1 - Introduction to xDSL
What is xDSL?
[1.2] How do other residential and broadband
[1.3] What/Where are the xDSL standards?
[1.4] Should I get xDSL?
2 - General xDSL information
[2.1] How does xDSL work?
What are the various types of xDSL?
How much does xDSL cost?
[2.4] Is xDSL available in my area?
Why are some variations of xDSL asymmetric?
[2.6] What does a POTS splitter do and when
do I need one?
[2.7] What test equipment is available for
3 - Basic Data Communications
[3.1] What is analog?
[3.2] What is digital?
What is modulation?
[3.4] What is attenuation?
[3.5] What is crosstalk?
[3.6] What is the effect of noise?
4 - The Local Loop
[4.1] What is the local loop?
[4.2] What is a bridge tap?
[4.3] What are loading coils?
[4.4] What are echo suppressors and echo
[4.5] What is a CODEC?
[4.6] How do I determine how far I am from
[4.7] What do people mean by a "truck roll"?
[4.8] What is dry copper?
[4.9] What are binder groups and why are
5 - Encoding and Modulation
[5.1] What is QAM?
[5.2] What is PCM?
[5.3] What is PAM?
[5.4] What is V.90?
[5.5] What is CAP?
[5.6] What is DMT?
6 - Setup and Configuration
[6.1] What hardware does my home computer
[6.3] Can I use my 28.8K/56K modem with my
Appendix A - Acronym List
1 - Introduction to xDSL
What is xDSL? [top]
xDSL is a generic abbreviation for the many flavors of DSL or
Digital Subscriber Line technology. DSL refers to the technology
used between a customer's premises and the telephone company,
enabling more bandwidth over the already installed copper cabling
than users have traditionally had.
[1.2] How do other residential and broadband
technologies compare? [top]
Cable Modems ------------ Cable modems are devices that attach
to the cable TV network connection in a home. This broadband
technology is being driven by the cable companies to provide
services beyond traditional broadcast cable TV such as Internet
access. Along with xDSL, it is still in the early stages of
development. There are a number of challenges faced by this
industry, including return path capabilities, customer service
issues and standards. However, potential bandwidth estimates
range upwards of 30Mbps from the service provider to subscriber.
Cable networks are inherently different in design than telephone
networks. Cable networks are broadcast oriented, with each subscriber
in an area receiving the same signals as all others in that
area. xDSL is circuit oriented so that each connection is independent
of all others. Cable networks are inherently hierarchical in
nature and thus require two paths, one for downstream and one
for upstream. This requires either a second cable plant for
upstream or a second frequency band allocated onto the existing
system. Wireless There are a number of different wireless schemes
proposed, planned and implemented throughout the world. Wireless
access technology takes shape in a number of different forms
such as via a satellite TV service provider or a cellular phone
network. Wireless systems can provide ubiquitous access to a
large number of subscribers in a relatively large area. Bandwidth
can range from a few kilobits a second to many megabits and
be either symmetrical or asymmetrical. Like all other technologies,
there can be deployment issues which may include spectrum licensing,
interference and noise problems, or bandwidth limitations. Analog
Modems Analog modems use a telephone network as is. That is,
there are no special provisions that are required to use analog
modems in today's telephone networks. Analog modems simply allow
digital data to flow over the telephone company's already analog
network by performing a digital to analog conversion for transmission
onto the network and vice versa on the receiving end. The only
necessity for analog modems is that each end of the call must
have a compatible modem. In essence, this makes analog modem
connections the most ubiquitous form of data communications
available today. However, analog modems are thus limited by
the telephone company's voice bandwidth service. Current analog
modems are struggling to achieve rates of only 56Kbps. With
only a bandwidth of about 3,000 Hz, there is a extremely small
finite limit on the amount of data that may be encoded and sent
reliably through this network. User requirements far outstrip
what analog modems can obtain today. ISDN ISDN is a telephone
company technology that provides digital service typically in
increments of 64Kbps channels. ISDN has been around for many
years, but it's popularity is now only beginning to increase
due to the limitations of analog modems and the rise of Internet
usage. ISDN requires the phone company to install services within
their phone switches to support this digitally switched connection
service. Roll out of this service initially got off to a slow
start and was stalled by high costs, lack of standards and low
acceptance rate by consumers. xDSL is technology backed by telephone
companies to provide next generation high bandwidth services
to the home and business using the existing telephone cabling
infrastructure. xDSL to the home over existing phone lines promises
bandwidths up to 9Mbps, but distance limitations and line quality
conditions can reduce what will actually be achievable. xDSL
technologies will use a greater range of frequencies over the
cable than traditional telephone services which in turn allow
for greater bandwidth with which to send and receive information.
This technology is still in the early stages of roll out with
standards and products just getting under way. Driving this
market is the competition from competing access providers and
the pursuit of your Internet access dollar.
