A modem (a portmanteau constructed from modulate and demodulate) is a device that
modulates an analog carrier signal to encode digital information, and also demodulates such a
carrier signal to decode the transmitted information. The goal is to produce a signal that can
be transmitted easily and decoded to reproduce the original digital data. Modems can be used
over any means of transmitting analog signals, from driven diodes to radio. Experiments have
even been performed in the use of modems over the medium of two cans connected by a
The most familiar example of a modem turns the digital '1s and 0s' of a personal computer
into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems
(POTS), and once received on the other side, converts those sounds back into 1s and 0s.
Modems are generally classified by the amount of data they can send in a given time,
normally measured in bits per second, or "bps".
Far more exotic modems are used by Internet users every day, notably cable modems and
ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at
very high data rates over microwave radio links. Some microwave modems transmit more
than a hundred million bits per second. Optical modems transmit data over optic fibers. Most
intercontinental data links now use optic modems transmitting over undersea optical fibers.
Optical modems routinely have data rates in excess of a billion (1x109) bits per second.
1957 AT&T Dataphone
Modems in the United States were first introduced as a part of the SAGE air-defense system
in the 1950s, connecting terminals at various airbases, radar sites, and command-and-control
centers to the SAGE director centers scattered around the U.S. and Canada. SAGE ran on
dedicated communications lines, but the devices at each end were otherwise similar in
concept to today's modems. IBM was the primary contractor for both the computers and the
modems used in the SAGE system.
A few years later, a chance meeting between the CEO of American Airlines and a regional
manager of IBM led to development of a "mini-SAGE" as an automated airline ticketing
system. The terminals were at ticketing offices, tied to a central computer that managed
availability and scheduling. The system, known as SABRE, is the ancestor of today's Sabre
AT&T monopoly in the United States
For many years, AT&T maintained a monopoly in the United States on the use of its phone
lines, allowing only AT&T-supplied devices to be attached to its network. For the growing
group of computer users, AT&T introduced two digital sub-sets in 1958. One is the
wideband device shown in the picture to the left. The other was a low-speed modem, which
ran at 200 baud.
In the summer of 1960, the name Data-Phone was introduced to replace the earlier term
digital subset. The 202 Data-Phone was a half-duplex asynchronous service that was
marketed extensively in late 1960. In 1962, the 201A and 201B Data-Phones were
introduced. They were synchronous modems using two-bit-per-baud phase-shift keying
(PSK). The 201A operated half-duplex at 2000 bit/s over normal phone lines, while the 201B
provided full duplex 2400 bit/s service on four-wire leased lines, the send and receive
channels running on their own set of two wires each.
The famous 103A was also introduced in 1962. It provided full-duplex service at up to 300
baud over normal phone lines. Frequency-shift keying (FSK) was used with the call
originator transmitting at 1070 or 1270 Hz and the answering modem transmitting at 2025 or
2225 Hz. The readily available 103A2 gave an important boost to the use of remote low-
speed terminals such as the KSR33, the ASR33, and the IBM 2741. AT&T reduced modem
costs by introducing the originate-only 113D and the answer-only 113B/C modems.
The Carterfone decision
The Novation CAT acoustically coupled modem
Before 1968, AT&T maintained a monopoly on what devices could be electrically connected
to its phone lines. This led to a market for 103A-compatible modems that were mechanically
connected to the phone, through the handset, known as acoustically coupled modems.
Particularly common models from the 1970s were the Novation CAT (shown in the image)
and the Anderson-Jacobson, spun off from an in-house project at the LLNL.
In 1968, the U.S. Supreme Court broke AT&T's monopoly on the lines in the landmark
Carterfone decision. Now, the lines were open to anyone, as long as they passed a stringent
set of AT&T-designed tests. AT&T made these tests complex and expensive, so acoustically
coupled modems remained common into the early 1980s.
In December 1972, Vadic introduced the VA3400. This device was remarkable because it
provided full duplex operation at 1200 bit/s over the dial network, using methods similar to
those of the 103A in that it used different frequency bands for transmit and receive. In
November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar
in design to Vadic's model, but used the lower frequency set for transmit from originating
modem. It was also possible to use the 212A with a 103A modem at 300 bit/s. According to
Vadic, the change in frequency assignments made the 212 intentionally incompatible with
acoustic coupling, thereby locking out many potential modem manufacturers.
In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to
computer center operators that supported Vadic's 1200-bit/s mode, AT&T's 212A mode, and
The next major advance in modems was the Smartmodem, introduced in 1981 by Hayes
Communications. The Smartmodem was an otherwise standard 103A 300-bit/s modem, but
was attached to a small controller that let the computer send commands to it to operate the
phone line. The command set included instructions for picking up and hanging up the phone,
dialing numbers, and answering calls. The basic Hayes command set remains the basis for
computer control of most modern modems.
