TechOnLine Publication Date: Oct. 24, 1997
DMT Versus CAP: A Counterproductive
ADSL Product Manager
With the internet explosion of the early 1990s, the global
demand for high speed data delivery began to grow exponentially, in
many different directions at once, virtually out of control. As we
approach the new millennium, that growth seems to be reaching
critical mass, and shows no signs of abating. Meanwhile, the
present decade has presented the broadband industry with a
staggering array of exciting opportunities.
So it seems almost a tragedy that we're still discussing the
issue of DMT versus CAP. PHY layer discussions and line code
squabbles are counterproductive given the many challenges we face
in trying to commercialize broadband and successfully deliver high
speed data access to millions of consumers worldwide. There are so
many more important issues facing the broadband industry-billing,
service management, backbone capacity and infrastructure, service
architecture-why divert attention, effort, and resources away from
them with unnecessary debate?
Unfortunately, the focus paid to this basic technology issue has
perpetuated the mistaken belief that ADSL is an immature,
laboratory technology not yet suited to mass deployment. Some of
these claims have spread unfounded concerns about the suitability
of ADSL as a broadband solution for operators (comments regarding
power consumption or internet access suitability, for example).
Choice of line-code (modulation) method is an engineering
decision, not one to be made on faith or gut instinct. Different
situations suggest different choices. Certainly there are
applications where single-carrier technology is well suited, and
should be considered. But as we'll see in the next few pages, ADSL
access to broadband services (internet access, LAN access as well
videoconferencing, tele-learning, and video-on-demand) over the
copper network is far better served by DMT.
This is not just our opinion; it is the consensus of experts
around the world, global standards organizations that include ANSI,
ETSI, and the ITU. And it's the opinion of many independent
manufacturers who have the experience and expertise to design
products using whatever code they deem best (they all selected
DMT). Table 1 addresses some of the myths that have grown
up around the debate, and answers each with what we believe to be
the reality of the issue.
|DMT has higher consumption than CAP.
||At equivalent rates, CAP requires more power than
|DMT was intended for Video on Demand (VoD) and
has been made obsolete by internet access.
||DMT is very well suited for internet support. The
ANSI standard explicitly addressed data access. The upstream rate
was chosen to reflect the 10:1 ratio that is optimum for internet
|CAP "invented" rate adaptation.
||DMT has always been rate adaptive. It takes it for
granted, and implements it in an elegant and highly flexible way.
The coarse granularity of CAP (steps of 300 kbps and no downstream
rates of <640 kbps) renders it essentially useless for rural
low-rate/long-reach applications. In contrast, DMT steadily adapts
to support optimum rates on all loops. This allows it to deliver
better services to a larger area, and justifies higher end-user
|Performance is equivalent.
||DMT is demonstrably more robust and has much
better performance-delivering higher rates, much longer reach, or
|DMT is heavily patented or inaccessible.
||DMT is defined in an open international standard
that mandates fair access. Many manufacturers have independently
developed their own solutions. CAP technology has remained
proprietary, with only one supplier.
|DMT is less available than CAP.
||Solutions designed to the ANSI standard are
available now from several suppliers. Yet there is only source of
CAP chipsets, and it will not comply with proposed future
|CAP is simpler than DMT.
||DMT uses more digital smarts and MIPS to deliver
higher performance with less wasted energy. In the past, when
digital technology was expensive, this was a valid criticism.
Today, it no longer holds.
|CAP and DMT are equivalent in
||DMT is the only technology standardized by ANSI,
ETSI or (by reference) the ITU.
|CAP is more robust than RFI.
||CAP can 'power through' RFI; DMT simply sidesteps
it, and doesn't waste energy fighting with a powerful
Table 1: The myths and realities of CAP versus
CAP is closely related to QAM-mathematically, the two can be
considered simple transforms of each other. But they're not
identical, and they can't talk to each other directly without
translation. DMT, closely related to OFDM, which was recently
selected for Europe's Digital Audio Broadcast (DAB) for CD quality
sound and multimedia, differs in that it uses many narrow-band
carriers. They all transmit at once, in parallel, each carrying a
small fraction of the total information. If something happens to
one carrier, it's easy to switch it off and avoid the trouble
(admittedly, multi-carrier techniques require much digital
processing, and weren't commercially feasible until recently, when
IC technology could implement them economically and reliably).