[1.3] What are the xDSL standards? [top]
American National Standards Institute ANSI
TI.413-1995 Asymmetric Digital Subscriber Line (ADSL) Metallic
Interface Note: ANSI TI.413 Issue 2 was released September 26,
1997 Also of interest: Standards Committee T1-Telecommunications Note: relevant documents
are from the T1E1 subcommittee European Telecommunications Standards Institute .
[1.4] Should I get xDSL? [top]
That depends on a number of answers to questions which you'll
need to ask yourself. First and foremost you need to determine
if DSL is even available in your area. You may not have a choice.
By reading this FAQ, you can hopefully learn enough about xDSL
and how to get more information to make an informed decision.
Although there are merits to all competing technologies, we
make no recommendation in this FAQ to specify which one is right
3.0 General xDSL information
[3.1] How does xDSL work? [top]
xDSL utilizes more of the bandwidth on copper phone lines than
what is currently used for plain old telephone service (POTS).
By utilizing frequencies above the telephone bandwidth (300Hz
to 3,200Hz), xDSL can encode more data to achieve higher data
rates than would otherwise be possible in the restricted frequency
range of a POTS network. In order to utilize the frequencies
above the voice audio spectrum, xDSL equipment must be installed
on both ends and the copper wire in between must be able to
sustain the higher frequencies for the entire route. This means
that bandwidth limiting devices such as loading coils must be
removed or avoided.
[3.2] What are the various types of xDSL? [top]
There are several forms of xDSL, each designed around specific
goals and needs of the marketplace. Some forms of xDSL are proprietary,
some are simply theoretical models and some are widely used
standards. They may best be categorized within the modulation
methods used to encode data. Below is a brief summary of some
of the known types of xDSL technologies. ADSL Asymmetric Digital
Subscriber Line (ADSL) is the most popular form of xDSL technology.
The key to ADSL is that the upstream and downstream bandwidth
is asymmetric, or uneven. In practice, the bandwidth from the
provider to the user (downstream) will be the higher speed path.
This is in part due to the limitation of the telephone cabling
system and the desire to accommodate the typical Internet usage
pattern where the majority of data is being sent to the user
(programs, graphics, sounds and video) with minimal upload capacity
required (keystrokes and mouse clicks). Downstream speeds typically
range from 1.5Mbps to 9Mbps. Upstream speeds typically range
from 64Kbps to 1.5Mbps. ADSL Lite A lower data rate version
of Asymmetric Digital Subscriber Line (ADSL) has also been proposed
as an extension to ANSI standard T1.413 by the UAWG (Universal
ADSL Working Group) led by Microsoft, Intel, and Compaq. This
is known as G.lite in the ITU standards committee. It uses the
same modulation scheme as ADSL (DMT), but eliminates the POTS
splitter at the customer premises. As a result, the ADSL signal
is carried over all of the house wiring which results in lower
available bandwidth due to greater noise impairments. CDSL Consumer
Digital Subscriber Line (CDSL) is a proprietary technology trademarked
by Rockwell International. EtherLoop EtherLoop is currently
a proprietary technology from Nortel, short for Ethernet Local
Loop. EtherLoop uses the advanced signal modulation techniques
of DSL and combines them with the half-duplex "burst" packet
nature of Ethernet. EtherLoop modems will only generate hi-frequency
signals when there is something to send. The rest of the time,
they will use only a low-frequency (ISDN-speed) management signal.
EtherLoop can measure the ambient noise between packets. This
will allow the ability to avoid interference on a packet-by-packet
basis by shifting frequencies as necessary. Since EtherLoop
will be half-duplex, it is capable of generating the same bandwidth
rate in either the upstream or downstream direction, but not
simultaneously. Nortel is initially planning for speeds ranging
between 1.5Mbps and 10Mbps depending on line quality and distance
limitations. G.Lite (see ADSL Lite) HDSL High Bit-rate Digital
Subscriber Line (HDSL) is generally used as a substitute for
T1/E1. HDSL is becoming popular as a way to provide full-duplex
symmetric data communication at rates up to 1.544 Mbps (2.048
Mbps in Europe) over moderate distances via conventional telephone
twisted-pair wires. Traditional T1 (E1 in Europe) requires repeaters
every 6000 ft. to boost the signal strength. HDSL has a longer
range than T1/E1 without the use of repeaters to allow transmission
over distances up to 12,000 feet. It uses pulse amplitude modulation
(PAM) on a 4-wire loop. IDSL ISDN based DSL developed originally
by Ascend Communications. IDSL uses 2B1Q line coding and typically
supports data transfer rates of 128 Kbps. RADSL Rate Adaptive
Digital Subscriber Line (RADSL) is any rate adaptive xDSL modem,
but may specifically refer to a proprietary modulation standard
designed by Globespan Semiconductor. It uses carrier less amplitude
and phase modulation (CAP). T1.413 standard DMT modems are also
technically RADSL, but generally not referred to as such. The
uplink rate depends on the downlink rate, which is a function
of line conditions and signal to noise ratio (SNR). SDSL Symmetric
Digital Subscriber Line (SDSL) is a 2-wire implementation of
HDSL. Supports T1/E1 on a single pair to a distance of 11,000
ft. The name has become more generic over time to refer to symmetric
service at a variety of rates over a single loop. VDSL Very
High Bit-rate Digital Subscriber Line (VDSL) is proposed for
shorter local loops, perhaps up to 3000 ft. Data rates exceed
[3.3] How much does xDSL cost? [top]
It varies. xDSL service availability still in the early stages,
but pricing in some areas has been very aggressive. Prices can
change overnight and differ significantly depending on the service
provider and surrounding area. Local tariffs and government
regulations may also play a role in determining end user cost.
