techtrends By Joshua Israelsohn, Technical Editor
FITTING BROADBAND SERVICES ONTO A
UBIQUITOUS NETWORK DESIGNED FOR
NARROWBAND HAS BEEN NO SIMPLE MATTER. AFTER YEARS OF EMBARRASSING FALSE
STARTS AND UNDELIVERED PROMISES, DSL
MAY FINALLY BE UP TO SNUFF FOR A USER
BASE INCREASINGLY ADDICTED TO SPEED.
Addicted to speed
W
ith more than a century of infrastructure development,
POTS connections are available to a larger fraction of the
world’s population than any other electronic communication system. Designed to carry 3.4-kHz-voiceband traffic, the network is a
hodgepodge of new and old cable, buried and overhead wiring, better and lesser quality cable splices, and unterminated taps, none of
At a glance ............................55
Acronyms ..............................56
For more information ........58
which prevent it from delivering its intended payload. But if the
designed service bandwidth isn’t enough to suggest a fundamental
incompatibility between POTS and datacommunications services, add to it those
quality variations, noise sources, signal
reflections, and even a reliable 48-kbps
modem connection is impressive. In that
context, datacomm services one to two
orders of magnitude faster would seem
impossible. Yet for millions of subscribers, DSL works, and, what’s better,
newer technologies and additional infrastructure improvements are extending
DSL’s reach.
OBSTACLES IN THE PHYSICAL LAYER
The most basic limitation wiring poses is signal attenuation. The distributed
cable impedance forms a lowpass filter
54 edn | September 13, 2001
whose attenuation of high-frequency information increases in proportion to cable length. Longer loops are possible at a
given bandwidth by increasing the wire
size or by selecting lower capacitance cable constructions, but for customers living in established communities, the infrastructure dominated by 24- and 26gauge wire is a given.
The electrical characteristics of local
loops pose challenges even in the voiceband when cable lengths extend to 18,000
ft or longer. Telephone companies install
loading coils every 6000 ft to improve
voiceband performance for these long
loops (Reference 1). Loading coils are incompatible with DSL, but depending on
www.ednmag.com
the telephone company and the geodistance over which DSL services can opgraphic area, as many as 20% of the loerate, are not the only physical-layer obAT A GLANCE
cal loops have them.
stacles. Telephone companies use DLC
The
access
network’s
copper
plant
still
e
As the local loops approach the central
remote terminals, which are incompatirepresents the greatest limitation to xDSL
office, they branch into large feeder cable with DSL, to terminate local loops in
access.
bles, which reduces the spacing between
the field, multiplex their signals, and conloops and increases interloop coupling.
nect to the central office over fiber-optic
New
xDSL
technologies
are
more
tolere
As you would expect, this crosstalk incable or four-wire copper circuits. DLC
ant of bridged taps.
terference poses a greater problem for
remote terminals service about 35% of
high-frequency signals than for those at
existing and 60% of newly installed loe CPE modems with autoconfiguration
the lower end of the spectrum. Crosstalk
cal loops (references 5 and 6).
may help eliminate installation visits.
is also a greater problem for upstream
DSL À LA CARTE
signals—those transmitted from the CPE
e VDSL promises speeds fast enough for
to the central office—than for the downSpeed, SNR, distortion, power dissipaconcurrent streaming video, high-speed
stream signals. This asymmetry in
tion, cost, form factor, and standards
data, and voice, but proponents of incomcrosstalk sensitivity is due to the fact that
compliance are important factors in DSL
patible signaling standards are waging a
the upstream signal is already attenuatphysical-layer designs for either centralstandards battle that could delay its deployed before it reaches the point at which
office or CPE applications. No one comment.
signals from adjacent cables can couple
bination of components simultaneously
in. The downstream path from the cenoptimizes all factors. The range of intetral office enjoys a better SNR, which par- These taps increase the cable attenuation gration runs from driver amplifiers to
allels the greater need for bandwidth in by splitting the incident-signal energy.
chip sets that comprise drivers, receivers,
the downstream channel than in the upBridged taps are not only a concern for filters, converters, data pumps, and bus
stream channel. However, as Internet use the access network; customer-premises interfaces.
