special
Reprint instructions inside the front cover.
Int. J. Network Mgmt 2001; 11:265 – 276