The two techniques can be seen as not-so-identical twins: CAP
operates in the time domain; DMT, in the frequency domain. QAM/CAP
techniques are time-domain based with fast symbols. Each symbol
(and there's only one) lasts a short time, and has a sizable
bandwidth. Each DMT subchannel (hundreds of them are communicated
in parallel) lasts a long time but occupies a narrow frequency
band. A DMT symbol, which is the combination of all the
subchannels, lasts 250 msonly a 4 kbaud symbol rate.
In principle, on a given channel, the two should achieve the
same throughput, (Shannon's law does not specify line code). In
practice, however, differences in the transmitter and receiver
architecture as well as implementation limitations due to cost
affect real-world performance.
Which Technique is More Robust?
A popular truism holds that the best transmission technique is
one which adapts its signal to "match" the channel. More
accurately, the best transmission technique will distribute its
transmit power in those parts of the frequency band that are best
received at the other end of the communications link. Therefore,
the characteristics of the communications link determine what
technique is best. For example, in broadcast systems, where all
frequencies are roughly similar, or mobile cellular, which is
characterized by rapid fades, a simple technique that 'powers the
signal through' is acceptable.
The ADSL environment is different: high frequencies are
attenuated more significantly than low frequencies; bridge taps can
cause frequency notches and resonances; radio stations cause strong
(but stable) RFI. It's a difficult channel in that it treats
dissimilar frequencies very differently-but the channel properties
don't change with time (or change very slowly). A DMT transmitter
can easily monitor the channel, adapt its transmission to the
characteristics of the phone line, and continuously update (via
bit-swapping) to maintain the optimum. For every line, the DMT
system transmits the "best" possible signal. A CAP system can't
modify its transmitter, so it tries to undo all the attenuation and
notches in one fell swoop at the receiver-a tough challenge. That's
why, in evaluations, CAP systems are often described as being less
robust than DMT systems, or not as tolerant to bridge taps. DMT
uses more 'smarts' to tailor its signal to the channel.
Another challenge communications systems must overcome is noise.
In addition to universal thermal noise, the phone channel is
impaired by impulse noise and radio frequency interference. Impulse
noise from electrical appliances, lightning or, most relevantly,
with a ringing or off-the-hook phone, is wideband in frequency and
narrow in time. So it's averaged across a small percentage of a
symbol, and across many DMT subchannels; but it will really clobber
a number of CAP symbols. Essentially, because a DMT symbol is much
longer than a CAP one, an impulse event hurts it less, thus the
technique is inherently more robust.
The most significant sources of RFI are radio stations, because
the ADSL band of 1 MHz sits on top of a portion of the AM band, and
signals leak into the phone wires. These are extremely predicable
(otherwise it would be hard to use your car radio), so the DMT
modem puts signal power where it pays off most-at the receiver-and
not, for example, where it will get wiped out by a radio
interferer. While CAP merely 'powers through' the RFI, DMT simply
avoids it, putting energy into frequency areas that can use it
instead of wasting energy fighting a powerful AM broadcaster.
Thus in the ADSL environment, DMT copes with RFI in a much more
efficient and intelligent way than CAP. And it copes with impulse
noise better than CAP or QAM, because it's inherently more immune
Everyone agrees that standards are important. Because of them, a
consumer can buy a modem in any retail store, dial his or her ISP
with confidence, and get a reliable connection, without ever having
to worry about whether the two will be compatible.
The standards process works through consensus: experts from
across an industry meet, debate, and share opinions and research in
order to arrive at a solution that's greater than the sum of its
parts. The disadvantage is that it can be time-consuming; a
consensus-based solution will arrive on the market later than an
in-house approach that doesn't concern itself with the needs of
others (since CAP technology doesn't adhere to any standard,
proprietary chipset solutions shipped before optimized
standards-based DMT chipsets. However, without interoperability,
the usefulness of these proprietary products ended when the trials
ended. Now, standard DMT chips are available).
However, there are far-reaching benefits. When a standards-based
solution does arrive, it is supported by a wide base of
manufacturers, bringing interoperability between them, and fruitful
competition among them. Further, the process of experts from a
variety of companies working together and discussing the technology
usually leads to a solution that's much better than any one of
those companies could have developed alone (the classic example:
The Adoption of DMT
It was this process that, in 1993, led to the selection of DMT
as the preferred line code. Due to vigorous debate, the impartial
Bellcore (with Bell Atlantic and NYNEX) organized the 'ADSL
Olympics' to evaluate and compare three line code contenders. The
trial results indicated conclusively that DMT had performed better
than CAP and QAM.