Before the Smartmodem, modems almost universally required a two-step process to activate
a connection: first, manually dial the remote number on a standard phone handset, and then
plug the handset into an acoustic coupler. Because the modem could not dial the phone, the
acoustic coupler remained common to allow users needed to dial the phone. Hardware add-
ons, known simply as dialers, were used in special circumstances, and generally operated by
emulating someone dialing a handset.
With the Smartmodem, the computer could dial the phone directly by sending the modem a
command. This eliminated the need for an associated phone to dial and the need for an
acoustic coupler. The Smartmodem instead plugged directly into the phone line. This greatly
simplified setup. Terminal programs that maintained lists of phone numbers and sent the
dialing commands became common.
The Smartmodem and its clones also aided the spread of bulletin-board systems (BBSs).
Modems had previously been typically either the call-only, acoustically coupled models used
on the client side, or the much more expensive, answer-only models used on the server side.
The Smartmodem could operate in either mode depending on the commands sent from the
computer. There was now a low-cost server-side modem on the market, and the BBSs
Modems generally remained at 300 and 1200 bit/s into the mid 1980s, although, over this
period, the acoustic coupler disappeared, seemingly overnight, as Smartmodem-compatible
modems flooded the market.
An external 2400 bit/s modem for a laptop.
A 2400-bit/s system similar in concept to the 1200-bit/s Bell 212 signalling was introduced in
the U.S., and a slightly different, and incompatible, one in Europe. By the late 1980s, most
modems could support all of these standards, and 2400-bit/s operation was becoming
Many other standards were also introduced for special purposes, commonly using a high-
speed channel for receiving, and a lower-speed channel for sending. One typical example was
used in the French Minitel system, in which the user's terminals spent the majority of their
time receiving information. The modem in the Minitel terminal thus operated at 1200 bit/s for
reception, and 75 bit/s for sending commands back to the servers.
Such solutions were useful in many circumstances in which one side would be sending more
data than the other. In addition to a number of "medium-speed" standards, like Minitel, four
U.S. companies became famous for high-speed versions of the same concept.
Telebit introduced its Trailblazer modem in 1984, which used a large number of low-speed
channels to send data one-way at rates up to 19,200 bit/s. A single additional channel in the
reverse direction allowed the two modems to communicate how much data was waiting at
either end of the link, and the modems could switch which side had the high-speed channels
on the fly. The Trailblazer modems also supported a feature that allowed them to "spoof" the
UUCP "g" protocol, commonly used on Unix systems to send e-mail, and thereby speed
UUCP up by a tremendous amount. Trailblazers thus became extremely common on Unix
systems, and maintained their dominance in this market well into the 1990s.
U.S. Robotics (USR) introduced a similar system, known as HST, although this supplied
only 9600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than
offer spoofing, USR instead created a large market among Fidonet users by offering its
modems to BBS sysops at a much lower price, resulting in sales to end users who wanted
faster file transfers.
Hayes was forced to compete, and introduced its own 9600-bit/s standard, Express 96 (also
known as "Ping-Pong"), which was generally similar to Telebit's PEP. Hayes, however,
offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained
Operations at these speeds pushed the limits of the phone lines, and would have been
generally very error-prone. This led to the introduction of error-correction systems built into
the modems, made most famous with Microcom's MNP systems. A string of MNP standards
came out in the 1980s, each slowing the effective data rate by a smaller amount each time,
from about 25% in MNP 1, to 5% in MNP 4. MNP 5 took this a step further, adding data
compression to the system, thereby actually increasing the data rate: generally, the user could
expect an MNP modem to transfer at about 1.3 times the normal data rate of the modem.
MNP was later "opened" and became popular on a series of 2400-bit/s modems, although it
was never widespread.
Another common feature of these high-speed modems was the concept of fallback, allowing
them to talk to less-capable modems. During the call initiation the modem would play a series
of signals into the line and wait for the remote modem to "answer" them. They would start at
high speeds and progressively get slower and slower until they heard an answer. Thus, two
USR modems would be able to connect at 9600 bit/s, but, when a user with a 2400-bit/s
modem called in, the USR would "fall back" to the common 2400-bit/s speed. Without such a
system, the operator would be forced to have multiple phone lines for high- and low-speed
Echo cancellation was the next major advance in modem design. Normally the phone system
sends a small amount of the outgoing signal, called sidetone, back to the earphone, in order to
give the user some feedback that their voice is indeed being sent. However this same signal
can confuse the modem, is the signal it is "hearing" from the remote modem, or its own
signal being sent back to itself? This was the reason for splitting the signal frequencies into
answer and originate; if you received a signal on your own frequency set, you simply ignored
it. Even with improvements to the phone system allowing for higher speeds, this splitting of
the available phone signal bandwidth still imposed a half-speed limit on modems.