To find out more about how much xDSL service may cost, check
with the service providers listed in section [8.8] or ask in
the newsgroup(s) or mailing list(s) for the most up to date
[3.4] Is xDSL available in my area? [top]
To find out, you can check a number of sources. First, you can
check with your local telephone company to see if they are providing
xDSL services. Second, check around with your local Internet
Service Providers (ISPs). Thirdly, try the competitive local
exchange companies (CLECs) in your area. A good resource for
CLECs is at <http://www.clec.com>. Fourth, try perusing
some of the resources listed in section [8.8] of this FAQ. Fifth,
ask around in the xDSL newsgroup(s) or mailing list(s). Lastly,
you can try a few of these links, although you are cautioned
that their accuracy or completeness can not be vouched for by
the maintainers of this FAQ. http://www.getspeed.com/htm/set_input.html
For those who really want to go all out, you can install the
FCC's Benchmark Cost Proxy Model from bcpm2.com. According to
a recent posting by Fred R. Goldstein, "once you start playing
with its 'transport' module, you'll find that it includes the
addresses of every CO."
[3.5] Why are some variations of xDSL asymmetric? [top]
It is primarily due to near-end crosstalk (NEXT). The large
bundle of wire at the CO is heavily susceptible to crosstalk
when the data is traversing from the far end (the end user).
At the far end, there are fewer problems with NEXT so bandwidth
is greater from the CO to the user. High bit rates, or in this
case, higher frequencies suffer a greater amount of attenuation.
The reason that the upstream speed in ADSL is generally much
less than the downstream rate is due to this fact. When the
high frequencies have attenuated at the CO end, they are very
susceptible to all the other signals in the binder group due
to EMI. In the downstream direction, the high frequencies still
attenuate, but at the customer end, they have a better chance
of avoiding crosstalk since most subscribers will not have large
bundles of cables running into their premises.
[3.6] What does a POTS splitter do and when do I need
A POTS splitter uses a low pass filter to separate the low end
frequencies of the telephone audio spectrum from the higher
frequencies of the xDSL signals. The splitter should be a passive
device, not requiring power so that "life-line" voice service
can be provided as has been in the past. This splitter allows
for the traditional voice service that consumers are accustomed
to. A splitter is required at both the customer premises and
at the far end (CO). xDSL that does not use a POTS splitter
is termed "splitter-less xDSL". Whether a POTS splitter is required
or not depends on the xDSL service being provided.
[3.7] What test equipment is available for xDSL? [top]
Fluke One Touch <http://www.fluke.com/nettools/> Fluke
Corporation sells an add-on to their One Touch LANMeter product
that can perform asymmetric bandwidth testing.
4.0 Basic Data Communications
[4.1] What is analog? [top]
A good starting point in order to understand analog communications
is to first take in the picture below. + ,'^', + / \ + / \ +
/ \ ++++++++++++++\++++++++++++ + \ / + \ / + \ / + `._.` Although
my artistic ability leaves much to be desired, this wave form
is a depiction of a simple analog signal. The key to the analog
signal is that it is *continuous*. In other words, notice how
the wave slowly rises, peaks, slowly descends, bottoms out and
slowly climbs again. Taken as a simple example, imagine many
forms of this wave signal. Some of the waves are closer together
than others, some may have more height, still others may actually
start their peaks and descents in entirely different places!
Encoding data can be done based on these various kinds of wave
changes. One of the important considerations in analog communications
is the ability to decode these continuous wave forms. With the
introduction of noise, or other signal disturbance, decoding
a analog signal properly can be difficult. This is why we turn
to the digital communications system (see next question).