Consequently, both ANSI (T1.413 for the U.S.) and ETSI (TR238
for Europe) adopted DMT and the same standard. This is now endorsed
by the ITU: "Q4/15: The initial work will focus on developing
Recommendations for ADSL and HDSL based on the existing ANSI
Standard T1.413 [for ADSL] and ETSI ETR152 Edition 3 [for HDSL]
respectively. These proposed Recommendations will adopt these
existing standards by reference."
At least eight different companies have developed independent
solutions, all to comply with the same definition. Issue 2 of the
standard is now being prepared, resolving many minor editorial
changes and updating the body to formalize new advances (e.g.,
protocols for rate adaptation, ATM cell transport or higher-speed
ADSL) that were implicit in the previous draft, but may not have
been codified. None of these changes hinder compatibility or
In contrast, CAP remains a single-source proprietary technology
that not has been standardized. Late last year, an ad hoc group was
set up to document a single-carrier approach. To date, the group
has made no significant progress. In fact, it wasn't until May 1997
that the most fundamental choice was made: because there was no
agreement about whether CAP or QAM was the better choice, all
modems will be designed as dual-mode to support both, inevitably
requiring greater complexity.
Given the amount of other details still to be resolved (it took
more than two years to document DMT), it is obvious that
single-carrier solutions will not be codified in the near future.
This virtually guarantees there will be no interoperability or
backwards compatibility between them.
Interoperability Drives The Sale
Interoperability is a requirement of any successful
communications technology. Consumers must have confidence that the
products they wish to purchase will work with existing systems. In
part, their readiness to buy depends on knowing that their V.34
modem will connect to their ISP; that their NIC card will connect
to the server; and that their cell phone will work during a
Unfortunately, in the two years that CAP chipsets have been
shipping, interoperability has never been demonstrated-not even
between two modem manufacturers using the same chipset! This is
astonishing, and it's primarily because CAP is not a specified
standard, so many essential features such as error-correction must
be individually developed by each manufacturer rather than within
the chipset (currently, CAP is a proprietary single-source
technology, so interoperability between different chipsets is
meaningless anyway). In contrast, interoperability between modems
from different manufacturers based around the same technology has
been demonstrated by DMT suppliers.
Because DMT is so adept at matching its transmission channels by
varying each of the 200-plus tones independently, it is both more
efficient in its use of bandwidth, and delivers higher performance
under any realistic circumstance.
Since the Bellcore Olympics, CAP modems have avoided independent
tests that produce open, public results. For example, GTE published
an audited performance test, and Tele.com magazine performed a
Consumer Reports-style trial of a number of modems from different
manufacturers. Several DMT modems (and even one HDSL modem) were
tested. All CAP suppliers declined.
Where test results have emerged, it is clear that CAP
consistently under-performs and is less robust. For example,
Network Computing recently completed a trial (testing was performed
by the independent MCI Test Labs):
Since some don't agree that DMT is better than CAP, we
decided to include both in our tests. Although the modem units used
in our tests are early releases, all performed at exceptional
levels. Overall, we found that the DMT-based ADSL modems were
more robust in signaling and were able to perform over longer
distances (up to 18,000 feet).
In fact, we found that most of the modems achieved distances of up
to 15,000 feet (-26 dB). The DMT-based ADSL modems in our tests
were able to operate rate adaptively up to our maximum cable
distance of 18,000 feet (with a measured line attenuation of 31
The CAP-based modems operated at a full speed of up to 4Mbps
downstream and 422Kbps upstream until 12,000 feet was reached for
one of the modems, and 15,000 feet for the other modem, which
reached lower speeds of up to 2.2 Mbps.
[The DMT-based modems had maximum speeds of up to 8Mbps/768Kbps and
achieved higher speeds than CAP at comparable rates]
In other words, despite the alleged maturity of CAP, an
impartial public test found DMT modems faster, more robust and
considerably longer in reach. While CAP modems needed different
models to be optimized for reach or rate, the DMT modems (from
different manufacturers) rate adapted well, achieving either 100%
faster rates with at least 50% extra reach than one specific CAP
model, or four times the maximum speed with 20% extra reach.
The maximum reach was limited by the test environment; however,
the DMT modems would have gone further-they still had plenty of
scope for rate adaption and further reach beyond 18,000 feet. This
is entirely consistent with predictions.