Echo cancellation was a way around this problem. By using the sidetone's well-known
timing, a slight delay, it was possible for the modem to tell if the received signal was from
itself or the remote modem. As soon as this happened the modems were able to send at "full
speed" in both directions at the same time, leading to the development of the 9600 bit/s v.32
Starting in the late 1980s a number of companies started introducing v.32 modems, most of
them also using the now-opened MNP standards for error correction and compression. These
earlier systems were not very popular due to their price, but by the early 1990s the prices
The "tipping point" occurred with the introduction of the SupraFax 14400 in 1991. Rockwell
had introduced a new chip-set supporting not only v.32 and MNP, but the newer 14,400 bit/s
v.32bis and the higher-compression v.42bis as well, and even included 9600 bit/s fax
capability. Supra, then known primarily for their hard drive systems for the Atari ST, used
this chip set to build a low-priced 14,400 bit/s modem which cost the same as a 2400 bit/s
modem from a year or two earlier (about 300 USD). The product was a runaway best-seller,
and it was months before the company could keep up with demand.
The SupraFax was so successful that a huge number of companies joined the fray, and by the
next year 14.4 modems from a wide variety of companies were available. The Rockwell chip
set, while not terribly reliable, became extremely common, but Texas Instruments and AT&T
Paragon quickly responded with similar chipsets of their own.
v.32bis was so successful that the older high-speed standards had little to recommend them.
USR fought back with a 16,800 bit/s version of HST, but this small increase in performance
did little to keep HST interesting. AT&T introduced a one-off 19,200 bit/s "standard" they
referred to as v.32ter (also known as v.32 terbo), but this also did little to increase demand,
and typically this mode came into use only when two users with AT&T-based modems just
happened to call each other. Motorola also introduced another, incompatible, 19.2 standard,
but charged very high prices for their modems, which they had previously sold into
commercial settings only.
An ISA modem manufactured to conform to the v.34 protocol.
Any interest in these systems was destroyed during the lengthy introduction of the 28,800
bit/s v.34 standard. While waiting, several companies decided to "jump the gun" and
introduced modems they referred to as "V.FAST". In order to guarantee compatibility with
v.34 modems once the standard was ratified (which happened in 1994), the manufacturers
were forced to use more "flexible" parts, generally a DSP and microcontroller, as opposed to
purpose-designed "modem chips".
A good example of this was USR, which changed their modems to use a DSP from Texas
Instruments, and introduced a top-of-the-line Courier product, the V.everything. As the name
implied, the new model supported practically every standard on the market, including all of
the HST modes, v.32bis, V.FAST and, later, v.34. Rockwell also introduced a V.FAST
chipset in late 1993, which they referred to as V.FC (for "Fast Class").
Rapid commoditization in 1994 forced almost all vendors out of the market; Motorola gave
up and disappeared without a trace, AT&T throwing in the towel soon after. Their attempts to
introduce their own standards were failures in both a technical and business sense.
With the rapid introduction of all-digital phone systems in the 1990s, it became possible to
use much greater bandwidth, on the assumption that users would generally be based on
digital lines – if not immediately, then in the near future. Digital lines are based on a standard
using 8-bits of data for every voice sample, sampling 8000 times a second, for a total data
rate of 64 kbit/s. However, many systems use in-band signaling for command data, inserting
one bit of command data per byte of signal, and thereby reducing the real throughput to 56k.
In 1996, modems began to appear on the market that took advantage of the widespread use of
digital phone systems at ISPs, in order to provide download speeds up to 56kbps. Originally,
there were two available protocols for achieving such speeds, K56flex, designed and
promoted by Rockwell and X2 (protocol), designed and promoted by U.S. Robotics. The
already widespread use of the Rockwell chip set made K56flex more popular. A
standardization effort started around 1996 with the intent to create a single standard for 56k
modems that would replace K56flex and X2. Originally known as V.pcm (PCM referring to
the pulse code modulation used in digital telephony), it became the v.90 protocol when
finalized in 1998.
There are certain special requirements and restrictions associated with v.90 modems. In order
for a users to obtain up to 56k upload speeds from their ISP, the telephone line had to be
completely digital between the ISP and the telephone company central office (CO) of the
user. From there the signal could be converted from digital to analog but only at this point. If
there was a second conversion anywhere along the line 56k speeds were impossible. Also, the
line quality of the user's telephone line could affect the speed of the 56k connection with line
noise causing slow downs, sometimes to the point of only being marginally faster the
33.6Kbps connection. An important restriction with v.90 is that while v.90 modems can
obtain up to 56Kbps download speeds, they are limited to 33.6Kbps upload speeds.
Prior to the adoption of the v.90 protocol, users were slow to adopt K56flex and X2 based
56K modems, many simply waiting for v.90 to arrive. Some modem manufacturers promised
and later offered firmware or driver upgrades for their modems so that users could add v.90
functionality when it was released. V.90 modems can be backwards compatible with K56flex
or X2. Thus users of non-upgradeable K56flex or X2 modems can often find ISP dial-up
numbers that will support at least one of the older 56K protocols along with v.90.