[4.2] What is digital? [top]
Again, with a picture let us look a simplistic view of a digital
signal. + .--------. .-- + | | | + | | | + | | | ++++|++++++++|++++++++|+++
+ | | | + | | | + | | | + --' '--------' Compared to the picture
of the analog signal above, there is a major difference in this
wave form. The transition from the peak of the wave to the bottom
of the wave is *discrete*. In this case, the only way to represent
data is by using the high or low point of the wave. For example,
the high point may represent a "on" signal and the low point
may represent a "off" signal. In the world of computers, this
is also known as a binary numbering system consisting of only
two digits. By using a digital signaling system in this fashion,
it makes encoding and decoding data very simple. Generally,
it will be very easy to determine where the peaks and valleys
are, even with some signal loss or disturbance. Digital methods
are used as long as frequency response (bandwidth) is not a
limitation. Analog methods are used only because multiple signal
levels must be exploited to communicate a higher data rate of
digital values in lieu of having adequate bandwidth. A digital
signaling system often has an analog component. Strictly speaking,
this means the a digital wave isn't as sharp cornered as the
picture shows above. The corners will likely be slightly rounded
and even more so as the signal travels over some distance. For
our purposes, this definition should give you a basic idea of
how a digitally encoded system works.
[4.3] What is modulation? [top]
Modulation is a prescribed method of encoding digital (or analog)
signals onto a waveform (the carrier signal). Once encoded,
the original signal may be recovered by an inverse process called
demodulation. Modulation is performed to adapt the signal to
a different frequency range than that of the original signal.
Here's how it flows: bits -> modulator -> audio ->
phone network -> audio -> demodulator -> bits Hence
the name MODEM short for modulator/demodulator. The modem is
necessary because the phone network transmits audio, not data
bits. The modem is for compatibility with existing equipment.
[4.4] What is attenuation? [top]
Attenuation is signal loss due to the diminishing availability
of signal energy, or signal power. As a analog or digital signal
traverses across a medium, it fades. High attenuation may lead
to the inability to recover the signal on the far end. Signal
repeaters may be used on the transmission path to periodically
boost the signal strength. Baseband transmission is extremely
limited to attenuation. Broadband much less so. In addition,
wireless communications is much less susceptible to attenuation
than is wireline communications such as xDSL or cable modems.
[4.5] What is crosstalk? [top]
Crosstalk refers to the interference between channels. In the
xDSL world, the interference between nearby cables can have
a negative impact on the performance of the affected cable(s).
Have you ever been on the phone and heard some other conversation,
not yours, in the background?
If so, you have experienced the effect of crosstalk. Near-end
crosstalk (NEXT) occurs when the transmitter sends a signal
and a nearby transceiver at the same end of link, through capacitive
and inductive coupling, "hears" the signal. Far-end crosstalk
(FEXT) occurs when the transmitter sends a signal and a transceiver
at the far end of the link, through capacitive and inductive
coupling, "hears" the signal. FEXT will be of more concern in
an asymmetrical system such as ADSL than symmetrical systems
like HDSL. This is because strong signals originating from the
near end, can interfere with the weaker signals originating
at the far end.
[4.6] What is the effect of noise? [top]
Noise may be defined as the combination of unwanted interfering
signal sources whether it comes from crosstalk, radio frequency
interference, distortion, or random signals created by thermal
energy. Noise impairs the detection of the smallest analog levels
which may be resolved within the demodulator. The noise level
along with the maximum clip level of an analog signal path set
the available amplitude dynamic range. The maximum data rate
of a modem is limited by the available frequency range (bandwidth)
and signal-to-noise ratio (SNR) which is amplitude dynamic range.
If more of either is available, more bits may be transferred
per second. The information carrying limit was discussed theoretically
by Claude Shannon and is known as Shannon's limit, or information
theory. Because modems run close to Shannon's limit today, no
further advances will be made to traditional telephone line
modems other than incremental improvement of V.90. The frequency
range of the audio channel is very limited at about 4KHz. V.34+
modems are limited to a maximum data rate of 33.6Kbps by an
SNR of about 36dB caused mostly by network PCM quantization
noise. While V.90 improves the SNR by utilizing the network
PCM levels directly, it is still subject to Shannon's limit.
xDSL modems take advantage of the spectrum above the telephone
audio channel. While operating with somewhat less amplitude
dynamic range they increase data rates by greatly increasing
the frequency range of the communication signal (from about
10KHz to over 1.0MHz). To do this they require the installation
of special equipment at the central office and customer premise.