moves away from a broadcast model to- wiring almost always includes a number
Due to the low system impedances and
ward peer-to-peer information sharing, of short taps as well. POTS splitters help long signal lines, power dissipation is a
this asymmetry becomes a renewed con- alleviate the problem but don’t allow ex- major factor for line drivers. Vendors ofcern.
isting site wiring to carry ADSL signals. ten package the drivers separately to alDepending on its build-out and main- Higher speed services, particularly low process optimizations that are inaptenance history, the network includes VDSL, are less tolerant of on-premises propriate for more densely integrated
bridged taps, which are unterminated wiring quality (Reference 4).
functions, such as analog front ends or
stubs of undetermined lengths. The imSignal attenuation, interference, and data pumps.
pact of a bridged tap on a signal is a func- crosstalk, which inconsistently limit the
Common line-interface designs intion of how the tap length compares with
the signal wavelength (references 2 and
L
4.57m
304.8m
3). At dc, bridged taps are innocuous. At
datacomm bandwidths, however, unterminated taps divide the signal, increasing the effective cable attenuation be0
yond that of an uninterrupted loop. In
cases in which the tap length coFigure 1
25
incides with a quarter of the sigL=0
nal’s wavelength, l/4, the reflection from
L=7.7m
210
the unterminated stub recombines with
L=37.5m
the original wavelength delayed by l/2.
L=757.5m
215
The corresponding 1808 phase shift in
the reflected signal destructively comATTENUATION (dB) 220
bines with the original signal, causing a
null in the line’s spectral response (Fig225
ure 1). Additional nulls are created at fre230
quencies that coincide with odd multiples of the quarter wavelength,
235
(2n11)l/4. Reflections from short taps
create deep nulls in the channel response.
240
0
2
4
6
8
10
12
14
16
18
20
Longer taps create a series of closer, regFREQUENCY (MHz)
ularly spaced nulls. Very long bridged
taps tend not to generate destructive reflections because the reflected energy is Bridged taps in the access network appear as unterminated stubs at broadband frequencies,
dissipated before it rejoins the main loop. increasing signal attenuation and creating spectral nulls (courtesy Analog Devices).
www.ednmag.com
September 13, 2001 | edn 55
techtrends xDSL PHY
clude dual drivers and dual receivers,
forming balanced I/O channels behind a
signal transformer. National Semiconductor has developed VIP10—a dielectrically isolated, complementary bipolar
process tailored for high-speed amplifiers and line drivers for applications
such as xDSL and cable modems. The
process uses a buried oxide layer and oxide-filled side trenches to reduce the collector-substrate parasitic and to isolate
devices. Other process features reduce
the collector-base capacitance and the
collector, base, and emitter resistances,
yielding fTs of 9 and 8 GHz for npns and
vertical pnps, respectively. The reduced
parasitic terms also improve the powerto-bandwidth ratio.
Among the first products available on
National’s VIP10 process is the LMH6672 dual high-speed amplifier for use as
a full-rate ADSL upstream line driver in
CPE modems. The 6672 can drive 6200
mA with a 9V p-p output operating on
12V supplies. It features 130-MHz 3-dB
bandwidth and 160V/msec slew rate but
develops only 4.5 nV/=Hz input-noisevoltage density at 100 kHz. The amplifier’s second harmonic distortion is typically 292 dBc driving 8.4V p-p at 1 MHz
into 100V. Under the same conditions,
the third harmonic distortion is 95 dBc.
The quiescent current for the dual line
driver is 6.2 mA per amplifier. Pricing
and packaging options include a $1.98
(1000) SOIC-8 and PSOP-8 and a $1.88
(1000) LLP-8 that reduces the devices
footprint to 434 mm, including termi-
nations. Both the PSOPs and LLPs are
equipped with thermal pads.
National’s LMH6654 and LMH6655
also offer single and dual low-noise xDSL
receivers, respectively, which are suitable
for central-office or CPE systems. The
250-MHz unity GBW parts draw 7 mA
maximum per amplifier over 240 to
185 8C. The input-referred voltage noise
is typically 4.5 nV/=Hz above 100 kHz.
The current noise is 1.7 pA/=Hz. Distortion is 280 dBc for the second harmonic and 285 dBc for the third harmonic at 2V p-p, 5 MHz into 100V.