Following the adoption of v.90, there was an attempt to adopt a protocol that would define a
standard to allow all-digital communications (i.e where both the ISP and the user had digital
connections to the telephone network). It was to be known as v.91 but the process appears to
be "dead", as the rapid introduction of short-haul high-speed solutions like ADSL and cable
modems offer much higher speeds from the user's local machine onto the Internet. With the
exception of rural areas, the need for point-to-point calls has generally disappeared as a
result, as the bandwidth and responsiveness of the Internet has improved so much. It appears
that v.90 will be the last analog modem standard to see widespread use.
V.92 is the standard that followed v.90. While it provides no speed increase when
downloading from the Internet (56kbps appears to be the maximum speed for analog based
modems), it does allow upload speeds to match the download speed provided both the ISP
and the caller both have fully v.92 compatible modems. It also adds two features. The first is
the ability for users who have call waiting to put their dial-up Internet connection on hold for
extended periods of time while they answer a call. The second feature is the ability to "quick
connect" to one's ISP. This is achieved by remembering key information about the telephone
line one is using to connect with and then using this saved information to help speed up
future calls made from the line to the ISP.
ISPs have been slow to adopt V.92 due to the high cost of upgrading their equipment and the
lack of demand by their customers to do so. With the rise in broadband take-up that has led to
declining numbers of dial-up users, some ISPs have decided not to bother ever upgrading to
Long haul modems
In the 1960s, Bell began to digitize the telephone system, and developed early high-speed
radio modems for this purpose. Once digital long-haul networks were in place, they were
leased for every other purpose.
Optic fiber manufacturing was mastered in the 1980s, and optic modems were first invented
for these early systems. The first systems simply used light-emitting diodes and PIN diodes.
Faster modulation was quickly adopted for long-haul networks. In the 1990s, multispectral
optical modems were adopted as well.
28.8kbit/s serial-port modem from Motorola
A standard modem of today is what would have been called a "smart modem" in the 1980s.
They contain two functional parts: an analog section for generating the signals and operating
the phone, and a digital section for setup and control. This functionality is actually
incorporated into a single chip, but the division remains in theory.
In operation the modem can be in one of two "modes", data mode in which data is sent to
and from the computer over the phone lines, and command mode in which the modem
listens to the data from the computer for commands, and carries them out. A typical session
consists of powering up the modem (often inside the computer itself) which automatically
assumes command mode, then sending it the command for dialing a number. After the
connection is established to the remote modem, the modem automatically goes into data
mode, and the user can send and receive data. When the user is finished, the escape sequence,
"+++" followed by a pause of about a second, is sent to the modem to return it to command
mode, and the command to hang up the phone is sent. One problem with this method of
operation is that it is not really possible for the modem to know if a string is a command or
data. When the modem misinterprets a string, it generally causes odd things to happen.
The commands themselves are typically from the Hayes command set, although that term is
somewhat misleading. The original Hayes commands were useful for 300 bit/s operation
only, and then extended for their 1200 bit/s modems. Hayes was much slower upgrading to
faster speeds however, leading to a proliferation of command sets in the early 1990s as each
of the high-speed vendors introduced their own command styles. Things became considerably
more standardized in the second half of the 1990s, when most modems were built from one
of a very small number of "chip sets", invariably supporting a rapidly converging command
set. We call this the Hayes command set even today, although it has three or four times the
numbers of commands as the actual standard.
The 300 bit/s modems used frequency-shift keying to send data. In this system the stream of
1's and 0's in computer data it translated into sounds which can be easily sent on the phone
lines. In the Bell 103 system the originating modem sends 0's by playing a 1070 Hz tone, and
1's at 1270 Hz, with the answering modem putting its 0's on 2025 Hz and 1's on 2225 Hz.
These frequencies were chosen carefully, they are in the range that suffer minimum distortion
on the phone system, and also are not harmonics of each other. For the 103F leased line
version, internal strapping selected originate or answer operation. For dial models, the
selection was determined by which modem originated the call. Modulation was so slow and
simple that some people were able to learn how to whistle short bits of data into the phone
with some accuracy.
In the 1200 bit/s and faster systems, phase-shift keying was used. In this system the two tones
for any one side of the connection are sent at the similar frequencies as in the 300 bit/s
systems, but slightly out of phase. By comparing the phase of the two signals, 1's and 0's
could be pulled back out, for instance if the signals were 90 degrees out of phase, this
represented two digits, "1, 0", at 180 degrees it was "1, 1". In this way each cycle of the
signal represents two digits instead of one, 1200 bit/s modems were, in effect, 600 bit/s
modems with "tricky" signalling.
It was at this point that the difference between baud and bit per second became real. Baud
refers to the signaling rate of a system, in a 300 bit/s modem the signals sent one bit per
signal, so the data rate and signalling rate was the same. In the 1200 bit/s systems this was no
longer true since the modems were actually 600 baud. This led to a series of flame wars on
the BBSes of the 80s.