5.0 The Local Loop
[5.1] What is the local loop? [top]
A pair of wires, moderately twisted for the entire length between
the telephone company's end office and the user premises (the
common telephone set) form a loop, so it is referred to as the
local loop. This loop provides a user with access to the global
telecommunications infrastructure that is installed all over
the world. The local loop has been historically designed to
provide voice grade audio service. The circuit is powered from
the central office with 48V (open circuit voltage) limited in
current to a value somewhat higher than 20mA. This current is
used for signaling phone access, burning off moisture, breaking
through metallic oxides caused by corrosion, and powering a
carbon microphone. The original telephone equipment contained
no active electronics. The actual wiring of the local loop may
be considered to be a lousy transmission line. xDSL uses whatever
frequencies will propagate on this line for purposes of digital
data transmission. T1 modulation (alternate mark inversion)
has been doing this for years. xDSL extends the capability by
using modern technology to increase the data rates and distances
[5.2] What is a bridge tap? [top]
A bridge tap is an accidental connection of another local loop
to the primary local loop. Generally it behaves as an open circuit
at DC, but becomes a transmission line stub with adverse effects
at high frequency. It is generally harmful to xDSL connections
and should be removed. Extra phone wiring within one's house
is a combination of short bridge taps. A POTS splitter isolates
the house wiring and provides a direct path for the xDSL signal
to pass unimpaired to the ATU-R modem.
[5.3] What are loading coils? [top]
Loading coils are used to extend the range of a local loop for
voice grade communications. They are inductors added in series
with the phone line which compensate for the parallel capacitance
of the line. They benefit the frequencies in the high end of
the voice spectrum at the expense of the frequencies above 3.6KHz.
Thus, loading coils prevent xDSL connections.
[5.4] What are echo suppressors and echo
These are active devices used by the phone company to suppress
the reflection of an analog signal or positive feedback (singing)
on the phone network. The effect of the echo on a voice connection
is undesirable. Imagine that as you spoke into the phone's microphone,
there was a short delay and you hear your own voice back over
the earpiece. A soft echo that comes back fast enough is not
bothersome to the average person. A more delayed echo is annoying.
A echo suppressor works by allowing only one direction to transmit
at a time so as to entirely eliminate the effect of an echo.
An echo suppressor is able to switch between each end very rapidly,
typically within 5msec. Network echo suppressors make full-duplex
communication impossible. However, modems can deactivate these
devices by sending the 2100Hz answer tone at the beginning of
the connection. An echo canceller subtracts a locally generated
replica of the predicted echo based on the signal propagating
in the forward direction. Echo cancellers do allow full-duplex
operation and are generally preferred over echo suppressors
in voice calls. But when network echo cancellers compete with
echo cancellers within the modem they are problematic. Typically
they reduce data rates to 9.6Kbps or lower. Network echo cancellers
are deactivated by placing 180 degree phase reversals every
450msec on answer tone. As long as carrier is maintained, they
are supposed to remain deactivated. xDSL is not affected by
network echo suppressors/cancellers because they are part of
the CODEC signal processing.
[5.5] What is a CODEC? [top]
CODEC is an abbreviation for coder/decoder. Specifically it
converts a voice grade analog signal to u-law or A-law encoded
samples at an 8KHz sampling rate. xDSL bypasses the CODECs at
the central office by separating the xDSL signal and voice frequencies
in a POTS splitter. The voice signal is passed to a CODEC while
the xDSL signal terminates in a DSLAM, the xDSL equivalent of
[5.6] How do I determine how far I am from
my CO? [top]
You can call your service provider and ask them for the address
of your local CO. Using a map, you get an approximate distance
from your residence to the CO. However, these are very rudimentary
measurements because you can never be sure exactly what route
your line takes between the two points. It may not be a direct
route. If you're interested in whether you will qualify for
high speed broadband service or if you're just wondering what
the potential speed you could attain may be, there are other
factors to consider (i.e. wire gauge, element continuity, environments,
etc.) In a nutshell, just knowing the where the CO is, may not
tell you much at all.
[5.7] What do people mean by a "truck roll"?
Anytime a service technician needs to be dispatched in order
to install, configure or troubleshoot a line installation, it
is referred to as a "truck roll". The significance of this term
implies a real cost to the service provider whenever a technician's
time is required. The term derives from the scene of a technician
driving the familiar "company truck" and pulling up to the curb
of your premises with the intention to install, configure or
troubleshoot a line.
[5.8] What is dry copper? [top]
Dry copper refers to twisted pairs that are not connected to
a telephone switch, battery or anything else between customer
locations. They are merely cross-connected in between. The term
"dry" actually originated over 100 years ago, when batteries
were first used to power telephones. A dry pair had no power
applied to it from the CO and a "wet" one did. Some folks have
been able to implement xDSL via dry copper connection between
two sites. By simply placing xDSL modems at each end of the
dry copper connection, a xDSL may be possible with little intervention
from the perspective of the CO. However, this is a risky method
of deploying xDSL, especially asymmetrical versions. The problems
occur when there is interference between the dry copper xDSL
lines and other lines nearby, such as T1 and POTS. Typically
dry copper has been used for low speed alarm circuits. By implementing
xDSL service over dry copper, you run the risk of future problems.