Power Consumption: A Reality Check
It's a widespread misconception that DMT uses more power than
CAP. Though it doesn't, there is some historical justification for
the misconception. Obviously, a low data rate system will use less
power than a faster one. And a chipset will draw less power than a
non-optimized solution based on discrete general-purpose ICs. So a
comparison of a 1.5 Mbps CAP chipset with a 8 Mbps DMT solution
assembled using many discrete general-purpose devices will give the
expected result: the former draws less power than the latter.
However, this doesn't mean that at the same data rate, under the
same test conditions, CAP will use less power. In fact, a fair
comparison will show that DMT ADSL actually requires about the same
power as CAPor less. Here's why:
The signal processing complexity of the two is comparable in
terms of MIPS or die area, although DMT is more 'digital oriented'
than CAP (which uses more precise analog filtering), and will
benefit faster from process technology advances.
The energy used in the driver dominates the system (roughly 50%
of the total power used by the chipset. This depends on the power
spectral density (how many watts are put into each Hz of bandwidth)
and the bandwidth used. Since CAP is less well matched to the line,
and may not benefit from FEC, to obtain equivalent data rates it
needs to relax the psd mask and transmit at a greater power
spectral density. In effect, it squeezes more energy into each Hz
to overcome noise and the weakness of its implementation, at the
expense of power dissipation and cross talk.
It is hard to generalize, as each system can set its own driver
power to meet its specific needs. However, according to the
specifications in the ad hoc proposal a CAP solution transmits at a
much higher power level per Hz. The CAP signal at up to -34dBm/Hz,
where a DMT signal is limited to the lower -40dBm/Hz. CAP's extra
6dBm/Hz means more signal power must be placed per Hertz, all the
What about the peak-to-average ratio (PAR) or crest factor? The
power discussed above is the average level drawn
continuously by the driver and sent on to the line. At times there
will be a need for more, as the signal hits a particular pattern,
and the peak power will be very much higher than the average level.
For CAP, the ratio between these two is 4.0 (in other words, the
peaks are four times the level of the average); for DMT the ratio
is 5.3. In a piece of marketing spin it is sometimes claimed that
because DMT has a higher crest factor it draws more power. This
utterly ignores the fact that PAR is a ratio and that without
knowing the average, you can make no conclusion about the
The actual specifics, for two systems with comparable net (after
overhead) throughput are:
||Category 1 T1.413 DMT
Pseudo-scientific references to 'peak to average ratio'
notwithstanding, the simple physical fact is due to its lower
efficiency, for like data rates and equivalent loops, a CAP system
will require more power than a DMT system.
While most DMT systems are designed for maximum rate and reach,
per T1.413, it is possible to dramatically reduce driver
power for other configurations, if rate or reach can be relaxed. In
these cases, the signal processing and error-correction "smarts"
are employed to reduce power, rather than squeeze out the last drop
of performance. For example, reducing the data rate to achieve 1.5
Mbps at 12,000ft could save more than 1W in the driver.
Additionally, 5.3 is PAR for a raw or 'naïve' DMT solution.
A number of digital techniques exist for gain scaling and peak
mitigation that can significantly reduce this, which would reduce
the peak power needs. Once more, DMT uses digital technology and
algorithms to significantly improve system performance over the
'obvious' implementation. Other technologies don't have the scope
to use these opportunities, and faultfinding may arise from
critics' lack of familiarity with such techniques.
Spectral Compatibility and
Though it may sound like an abstruse or bureaucratic issue,
spectral compatibility is of great importance to any real system
that will be deployed in the network. It defines how much energy a
system will put out, and to what extent it will interfere with
other systems trying to use other copper pairs in the area. This
cross-talk will easily affect other copper pairs in the same binder
group, and in severe cases, can affect pairs in another binder (a
badly designed system can literally prevent other users from
communicating). As the network moves towards deregulation, having
clear and universally respected rules becomes even more important
to prevent conflict (and avoid lawsuits).
This psd mask was defined by T1E1 for ADSL as a generic
technology not specific to any one line code, in other words,
whatever the implementation, it must meet the requirements and
strictures to not cause interference. Unfortunately, the current
CAP document and existing implementations do not follow this mask.
Instead of stopping at 138 KHz, the upstream continues up to 180
KHz. This will sabotage the performance of standard DMT to such an
extent it is unlikely it could be deployed in the same binder for
normal length loops. A standards-compliant DMT system is better
behaved, and will not interfere with other technologies, including
Similarly, on the downstream, CAP continues out to 1.5 MHz,
instead of 1.1 MHz. This extra bandwidth places it directly into
the VDSL band, causing significant interference and cross-talk
there. It appears that a single ADSL system with these properties
would dominate all other noise sources and potentially make VDSL
unusable on all but the very shortest of loops.