National offers the 79-cent (1000)
LMH6654 single receiver in either SOIC8 or SOT23-5 packages. The dual LMH6655 is available in an SOIC-8 package
for $1.05 (1000).
Linear Technology offers the LT1794
dual amplifier in a TSSOP-20 for centraloffice-based xDSL line-driver applications. The IC has a resistor programmable bias current that allows you to set the
performance-versus-supply-current
trade-off to suit your design or switch biasing on the fly to save 80% of its quiescent power when the devices aren’t driving the line. In normal operation, each
amplifier draws 2 to 10 mA, depending
on biasing. In shutdown mode, the current load drops to 1 mA maximum and
100 mA typically. The amplifier can drive
6500 mA into 1V and swings 100V to
within 1.2V of 15V rails.
The 1794 is gain-of-10 stable and features a 200-MHz GBW. You can compensate the amplifier for operation at
lower gains. Distortion is rated at 277
dBc at 1 MHz driving 2V p-p into 100V,
and the input-noise voltage and current
densities are 8 nV/=Hz and 0.8
pA/=Hz, both at 10 kHz with a low 1/f
corner. Linear Technology characterizes
the $4.95 (1000) dual device for operation with 615, 612, and65V supplies.
Texas Instruments’ THS6042 dual line
driver for CPE applications operates on
65 to 615V supplies. Designed to operate into a 1-to-1 transformer, the
THS6042 has a 100-MHz small signal
bandwidth at a gain of four with a 100V
load. The input-voltage-noise density is
2.2 nV/=Hz at 10 kHz. But the asymmetrical inputs and their current noise
densities—2.1 and 11 pA/=Hz for the
noninverting and inverting inputs—can
dominate the total noise performance
depending on the impedances at those
nodes. Even so, the total input-referred
noise density of a typical THS6042 application is lower than many of its competitors. THD is rated at 279 dBc at 250
kHz driving 2V p-p into 100V at a gain
of four.
The $2.65 (1000) THS6042 dual driver, available in standard and thermally enhanced SOIC-8 packages, adheres to the
standard pinout for dual op amps and
draws 10.5 mA when it is powered from
612V rails. The THS6043, a sibling part
packaged in standard and thermally enhanced SOIC-14 package, adds a shutdown mode in which the current drops to
0.7 mA. The larger pinout’s “upper”
end—pins 1 through 4 and 11 through
ACRONYMS
ADC: analog-to-digital converter
ADSL: asymmetrical digital-subscriber line
ANSI: American National Standards Institute
CABGA: chip-array ball-grid array
CPE: customer-premises equipment
DAC: digital-to-analog converter
DLC: digital loop carrier
DMT: discrete multitone
DSL: digital subscriber line
DSLAM: digital-subscriber-line-access
multiplexer
DSP: digital-signal processor
GBW: gain bandwidth
HTSSOP: thermally enhanced thin shrink
small-outline package
ISDN: integrated services digital network
56 edn | September 13, 2001
ITU: International Telecommunication Union
LFBGA: low-profile fine-pitch ball-grid array
LGA: land-grid array
LLP: leadless leadframe package
LQFP: low-profile quad flat pack
MCM: multicarrier modulation
MSOP: mini small-outline package
PBGA: plastic ball-grid array
PLGA: plastic land-grid array
POTS: plain-old telephone service
PQFP: plastic quad flatpack
PSOP: power small-outline package
QAM: quadrature-amplitude modulation
QFN: quad flat nonlead
SCM: single-carrier modulation
SDSL: symmetric digital subscriber line
SFDR: spurious-free dynamic range
SHDSL: symmetrical high-bit-rate digital subscriber line
SNR: signal-to-noise ratio
SOIC: small-outline integrated circuit
SOT: small-outline transistor
STDR: sequence time-domain reflectometer
THD: total harmonic distortion
TDR: time-domain reflectometry
TQFP: thin quad flatpack
TSSOP: thin shrink small-outline package
Utopia: universal test and operation physical
interface for ATM (asynchronous transfer
mode)
VCXO: voltage-controlled crystal oscillator
VDSL: very high bit-rate DSL
www.ednmag.com
techtrends xDSL PHY
14—follows the same
connection pattern for
the dual amplifiers as
TRANSMIT
LINE DRIVER
ADSL
TIP
DAC
DIGITAL
AND FILTERS
DATA
the SOIC-8 package.