Increases in speed have since used increasingly complicated communications theory. The
Milgo 4500 introduced the 8 phase shift key concept. This could transmit three bits per
signaling instance (baud.) The next major advance was introduced by the Codex Corporation
in the late 1960's. Here the bits were encoded into a combination of amplitude and phase.
Best visualized as a two dimensional "eye pattern", the bits are mapped onto points on a
graph with the x (real) and y (quadrature) coordinates transmitted over a single carrier. This
technique became very effective and was incorporated into an international standard named
V.29, by the CCITT (now ITU) arm of the United Nations. The standard was able to transmit
4 bits per signalling interval of 2400 Hz. giving an effective bit rate of 9600 bits per second.
For many years, most considered this rate to be the limit of data communications over
In 1980 Godfried Ungerboek from IBM applied powerful channel coding techniques to
search for new ways to increase the speed of modems. His results were astonishing but only
conveyed to a few colleagues. Finally in 1982, he agreed to publish what is now a landmark
paper in the theory of information coding. By applying powerful parity check coding to the
bits in each symbol, and mapping the encoded bits into a two dimensional "eye pattern",
Ungerboek showed that it was possible to increase the speed by a factor of two with the same
error rate. The new technique was called mapping by set partitions (now known as trellis
modulation). This new view was an extension of the "penny packing" problem and the related
and more general problem of how to pack points into an N-dimension sphere such that they
are far away from their neighbors (so that noise can not confuse the receiver.)
The industry was galvanized into new research and development. More powerful coding
techniques were developed, commercial firms rolled out new product lines, and the standards
organizations rapidly adopted to new technology. Today the ITU standard V.34 represents the
culmination of the joint efforts. It employs the most powerful coding techniques including
channel encoding and shape encoding. From the mere 16 points per symbol, V.34 uses over
1000 points and very sophisticated algorithms to achieve 33.6 kbit/s.
In the late 1990's Rockwell and U.S. Robotics introduced new technology based upon the
digital transmission used in modern telephony networks. The standard digital transmission in
modern networks is 64 kbit/s but some networks use a part of the bandwidth for remote office
signalling (eg to hang up the phone), limiting the effective rate to 56 kbit/s DS0. This new
technology was adopted into ITU standards V.90 and is common in modern computers. The
56 kbit/s rate is only possible from the central office to the user site (downlink). The uplink
(from the user to the central office) still uses V.34 technology. Later, in V.92, upload speed
increased to a maximum of 48 kbit/s.
It is guessed that this rate is near the theoretical Shannon limit. Higher speeds are possible but
may be due more to improvements in the underlying phone system than anything in the
technology of the modems themselves.
Software is as important to the operation of the modem today as the hardware. Even with the
improvements in the performance of the phone system, modems still lose a considerable
amount of data due to noise on the line. The MNP standards were originally created to
automatically fix these errors, and later expanded to compress the data at the same time.
Today's v.42 and v.42bis fill these roles in the vast majority of modems, and although later
MNP standards were released, they are not common.
With such systems it is possible for the modem to transmit data faster than its basic rate
would imply. For instance, a 2400 bit/s modem with v.42bis can transmit up to 9600 bit/s, at
least in theory. One problem is that the compression tends to get better and worse over time,
at some points the modem will be sending the data at 4000 bit/s, and others at 9000 bit/s. In
such situations it becomes necessary to use hardware flow control, extra pins on the
modem–computer connection to allow the computers to signal data flow. The computer is
then set to supply the modem at some higher rate, in this example at 9600 bit/s, and the
modem will tell the computer to stop sending if it cannot keep up. A small amount of
memory in the modem, a buffer, is used to hold the data while it is being sent.
Almost all modern modems also do double-duty as a fax machine as well. Digital faxes,
introduced in the 1980s, are simply a particular image format sent over a high-speed
(9600/1200 bit/s) modem. Software running on the host computer can convert any image into
fax-format, which can then be sent using the modem. Such software was at one time an add-
on, but since has become largely universal.
A PCI Winmodem/Softmodem (on the left) next to a traditional ISA modem (on the right).
Notice the less complex circuitry of the modem on the left.
A Winmodem or Softmodem is a stripped-down modem for Windows that replaces tasks
traditionally handled in hardware with software. In this case the modem is a simple digital
signal processor designed to create sounds, or voltage variations, on the telephone line.
Modern computers often include a very simple card slot, the Communications and
Networking Riser slot (CNR), to lower the cost of connecting it up. The CNR slot includes
pins for sound, power and basic signaling, instead of the more expensive PCI slot normally
used. Winmodems are often cheaper than traditional modems, since they have fewer
hardware components. One downside of a Winmodem is that the software generating the
modem tones is not that simple, and the performance of the computer as a whole often suffers
when it is being used. For online gaming this can be a real concern. Another problem with
WinModems is lack of flexibility, due to their strong tie to the underlying operating system.