In addition to choosing a robust modulation technique,
error-correction coding provides a powerful technique to protect
against noise and interference. Virtually every communication
system in the world today employs error-correction and coding, and
it is almost inconceivable that any modern system would ignore the
benefits of including them. Probably the only exception is CAP
chipsets: the existing chipset supports error-correction only on
the downstream (in other words, the upstream is completely
unprotected). Worse, the proposed ad hoc report drops even that, so
there's no standard defined error-correction at all.
Since error-correction is essential in any practical ADSL
system, the fact that it is not implemented in the CAP chipset has
two critical implications:
- It must be implemented in the modem outside the datapump
chipset. This complicates the system design significantly. It is
also wasteful, expensive and inelegant, especially since
error-correction can be efficiently implemented in a comparatively
small silicon area as part of a datapump.
- It makes interoperability virtually impossible.
Error-correction is so fundamental to a connection that if it's
removed from a "standard" and made part of each modem
manufacturer's individual system, it is equivalent to abandoning
interoperability on anything other than a trivial system (one with
no error-correction is not realistically deployable).
It is important to remember this when comparing data rates. The
6.1Mbps defined in ANSI T1.413 is net effective
payloadthe actual line rate, with overhead, is higher
(approximately 6.9Mbps). In contrast, the "7 Mbps" of CAP is a
gross rate; the effective data rate will be much less, since all
error-correction will have to be subtracted from this. However,
since error-correction is not standardized, it is difficult to give
a specific answer as to what this will be, and the result will vary
with the implementation.
Rate-Adaptivity and Reach
In marketing CAP, the "RADSL" acronym (Rate-Adaptive ADSL) was
created, implying that DMT is not rate adaptive. Actually, the
reverse is true: DMT is inherently rate adaptive, and has always
been rate adaptive. No reference to this capability was attached to
the acronym because everyone took it for granted! In contrast, all
the original implementations of CAP were fixed rate. When it
finally achieved rate adaptation (1996), it was called an
DMT achieves rate adaptation easily and flexibly. It uses
hundreds of degrees of freedom (subchannels) to accomplish it, and
delivers the maximum data for any given line. This allows the
support of higher rates over shorter loops (>8 Mbps), or
sub-rate connections at very long reach (perhaps a few hundred Kbps
over many miles, load coils permitting of course). CAP can support
rate adaptation by only varying the constellation and the bandwidth
of a single carrier. This requires very careful analog design, and
the rates have much poorer granularity. DMT steps smoothly in 32
Kbps steps from 64 Kbps to >8 Mbps actual payload, while CAP has
coarse and erratic steps from 640 Kbps to 7 Mbps (gross rate
DMT is like a mountain bike: it has lots of gears to adapt to
different terrain. Some gears are suited for low rates and long
reach of rural areas, others for very high speed (T1.413 Issue 2
allows up to 16 Mbps) on the short loops of urban environments.
This is especially important when discussing rate adaptation for
lower rates and longer reaches, a key concern for U.S. operators.
While a 'coarse' step of >300 Kps may not matter in stepping
between multi-megabit speeds, it definitely matters at lower rates.
In fact, CAP simply can't support these applications: its rate
adaption steps go from 960 to 680 to 640 Kbps, then drop off to
nothing-meaning that no service at all can be delivered!
This is disastrous for operators who need to cover long reaches
and rural areas, even if at lower rates. In cases involving
distances of several miles, a rate-adaptive modem that can support
a few hundred Kbps is a major attraction. CAP cannot serve these
remote customers, whereas DMT will steadily scale to support very
long reach, its rate dropping smoothly as reach is increased, from
960 Kbps to 928, 896, and so on down to 64 Kbps at very long range,
allowing an operator to cover more area and offer service to more
This becomes even more important when you consider the effect of
area. Suppose that, due to DMT's greater efficiency and channel
matching, it can deliver a like rate at 25% greater range than CAP.
This allows it to serve almost 60% more area! When you consider the
many thousands of feet that DMT can serve at low-speed/long reach,
that's a huge areaand a huge number of subscribersthat
can't be served by CAP.