FILTERS
LINE
The “lower”end—pins
ADSL
HYBRID
TRANSFORMER
INTERFACE
5 through 10—allows
RECEIVE
ADSL
for the extra shutdown
RECEIVE PGA
DIGITAL
`
RING
CONTROL
ADC
AND FILTERS
FILTERS
function. The similar
pinouts allow you to
5 AND 3.3V
make the imVCXO
POWER
Figure 2
AND PLL
SUPPLIES
plementation
of the shutdown function a board-popula- The Si3101 analog front end from Silicon Laboratories integrates line drivers, receivers, converters, and filters for CPE
tion option. TI also of- modems.
fers lower power, lower
drive versions—the $2.28 (1000)
Another low noise dual receiver is
For central-office-side line drivers in
THS6052 and ‘53—and the single-supply Elantec’s $2.13 (1000) EL2227C with in- DSLAMs, Tripath makes the TLD401x
$2.14 (1000) THS6092 and ‘93.
put-noise densities of 1.9 nV/=Hz and series with programmable gain. The parts
TI specifies its $2.19 (1000) OPA2822 1.2 pA/=Hz. The amplifiers have mini- require 615 and 65V supplies but can
balanced receiver for either 5V single- mum open loop gain of 70 dB, 115-MHz drive at least 500 mA into 50V. At idle,
supply or 66V split-supply operation. Be typical 23dB bandwidth, and slew faster the driver IC dissipates 250 mW, which
aware, however, that with an absolute than 40 V/msec. They are stable at a gain drops by almost half in its logic-conmaximum supply rating of 66.5V, the of two. Second and third harmonic dis- trolled low-power mode during which it
2822 affords little tolerance for power- tortion is 293 and 294 dBc, respective- maintains its output-impedance characsupply overvoltage. What the IC does af- ly, driving 500V 2V p-p at 1 MHz, drop- teristics but does not drive the line. When
ford, however, is low noise—2.2 ping to 283 and 276 dBc with 150V. a controller disables the IC by holding its
nV/=Hz and 2.0 pA/=Hz—and a 150- The 2227, which follows the standard RESET line low, the line driver no longer
MHz GBW at gains of 20 or more. The dual op-amp pinout in SOIC-8s and maintains the line impedance but drops
dual receiver, available in an eight-lead MSOP-8s, operates supplies as low as its power dissipation further to 10 mW.
SOIC or MSOP, draws a maximum of 62.5V and draws 6.5 mA per amplifier
The TLD401x typically has a 10-MHz
11.2 mA from 66V rails.
at 612V.
small signal bandwidth at a gain of 20.8
FOR MORE INFORMATION...
For more information on products such as those discussed in this article, go to www.ednmag.com. When you contact any of the following manufacturers
directly, please let them know you read about their products in EDN.
Elantec
1-408-945-9305
www.elantec.com
Enter No. 338
National Semiconductor
1-408-721-5000
www.national.com
Enter No. 342
Tripath
1-408-567-3000
www.tripath.com
Enter No. 346
Broadcom
1-949-450-8700
www.broadcom.com
Enter No. 349
Legerity
1-512-228-5400
www.legerity.com
Enter No. 339
Silicon Laboratories
1-512-416-8500
www.silabs.com
Enter No. 343
OTHER COMPANIES MEN-
Infineon
1-888-782-8411
www.infineon.com
Enter No. 350
Linear Technology
1-408-432-1900
www.linear.com
Enter No. 340
STMicroelectronics
1-781-861-2650
www.st.com
Enter No. 344
Mindspeed Technologies/
Conexant Systems
1-949-579-3000
www.mindspeed.com
Enter No. 341
Texas Instruments
1-800-336-5236
www.ti.com
Enter No. 345
58 edn | September 13, 2001
TIONED IN THIS ARTICLE
Alcatel Microelectronics
1-919-850-5515
www.alcatel.com
Enter No. 347
Analog Devices
1-800-262-5643
www.analog.com
Enter No. 348
Lucent
1-888-458-2368
www.lucent.com
Enter No. 351
Marvell Semiconductor
1-408-222-2500
www.infineon.com
Enter No. 352
STANDARDS AND INDUSTRY
ORGANIZATIONS
American National
Standards Institute
www.ansi.org
DSL Forum
www.adsl.com
European Telecommunications Standards
Institute
www.etsi.org
International Telecommunications Union
www.itu.int
Open DSL
www.opendsl.org
VDSL Alliance
www.vdslalliance.com
VDSL Coalition
www.vdsl.org
SUPER INFO NUMBER
For more information on the products available from all of the vendors listed in this box, go to
www.ednmag.com, click on the Reader Service link, and enter no. 353
www.ednmag.com
techtrends xDSL PHY
dB and 280 dBc SFDR over the 26- to
138-kHz band while it delivers 20.4 dBm
into 100V. You can program the gain
from 12.8 to 27.8 dB in 1-dB increments
with a 4-bit code. The TLD4011 uses
standard termination resistors, but the
other three family members synthesize
their output impedance, in effect multiplying their termination resistors and,
consequently, reducing the terminator
dissipation. All four of the $3.50 (5000)
line drivers provide resistor-programmable overcurrent protection, overtemperature protection, and a Fault status
signal. They are packaged in a thermally
enhanced 32-lead TQFP.