A given Winmodem might not be supported by other operating systems (such as Linux),
because their manufacturers may neither support the other operating system nor provide
enough technical data to create an equivalent driver. A Winmodem might not even work (or
work well) with a later version of Microsoft Windows, if its driver turns out to be
incompatible with that later version of the operating system.
Apple's GeoPort modems from the second half of the 1990s were similar, and are generally
regarded as having been a bad move. Although a clever idea in theory, enabling the creation
of more-powerful telephony applications, in practice the only programs created were simple
answering-machine and fax software, hardly more advanced than their physical-world
counterparts, and certainly more error-prone and cumbersome. The software was finicky and
ate up significant processor time, and no longer functions in current operating system
Today's modern audio modems (ITU-T V.92 standard) closely approach the Shannon
capacity of the PSTN telephone channel. They are plug-and-play fax/data/voice modems
(broadcast voice messages and records touch tone responses).
Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do
most other wireless services today. Modern telecommunications and data networks also make
extensive use of radio modems where long distance data links are required. Such systems are
an important part of the PSTN, and are also in common use for high-speed computer network
links to outlying areas where fibre is not economical.
Even where a cable is installed, it is often possible to get better performance or make other
parts of the system simpler by using radio frequencies and modulation techniques through a
cable. Coaxial cable has a very large bandwidth, however signal attenuation becomes a major
problem at high data rates if a digital signal is used. By using a modem, a much larger
amount of digital data can be transmitted through a single piece of wire. Digital cable
television and cable Internet services use radio frequency modems to provide the increasing
bandwidth needs of modern households. Using a modem also allows for frequency-division
multiple access to be used, making full-duplex digital communication with many users
possible using a single wire.
Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are
often referred to as transparent or smart. They transmit information that is modulated onto a
carrier frequency to allow many simultaneous wireless communication links to work
simultaneously on different frequencies.
Transparent modems operate in a manner similar to their phone line modem cousins.
Typically, they were half duplex, meaning that they could not send and receive data at the
same time. Typically transparent modems are polled in a round robin manner to collect small
amounts of data from scattered locations that do not have easy access to wired infrastructure.
Transparent modems are most commonly used by utility companies for data collection.
Smart modems come with a media access controller inside which prevents random data from
colliding and resends data that is not correctly received. Smart modems typically require
more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE
802.11 standard defines a short range modulation scheme that is used on a large scale
throughout the world.
WiFi and WiMax
Wireless data modems are used in the WiFi and WiMax standards, operating at microwave
WiFi could be used in laptops for Internet connections (wireless access point and wireless
application protocol (WAP).
Modems for mobile phone lines (GPRS and UMTS) comes generally in a PC card, where a
phone card is included. Nowadays are appearing USB modems too.
ADSL modems, a more recent development, are not limited to the telephone's "voiceband"
audio frequencies. Some ADSL modems use coded orthogonal frequency division
Cable modems use a range of frequencies originally intended to carry RF television channels.
Multiple cable modems attached to a single cable can use the same frequency band, using a
low-level media access protocol to allow them to work together within the same channel.
Typically, 'up' and 'down' signals are kept separate using frequency division multiplexing.
New types of broadband modems are beginning to appear, such as doubleway satellite and
Broadband modems should still be classed as modems, since they do utilise analog/digital
conversion. They are more advanced devices than traditional telephone modems as they are
capable of modulating/demodulating hundreds of channels simultaneously.
Many broadband "modems" include the functions of a router and other features such as
DHCP, NAT and firewall features.
When broadband technology was introduced, networking and routers were not very familiar
to most people. However, many people knew what a modem was as most internet access was
through dialup. Due to this familiarity, companies started selling broadband adapters using
the familiar term "modem".
Voice modems are regular modems that are capable of playing audio over the telephone line.
They are used for telephony applications.
A CEA study in 2006 found that dial-up Internet access is on a notable decline in the U.S. In
2000, dial-up Internet connections accounted for 74% of all U.S. residential Internet
connections. This figure dropped to 60% by 2003, and currently stands at 36%. Modems
were once the most popular means of Internet access in the U.S., but with the advent of new
ways of accessing the Internet, the traditional 56K modem is losing popularity.
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Wechseln zu: Navigation, Suche
Vorderseite eines V.90-Modems
Ein Modem (zusammengesetztes Wort aus Modulator/Demodulator) dient dazu, digitale
Daten in für eine vorhandene analoge Leitung geeignete Signale umzuwandeln und auf der
anderen Seite wieder in digitale Daten zurückzuwandeln. Die dafür verwendete Modulation
ist auf die analoge Leitung abgestimmt.
Mit einem Modem werden digitale Daten durch Modulation eines analogen Signals über
analoge Kommunikationsnetze (Telefonnetz, Kabel-TV), Standleitungen und per Funk
übertragen. Am anderen Endpunkt der Kommunikation werden die digitalen Daten durch
Demodulation aus dem analogen Signal wieder zurückgewonnen.