One final point: DMT varies the rate by digitally adjusting the
individual sub-carriers. It is very flexible and efficient, and
easily scales (costs reduce) with process technology and Moore's
law. In contrast, CAP uses an analog technique (with different
filters) that is far less flexible, and less amenable to cost
"But Wasn't DMT Designed for VoD?"
There is a misconception that DMT was a specific solution for
video-on-demand, and that CAP is the internet access solution of
choice. Again, precisely the opposite is true. DMT was designed for
data access from the start, whereas CAP was initially totally
unsuitable, requiring a drastic redesign (and is still less
well-suited for the task).
CAP systems were originally built for video only as evidenced by
their minimal upstream capability (16 Kbps, increased to 64 Kbps in
1996enough to pick a movie but not enough to provide PPP
handshakes during an IP session). Not until two years after release
of the first products was the upstream redefined to support data
services. The original DMT standard, as defined, specifically
recognized data services (Annex G of T1.413-95 explicitly discusses
data access, remote LAN access and telecommuting as key
applications, and their requirements), and defined an upstream
DMT has higher bandwidth efficiency, which translates to higher
speed (and bandwidth is like PC memory: we always seem to need
more) only at the expense of end-to-end latency, which doesn't
matter for internet access. Within the standard document, the
system is defined to provide a 10:1 ratio in data rates, which
studies have confirmed is optimal for internet access.
(CAP is inherently suitable for internet access? Its supporters
spent much of 1995 and 1996 struggling to support internet services
with the antiquated upstream of 64 Kbps, while trying to justify a
20:1 or even 30:1 ratio as acceptable.)
Further, the latest version of the DMT standard goes even
further, explicitly documenting the ways that DMT and T1.413 will
grow, while remaining backward-compatible, to support rate
adaption, ATM and packet mode (IP and frame relay) data services in
an efficient, versatile and interoperable way. These developments
have no counterpart within the still-incomplete CAP
It is claimed that DMT is heavily patented, and enclosed in a
thicket of intellectual property. In fact, the situation of both
DMT and CAP is remarkably similar. The principle difference is that
DMT is documented in a standard, open forum, and independent
vendors can use this to design compatible systems. However, the
fact that all the details for DMT are publicly available encourages
independent developers. Several developers (including Alcatel,
ADI/Aware, and Orckit & Pairgain) have, completely
independently, developed systems designed to the T1.413
In contrast, CAP has been a proprietary, 'closed' architecture,
confirmed by the fact that not a single alternative supplier has
delivered an open-market alternative.
When Not To Choose DMT
We're not saying DMT is suitable for all applications-it isn't.
The goal should be to choose the best technology for a given
application. Therefore, perhaps it's worth describing when and
where DMT is not
DMT has long latency (2 ms). If this is important, another
technology is more relevant (e.g., HDSL2 uses single carrier
technology). However, for ADSL and its intended
applications-internet access and broadband services-latency is a
One of the strengths of DMT is its versatility, rate
adaptability, and flexibility in coping with a vast range of
environments found in the 700 million existing copper lines
(efficiently matching its properties to the channel, and coping
elegantly with noise). In some cases, where that versatility is not
required, a less adaptable technique may be more appropriate.
Finally, in applications where extreme power efficiency is
crucial and takes precedence over data-rate or information density
(e.g., battery powered wireless with data rates of just a few
Kbps), single carrier techniques can be optimized (e.g., OPQSK or
GMSK used in digital cellular) for efficiency. However, the high
data rates required of ADSL make it an entirely different
application (in any case, the CAP used in ADSL is a different
technology; it cannot achieve this optimization).
Moving Forward on Facts, Not Faith
We must remember that line code is an engineering issue, not a
matter of allegiance, instinct, or faith, so there's always room
for rational discussion. And we should try to avoid forcing
applications to fit existing technologies ("When the only tool you
have is a hammer, every problem looks like a nail.").
It is notable that most of the suppliers of DMT, including ADI,
Alcatel, Motorola, and Orckit & Pairgain, are not confined to a
single choice: they have the experience and skills to manufacture a
variety of products using a number of different line codes. This
suggests they are well positioned to recognize the best technology
for an application. And each of them selected DMT.
We believe the standards process and industry experts have done
a good job over the past five years of discussion. They concluded
that DMT is the optimum technology for ADSL. Why? Because in the
critical areas-communications speed, bandwidth efficiency, spectral
compatibility, performance, robustness and power consumption, and
delivery of a better service to more customers-DMT has proven
superior to alternative single-carrier technologies.