IT’S A TWO-WAY STREET
Beyond individual line drivers and receivers, the next step in functional integration is to combine the two functions,
often with active line termination to reduce the total system-power dissipation.
The system-power budget is always a
consideration, but it is particularly so for
central-office and remote terminal systems in which power-system capacity,
channel density, and rack cooling impact
capital equipment and operating costs.
Legerity exploits its high-voltage dielectrically isolated bipolar process in the
Le87S11 central-office-side ADSL transceiver, which operates from a 5V and a
standard 248V telephone-company supply instead of operating on an intermediate supply voltage, such as 615 or
612V. This distinction allows central-office- or remote-terminal-equipment designers to simplify their power-supply
designs. Like most gear designed for telephone-company power, the 87S11’s supply tolerance is substantial—stretching
from 230 to 268V.
The Le87S11 transceiver provides active line termination to halve dissipation
in the line-terminating resistors. The
transceiver buffers the upstream data to
drive a low-impedance-input analog
front end, but if your analog front end
has high-impedance inputs, a logic control allows you to shut down the buffers
and further reduce the transceiver’s power dissipation. Under normal operating
conditions, the transceiver’s dissipation
is 800 mW when active and 150 mW in
monitor mode.
Preliminary specifications give a typical downstream signal gain of 27 over a
138- to 1104-kHz band and a 100V/msec
60 edn | September 13, 2001
THE SYSTEM-POWER
BUDGET IS ALWAYS A
CONSIDERATION, ESPECIALLY FOR CENTRAL-OFFICE
AND REMOTE-TERMINAL
SYSTEMS.
slew rate. The $6.50 (10,000) transceiver
fits in a QFN-32 package and supports 8Mbps ANSI T1.413 issue 2 and ITUG.992.1 as well as 1.5-Mbps ITUG.992.2.
Each of TI’s THS7102 and 7103 transceivers integrates preamplifiers, downstream bandpass and upstream lowpass
filters, a balanced line driver, a balanced
receiver, and active termination in a
PQFP-32 or 535-mm PLGA. The 7102’s
filters are set for ADSL over POTS with
a 155-kHz to 1.3-MHz bandpass transmit filter and a 146-kHz lowpass corner
on the receive channel. The 7103 is set
up to piggyback ADSL service over
ISDN lines with a 276-kHz to 1.3-MHz
bandpass and a 238-kHz lowpass. Because ADSL over ISDN doesn’t have an
equivalent to the POTS splitter, such a
link requires filtering to remove ISDN
traffic from the downstream ADSL
channel.
The line driver’s bias current is adjustable using two control pins, so your
system can tailor the power use to loop
length. The driver’s output noise is 75
nV/=Hz from 30 to 130 kHz for the
7102 and 150 to 230 kHz for the 7103.
The receiver gain-of-two output noise is
45 nV/=Hz at 30 kHz and 150 kHz for
the 7102 and 7103, respectively, and 54
nV/=Hz at 130 kHz and 230 kHz.