You may disrupt service at the CO and hence, the CO's customers.
Unless you have specifically contracted for this method of xDSL
service in advance, beware.
[5.9] What are binder groups and why are they important?
A binder group is just a bunch of wires. More correctly in the
telco world, a collection of twisted pair wires will share a
common "sheath". The implementation of services within a binder
group needs to be considered so that the effect of interference
between services does not degrade nearby signals. Interference
between wire pairs in a binder group can be a major issue in
6.0 Encoding and modulation
[6.1] What is QAM? [top]
Quadrature amplitude modulation (QAM) is a method for encoding
data on a single carrier frequency. The modulation encodes
data (or bits) as discrete phase plus amplitude changes of
a carrier tone. The phase vectors are arranged in a pattern
of points called a constellation from which the transmitted
point is selected based on the data to be sent. The modem
sends the symbols as abrupt changes in phase and amplitude,
but only as what emerges from a sharp cutoff filter which
carefully limits the bandwidth. The transmitted signal occupies
slightly more than ±1/2 the modulation rate either side of
the carrier frequency. The excess bandwidth, perhaps as much
as 10%, is required for recovering symbol timing within the
remote receiver. The receiver has to pick which point was
transmitted with great reliability. It may employ adaptive
equalization or other methods to reduce inter-symbol interference
to levels which are acceptable for discriminating the received
point. The background noise level of the receiver limits the
number of distinct constellation points which may be reliably
determined, and hence limits the data rate for a given symbol
rate. QAM has become the dominate modulation for high speed
voice band modems. Examples are V.22bis, V.27, V.29, V.32bis,
V.34. About every 2/3 of a carrier cycle the phase or amplitude
is changed to a new value. This signaling rate is known as
the baud (or symbol) rate. The highest QAM baud rate in use
today for telephone line modems is 10/7 of 2400Hz or about
3429 baud on a 1920Hz carrier in V.34. By encoding something
between 9 to 10 bits per baud a final data rate of 33.6Kbps
is developed. To encode this number of bits, over 1000 different
phase/amplitude values must be resolved by the receiver. This
is a nontrivial process involving adaptive equalizers, trellis
coding, and other highly sophisticated signal processing.
Transmit path: scrambler -> symbol generator -> 3x upsample
(S1,0,0,S2,0,0,S3,...) -> complex transmit baseband FIR
filter -> e^jwt carrier modulation -> scale real signal
output -> DAC converter The baseband filter is about 3dB
down at ±1/2 symbol rate, so for 3429 baud the signal out
of the filter extends from -1715Hz to +1715Hz. This is shifted
by the positive 1920Hz carrier to +205Hz to +3635Hz. One can
see that this just fits in the frequency spectrum of the voice
band telephone network. This filter, the analog electronics
and the phone channel smear any given symbol over a 10msec
period of the signal (about 32 symbols). The scrambler is
very important. It randomizes the signal so an adaptive equalizer
in the remote modem can build the inverse channel response
(including the transmit filter). The smearing (or inter-symbol
interference) is largely eliminated by dynamically adjusting
adaptive equalizer coefficients with the goal of minimizing
least square error in the received points. The major adaptation
is done during the training phase, although the feedback loops
remain active throughout the connection. Other impairments
to be solved are gain normalization, timing recovery, carrier
offset frequency, phase jitter removal, and echo cancellation.
[6.2] What is PCM? [top]
Pulse code modulation (PCM) is used in the phone network to
reduce the data rate required for voice grade audio to less
than 64Kbps. It uses either u-law (North America) or A-law
(Europe) as the compression method. Any given 8KHz analog
audio sample is converted to 4 bits of mantissa, 3 bits of
exponent, and a sign bit. This code has a characteristic that
quantization noise is proportional to signal amplitude and
does not become objectionable to the average telephone user.
For a conventional modem this noise floor limits the available
dynamic range to about 36dB which sets the maximum data rate.
The least significant bit of the mantissa may be periodically
stolen for signaling within the phone network (called robbed-bit
signaling) further increasing the noise. The 8-bit codes are
processed through the telephone switching network in fixed
time slots. There exists an ever increasing hierarchy of data
rates to support this. A DS0 is a 64Kbps time slot. 24 DS0s
become a DS1. 4 DS1s become a DS2 (now obsolete). 7 DS2s become
a DS3, etc. The physical layer of a DS1 (T1) may be re-modulated
as alternate mark inversion for passing over a wire pair as
a method to concentrate local loops. Repeaters regenerate
the signal every 6000-9000'. These signals may coexist with
xDSL in the same wire bundle.
[6.3] What is PAM? [top]
Pulse amplitude modulation (PAM) is the physical layer of
an ISDN or HDSL connection. The modulation consists of sending
discrete amplitude levels (symmetric about 0 volts) at a regular
rate. Both use the two binary one quaternary (2B1Q) line code.