1 Geschichte des Modems
o 3.1 Telefonmodems
o 3.2 Standleitungsmodems
3.2.3 Kabelnetz-Modems (über Kabel-TV-Netze)
o 3.3 Funkmodems
o 3.4 Stromleitungsmodems
5 Hersteller von Modem-Hardware
6 Siehe auch
Geschichte des Modems [Bearbeiten]
Modulationsverfahren wurden seit langem in der Rundfunktechnik und später in der
Trägerfrequenztechnik der ehemaligen Deutschen Bundespost eingesetzt (wireless
modulation). Inzwischen werden sie auch sehr stark in der leitergebundenen Kommunikation
verwendet (wireline modulation)
Bis Mitte der 80er Jahre war es in Deutschland wie in vielen anderen Ländern nicht erlaubt,
andere als posteigene Modems an die Telefonleitung anzuschließen. Das Modem zählte als
Netzabschluss, der wie die Leitung auch zum Telefonnetz und damit zum Hoheitsbereich der
staatlichen Deutschen Bundespost gehörte. Als trotzdem zunehmend private Modems benutzt
wurden, weil diese erheblich billiger und außerdem noch schneller waren als die Post-
Modems, ließ sich das Modem-Monopol nicht mehr aufrechterhalten und wurde aufgehoben.
Telefon: ITU-T- (bis 1992 CCITT-) Standards,
Kabel: 2x Simplex (ab 2 Adernpaare), Halbduplex,
(Voll-)Duplex (benötigen nur 1 Adernpaar)
Telefonmodems sind an die Besonderheiten des Telefonnetzes angepasst. Für die
Übertragung steht allein das Frequenzband von 300 Hz bis 3400 Hz zur Verfügung.
Anschlüsse an einer digitalen Vermittlungsstelle erweitern dieses Band auf 0 bis 4000 Hz.
Die ersten Telefonmodems für den Computer-Massenmarkt enthielten noch keine Bauteile
der Telefontechnik und keine Telefonbuchse, sondern koppelten Schall in das Telefonnetz.
Deshalb musste also ein Akustikkoppler mechanisch an einem Telefon-Handapparat befestigt
werden, um eine Modem-Verbindung herzustellen. Damit konnten Nebengeräusche schnell
zu Übertragungsfehlern führen, weshalb die Schalen der Akustikkoppler oft in Kissen
eingehüllt wurden. Dabei wurde mit 110 baud, später 300 baud mit dem
Modulationsverfahren FSK übertragen. Da die zu übertragenden Daten typischerweise gemäß
dem Standard der seriellen Datenübertragung auf Leitungen V.24 bzw. RS232 kodiert waren,
ergab sich in der Praxis eine geringere effektive Datenrate. Diese frühen Modem-Typen
mussten von der Deutschen Bundespost, die sie einer Typmusterprüfung unterzog, für die
Anschaltung eine Zulassung erhalten.
Die Ungleichung Datenrate ist kleiner als Baudrate wurde erst durch die Realisierung von
intelligenteren Modems mit eingebauter Datenkompression aufgehoben. In dieser Zeit wurde
auch durch die Telekommunikationsfirma Hayes der heutige De-Facto-Standard für Modems,
der sogenannte AT-Befehlssatz in ihre Modems implementiert, der später von zahlreichen
Herstellern übernommen und erweitert wurde. Leistungsfähigere Modulationsverfahren wie
PSK und QAM, sowie eine intelligente Messung und Aushandlung der für die Leitung und
die Gegenstelle maximal möglichen Baudrate nach dem Verbindungsaufbau steigerten die
erreichbare Übertragungsgeschwindigkeit weiter.
Da die Prozessoren von Modems immer leistungsfähiger geworden sind, gibt es heute weitere
Leistungsmerkmale, die den Grundrahmen des Modem-Konzepts sprengen. Zu nennen sind
insbesondere die Fax-Funktion (Faxmodem) und die Anrufbeantworter-Funktion (Voice-
Modem). Es gibt sogar Modelle, die das computergestützte Telefonieren erlauben. Die
Unterstützung schlägt sich durch zusätzliche Befehle im Rahmen des AT-Befehlssatzes
nieder. Es gibt mittlerweile sogar Modems mit integrierten Fax und E-Mail Protokollen
(POP3/SMTP) sowie Short Message Service(SMS)-Funktionalität zum autonomen
Versand/Empfang von Nachrichten.