GAME, CHIP SET, AND MATCH?
In its Si3101 ADSL AFE, Silicon Laboratories comes close to a chip set of one
for CPE modems supporting ITU
G.992.1 (G.dmt) and ITU G.992.2
(G.lite). The chip integrates transmit
and receive digital and analog filters, a
14-bit DAC and ADC, a line driver and
receiver, and a VCXO within a TQFP-44
(Figure 2). With a crystal, a transformer,
and a handful of passives, the 3101 supports 8-Mbps downstream and 800-
kbps upstream data rates. The analog
front end implements the analog and
digital filters, so the data that the chip
passes to the ADSL DSP requires only
DMT processing. The 3101 typically dissipates less than 1W combined from its
5V analog and 3.3V logic supplies.
The receiver-channel gain is programmable from 22 to 142 dB in 0.5-dB increments with a typical input-noise-voltage density of 8 nV/=Hz. THD is 285
and 282 dB for the transmit and receive
channels. A reference design based on the
$14.00 (10,000) analog front end consumes 2.5 in.2, excluding the DSP.
A $20 (10,000) per line chip set from
STMicroelectronics for central-officeside equipment processes eight channels
from the local loop to a Utopia system interface in 1.2 in.2 and 1.1W per line. You
can choose the STLC60133 single Class
AB or the STLC60243 dual Class G line
drivers. The 60133 offers 4 nV/=Hz input-noise-voltage density and 260-dBc
harmonic distortion, driving 18V at 1
MHz into 100V. At a gain of six, the line
driver has a 23-dB point at 100 MHz,
and it can slew 400V/msec. The
STLC60133 draws 25 mA per line typically from 65 to 615V supplies. The
60133, packaged in a thermally enhanced
HTSSOP-28, has four operating modes
determined by two control bits. In addition to its normal operating mode with
full bias, it has a power-down mode in
which it maintains its output impedance
for proper line termination and two lowpower operating modes with reduced
bias.
The STLC60243 dual-channel Class G
line-driver’s gain is digitally programmable from 13 to 28 dB in 1-dB steps. Its
distortion is 270 dBc driving 40V at 1
MHz into 50V. The 60243 requires both
65 and 615V supplies. The Class G amplifier is a tricked-up Class D output
stage, which nominally connects to the
65V supply and switches to the 615V
rails as necessary to follow its input-signal peaks. The dual-channel driver internally manages its power-supply
switching without the need for external
control or signal monitoring. It’s available in a thermally enhanced TQFP-48.
The STL60844 octal analog front end
with integrated line receivers in an
LFBGA-196 and the STLC60845 octal
DMT data-pump processor in a PBGA292 form the rest of the chip set.
www.ednmag.com
techtrends xDSL PHY
With the advances in xDSL silicon, the
uncertain condition of the copper plant
remains a large challenge to provisioning
DSL services. Mindspeed incorporates
loop-test capabilities in its multimode
SHDSL chip set, alleviating the need for
expensive central-office-side test equipment or truck rolls to remote-terminal or
DLC cabinets. The LoopWizard chip set
implements STDR, a low-power version
of TDR that can detect and locate open
circuits, short circuits, bridged taps, and
load coils that interfere with DSL-service
deployment. Unlike standard TDR signals, which can disrupt service on adjacent lines, the STDR test stimulus resembles a pulse-amplitude-modulated
signal with a smaller crest factor than
most DSL line codes.
The $160 (10,000) CX28985 implements LoopWizard test capability in an
octal central-office-side chip set, comprising a DSP/framer/microprocessor
with a Utopia level II interface in a
27327-mm PBGA and a pair of quad
analog front ends or transceivers in a
737-mm LGA. The CPE-side mating
chip set, the $24 (100,000) CX28975, includes a DSP/framer/microprocessor in
either a 24324-mm LQFP or a 13313mm CABGA and an analog front end or
transceiver in a 737-mm LGA. The chip
sets support multiple SDSL standards.
BACK TO THE FUTURE
Local exchange carriers are looking to
two technology trends to solve the
biggest DSL-deployment challenges. Provisioners expect CPE modems with automatic configuration facilities to eliminate expensive house calls. This capability will finally enable DSL modems to
join the competitive retail market (Reference 7). An increase in remote DSLAM
deployment is, in effect, bringing the central office to the neighborhood and solving the loop-length limitation. Coinciding with this trend is the increased
interest in VDSL, which can support
asymmetrical and symmetrical line bit
rates of 6.5 to 52 Mbps over 100 to
1500m.