Four analog voltages (called quaternary symbols) are used
to represent the four possible combinations of two bits. These
symbols are assigned the names +3, +1, -1, and -3. So each
amplitude level being held for one symbol time communicates
two bits. The following diagram is typical of the 2B1Q waveform
at the transmitter: +3 = 2.64V + .--. .--. .-- + | | | | |
+1 = 0.88V + | `--. .--' | .--. | ++++|+++++|++|+++++|++++++++|++|++++++++|++++
-1 = -0.88V + --' | | | .-----' `--. | + | | | | | | -3 =
-2.64V + `--' `--' `-----' One might assume this is a digital
signal relative to the definition in [4.2] , but by the time
the signal has reached the receiver these discrete levels
have diffused into each other because of phone line induced
amplitude and phase distortion. This is called inter-symbol
interference. Therefore an adaptive equalizer must be used
to restore the levels to values which may be discriminated
for recovering the data. The symbol timing is recovered by
examining the squared signal energy for a tone at the modulation
rate. Transitions between levels cause the instantaneous power
to dip on average provided there is adequate excess bandwidth.
PAM differs from the other modulations in that it is baseband
modulation and does not use a carrier. Some versions of HDSL
increase the number of levels to 16 which communicates four
bits per symbol in the same bandwidth.
[6.4] What is V.90? [top]
V.90 is actually a variant of PAM. It has 256 PCM levels from
which to choose a more limited set. The spacing between levels
is set by the u-law or A-law characteristic described in [6.2]
. The inner levels become more closely spaced so some of these
must be excluded for reasons of limited signal-to-noise ratio.
In addition, outer codes are excluded to keep transmit power
on the local loop below -12dBm, a formal limit established
by the FCC. V.90 includes a spectral shaping algorithm to
prevent sending signal at DC. V.90 bypasses the problems associated
with a conventional modem. It recognizes that with enough
signal processing the original PCM samples sent by the phone
company may be resolved as individual levels using a 16-bit
A/D converter on the receiving end. Audio is sent through
the digital network as 8-bit u-law or A-law samples. Of course,
the telco D/A converter, reconstruction filter, and phone
network blur the levels into one continuous signal, so it's
up to the receiver to reconstruct what was sent. An additional
problem is recovering symbol (i.e. PCM sample) timing information
which must be inferred from the residue of modulation at a
frequency around 4KHz. By just selecting a limited set of
codes with say 64 levels, 6 bits per 8KHz symbol may be sent
for a data rate of 48Kbps. More levels, more data, but a maximum
of about 53.3Kbps is a practical limit.
[6.5] What is CAP? [top]
Carrierless amplitude and phase (CAP) modulation is a proprietary
standard implemented by Globespan Semiconductor. While the
name specifies that the modulation is "carrierless" an actual
carrier is imposed by the transmit band shaping filter through
which the outbound symbols are filtered. Hence CAP is algorithmically
identical to QAM. The upstream symbol rate is 136K baud on
a 113.2KHz carrier, while the downstream symbol rate is 340K
baud on a 435.5KHz carrier, 680K baud on a 631KHz carrier,
or 952K baud on a 787.5KHz carrier. This allows the modem
to be symbol rate adaptive to varying line conditions (see
RADSL). The QAM modulation is also rate adaptive by varying
the number of bits per symbol. One advantage CAP claims to
have is a lower peak-to-average signal power ratio relative
to DMT. This means that the drivers and receivers may operate
at lower power than DMT because they are not required to have
the peak signal capacity that is required in the DMT circuitry.
This is mitigated by the infrequency of the really high signal
peaks in DMT which may be just considered to be another form
of noise if they happen to clip. CAP's principle advantage
is its installed base of modems. It is actively being deployed
in many trial markets and is available from several manufacturers.
[6.6] What is DMT? [top]
Discrete multitone (DMT) modulation is a method by which the
usable frequency range is separated into 256 frequency bands
(or channels) of 4.3125KHz each. These are intimately connected
to the FFT (fast Fourier transform) algorithm which DMT uses
as its modulator and demodulator. The FFT is not perfect in
separating the frequencies into individual bands, but it does
well enough, and it generates spectra which are fully separable
on the receiving end. By dividing the frequency spectrum into
multiple channels DMT is thought to perform better in the
presence of interference sources such as AM radio transmitters.
It is also better able to focus its transmit power on those
portions of the spectrum in which it is profitable to send
data. The assignment of channels is left flexible, but typical
settings might be channels 6-31 for upstream (24KHz-136KHz),
32-250 for downstream (136KHz-1.1MHz). The modulation used
on any given frequency channel is QAM. Channels 16 and 64
are reserved for pilot tones which are used to recover timing.