In what is shaping up to be a bad
broadband re-enactment of “the shootout at the VHS/Beta corral,” two groups
promoting competing, mutually incompatible signaling standards are trying to
dominate high-speed wired access. The
VDSL Coalition, including such semi-
62 edn | September 13, 2001
UNTIL DEPLOYABLE VDSL
SYSTEMS GO HEAD-TOHEAD, MUCH OF THE
SHOOT-OUT IS JUST A WAR
OF WORDS.
conductor vendors as Broadcom, Infineon, and Lucent, back SCM/QAM signaling. The VDSL Alliance, which includes Alcatel, STMicroelectronics, Texas
Instruments, Legarity, Tripath, Analog
Devices, and Marvell Semiconductor,
promotes MCM/DMT.
Both sides make claims and counterclaims about the superiority of their respective technologies. The forces behind
DMT point out that their approach allows for programmable frequency allocation. This feature allows DMT channels to operate on networks with bridged
taps (skipping over the spectral nulls)
and in the presence of fixed frequencyinterference sources, such as amateur and
AM-radio transmissions. DMT’s programmable frequency allocation also
provides the means for systems to update
themselves as future frequency plans
emerge without facilities resorting to
hardware replacements. Additionally,
systems can configure a DMT receiver as
an ersatz spectrum analyzer, a useful attribute during line provisioning and fault
detection.
The QAM camp points out that DMT
requires more processing power, which
translates into a larger silicon area, resulting in higher silicon costs and larger
power budgets (Reference 8). OEM engineers recognize low power dissipation
as a critical goal in remote rack designs
because the systems have to operate without forced cooling through the full range
of outdoor temperatures.
Both sides have compared the two
technologies but have done so largely in
advance of complete DMT VDSL field
trials. The availability of silicon implementing the DMT version of VDSL has
significantly lagged behind competing
QAM systems. Winning the time-tomarket race gives QAM and the members
of the VDSL Coalition a competitive advantage but not necessarily a sustainable
one: Witness ADSL where QAM systems
were out first but lost the standards battle to DMT. But not all of the technological issues scale directly, so until deployable VDSL systems go head-to-head,
much of the shoot-out is just a war of
words. That situation is about to change
over the next few quarters, as several
members of the VDSL Alliance announced products in Atlanta at last June’s
Supercomm.
Which new technologies telephone
companies adopt and how eager they are
to push DSLAMs into the neighborhoods will determine when subscribers
have access to what services. But with aggressive competition from cable companies and renewed interest in fiber-to-thecurb, the telephone companies are eager
to feed their customers most addicted to
speed.k
References
1. The DSL Sourcebook, Third Edition,
Paradyne Corp, 2000.
2. Friedman, Vladimir, “VDSL Technology Overview,” white paper, Analog
Devices, 2000.
3. Im, Gi-Hong, and JJ Werner, “Effect
of bridged taps at VDSL frequencies,”
ANSI T1E1.4/97-324, September 1997.
4. Huang, Gang, and JJ Werner, “Cable characteristics,”ANSI T1E1.4/97-169,
May 1997.
5.“2000 access network system market
forecast,” RHK, Feb 29, 2000.
6. “DSL Anywhere,” white paper, DSL
Forum, 2000.
7. Suydam, Margot, “Reload,” CommVerge, August 2001, pg 23.
8. Rokach, Ehud,“QAM: the choice for
VDSL transmission line code,” Orckit,
1998.
Author’s biography
Not long ago, Technical
Editor Joshua Israelsohn watched incredulously as installers
brought a new dedicated line to his house and
added new internal wiring so that he
could subscribe to DSL—the broadband
technology that uses your existing telephone line. Though he still shakes his
head at the memory, he’d rather not go
back to the 56-kbaud dial-up he keeps for
backup. You can reach Joshua Israelsohn
at 1-617-558-4427, fax 1-617-558-4470,
e-mail jisraelsohn@cahners.com.
www.ednmag.com