The number of bits per symbol within each channel may be independently
selected allowing the modem to be rate adaptive. The use of
the FFT is considered to be somewhat substandard to other
orthogonal transformations such as the discrete wavelet transform
which do a better job of isolating the individual frequency
spectra. The FFT is chosen for its computational efficiency.
While DMT is off to a slow start in the marketplace, it is
expected to dominate for two reasons: it is thought to perform
better for technical reasons and there is an ANSI standard
behind it (not to mention Intel/Microsoft support).
7.0 Setup and Configuration
[7.1] What hardware does my home computer need? [top]
Although it depends on your provider and the equipment they
use, typically you will need a 10Base-T adapter with which
to connect to the external DSL device. Typically the customer
DSL device is implemented as a bridge, router or both.
[7.2] <reserved> [top]
[7.3] Can I use my 28.8K/56K modem with my xDSL line?
Theoretically yes. However, most DSL providers have been installing
separate DSL circuits to the remote user without using a splitter
to separate out the voiceband bandwidth. If a splitter was
used, you could use a traditional POTS modem over the the
voiceband frequency spectrum of your phone line as you always
did. In most cases however, the line is dedicated for DSL.
Appendix A - Acronym List [top]
ADSL - Asymmetric Digital Subscriber Line ANSI
- American National Standards Institute ATM - Asynchronous
Transfer Mode ATU-C - ADSL Termination Unit - Central
Office ATU-R - ADSL Termination Unit - Remote AWG
- American Wire Gauge BERT - Bit Error Rate Test bps
- Bits Per Second BRI - Basic Rate Interface CAP
- Carrierless Amplitude and Phase CATV - Cable TV CBR
- Constant Bit Rate CCITT - Consultative Committee
for International Telegraph and Telephone CLEC - Competitive
Local Exchange Carrier CO - Central Office CODEC
- Coder/Decoder CPE - Customer Premise (or Provided)
Equipment CSU - Channel Service Unit DCE - Data
Communication (or Circuit-Terminating) Equipment DLC
- Digital Loop Carrier DMT - Discrete Multi-tone DSL
- Digital Subscriber Line DSLAM - Digital Subscriber
Line Access Multiplexer DSP - Digital Signal Processor
DSU - Data Service Unit DTE - Data Terminal
(or Termination) Equipment EMI - Electromagnetic Induction
ETSI - European Telecommunications Standards Institute
FCC - Federal Communications Commission FDM
- Frequency Division Multiplexing FEXT - Far-end crosstalk
FTTC - Fiber To The Curb FTTH - Fiber To The
Home HDSL - High bit-rate Digital Subscriber Line HFC
- Hybrid Fiber-Coax IEC - Inter-Exchange Carrier IEEE
- Institute of Electrical and Electronics Engineers IETF
- Internet Engineering Task Force ILEC - Incumbent
Local Exchange Carrier IP - Internet Protocol ISDL
- ISDN Digital Subscriber Line ISDN - Intergrated Services
Digital Network ISO - International Organization for
Standards ISP - Internet Service Provider ITU
- International Telecommunications Union IXC - Inter-exchange
Carrier Kbps - Kilobits Per Second LADC - Local
Area Data Circuit LADS - Local Area Data Service LAN
- Local Area Network LATA - Local Access and Transport
Area LEC - Local Exchange Carrier Mbps - Megabits
Per Second MDF - Main Distribution Frame MUX
- Multiplexer MVL - Multiple Virtual Line NAP - Network
Access Provider NEBS - Network Equipment Building Standards
NEXT - Near-end Crosstalk NIC - Network Interface
Card NID - Network Interface Device PBX - Public
Branch Exchange PCM - Pulse Code Modulation POP
- Point of Presence POTS - Plain Old Telephone Service
PPP - Point to Point Protocol PRI - Primary
Rate Interface PSTN - Public Switched Telephone Network
PTT - Postal, Telegraph and Telephone PVC -
Permanant Virtual Circuit QAM - Quadrature Amplitude
Modulation QoS - Quality of Service RADSL -
Rate Adaptive Digital Subscriber Line RBOC - Regional
Bell Operating Company SDSL - Symmetric Digital Subscriber
Line SNR - Signal-to-Noise Ratio SOHO - Small
Office/Home Office SVC - Switched Virtual Circuit TCP
- Transport Control Protocol TELCO - Telephone Company
TDM - Time Division Multiplexing UBR - Unspecified
Bit Rate UDSL - Unidirectional Digital Subscriber Line
UTP - Unshielded Twisted Pair VBR - Variable
Bit Rate VDSL - Very high bit-rate Digital Subscriber
Line VoIP - Voice over Internet Protocol VPN
- Virtual Private Network WAN - Wide Area Network xDSL
- (generic) Digital Subscriber Line - [top]