Im analogen Telefonnetz, in dem die Übertragungsbandbreite auf 3,1 kHz begrenzt ist, ist
nach dem Shannon-Theorem die maximale Datenübertragungsrate bei üblicher
Leitungsqualität auf 30 bis 40 kbit/s begrenzt. Eine Download-Datenrate von 56 kbit/s (V.90,
V.92) ist im Telefonnetz nur bei einem Analoganschluss möglich, der an eine digitale
Vermittlungsstelle gekoppelt ist. Dabei synchronisiert sich das Modem mit dem Wandler-
Takt der Vermittlung. Die sendende Gegenstelle, z. B. ein Einwahlknoten, muss hierzu
jedoch voll digital sein. Die Upload-Geschwindigkeit bleibt jedoch weiterhin analog
Modemverbindungen per Telefonnetz werden auch oft als Dial-Up-Verbindung bezeichnet,
da vor der Herstellung der Datenverbindung ein Wählvorgang notwendig ist. Beispiele für
Modem-Wählverbindungen sind z. B. BTX, Datex-P oder die analoge Einwahl ins Internet
über einen Internet-Provider. Die Einwahl per ISDN unterscheidet sich davon insofern, als
dort alles digital abläuft, also nicht mehr moduliert wird und somit auch kein Modem mehr
Während in der Frühphase vor allem Fernschreiber-Aufgaben an Modems delegiert wurden,
kam es später zur Entwicklung der Mailboxszene, die mit Protokollen, wie Kermit oder Z-
Modem arbeitete. Daneben fand sich im kommerziellen Bereich X.25 als Daten-
Vermittlungsschicht. In heutiger Zeit dominiert vor allem TCP/IP als Vermittlungs- und
Sicherungsschicht für den Datenaustausch per Telefonmodem.
Internes Fax-Modem (FerrariFax)
Ein Faxmodem ist ein Modem, das neben seiner gewöhnlichen Funktion zur
Datenübertragung auch ein Protokoll zur Übertragung von Faxen beherrscht. Die meisten
Faxmodems können Faxe mit 14400 bps (Bits pro Sekunde) übertragen. Die Übertragung
erfolgt dabei über gewöhnliche Telefonleitungen.
Mit Hilfe eines Faxmodems kann man einen Computer (meistens ein Personal Computer oder
auch ein Macintosh) also als Faxgerät verwenden – oft nur zum Senden, aber auch das
Empfangen ist möglich.
Softmodems sind spezielle Modems, bei denen einige Teile der Hardware-Funktionalität in
den Gerätetreiber ausgelagert worden sind. Meistens werden aus Kostengründen
Hardwarebestandteile weggelassen, und die dadurch entstehende Lücke muss durch Software
geschlossen werden. Winmodems sind Softmodems, die im besonderen für Microsoft
Windows hergestellt worden sind. Meistens kann das betreffende Gerät auf alternativen
Betriebssystemen, wie zum Beispiel Linux, nicht verwendet werden, da keine Treiber hierfür
Im Gegensatz zu den Telefonmodems bieten Standleitungsmodems eine Punkt-zu-Punkt-
Verbindung. Sie sind also fest mit immer derselben Gegenstelle verbunden; ein Wählvorgang
vor der Verbindungsaufnahme entfällt somit.
Standleitungen werden meistens von Banken und Großunternehmen genutzt.
Die hierzu vergleichbaren, kostengünstigeren Lösungen für den Heimbereich sind meistens
nur quasi-Standleitungen, da der Anbieter meistens eine Zwangstrennung der hergestellten
Verbindung im Tageszyklus vorsieht.
Während die Bandbreite im analogen Telefonnetz aus technisch wirtschaftlichen
Überlegungen heraus begrenzt ist, erlauben die normalen (teilweise geschirmten) Zweidraht-
Telefonleitungen durchaus höhere Bandbreiten.
Mit der geeigneten Gegenstelle sind so wesentlich höhere Übertragungsraten möglich. Dieses
wird bei DSL in den Varianten ADSL und SDSL umgesetzt. Das Endgerät beim Nutzer ist
weiterhin ein Modem, wenn auch mit erheblich größerer Bandbreite. Um die analoge und
digitale Telefonie auf derselben Leitung gleichzeitig übertragen zu können, wird die Leitung
durch einen sogenannten Splitter in zwei verschiedenen Frequenzbereichen genutzt.
Gebräuchliche Datenraten bei ADSL reichen bis 6 MBit/s im Download. Der Upload ist beim
ADSL auf einen niedrigeren Wert begrenzt. Die Datenrate kann jedoch bei großem Abstand
zur Vermittlungsstelle aus technischen Gründen Begrenzungen unterliegen. An der
Vermittlungsstelle wird meistens an einen rein digitalen Netzwerk-Backbone angekoppelt.
Für Details siehe: DSL-Modem
Kabelnetz-Modems (über Kabel-TV-Netze) [Bearbeiten]
Im weitesten Sinne sind auch die in Deutschland noch selten anzutreffenden Kabel-TV-
Modems als eine Art der Standleitungsmodems zu werten, wobei teilweise zudem reguläre
Telefonmodems verwendet werden, um einen Rückkanal zu ermöglichen, wenn das
vorliegende Kabel-TV-Netz keine bidirektionale Übertragung erlaubt.
Modems zur Datenübertragung per Funk unterscheiden sich zwar nicht prinzipiell von ihren
Leitungs-Vettern, sind jedoch als Einzelobjekt wesentlich seltener anzutreffen. Meistens sind
Funkmodems in anderen Geräten integriert, und der jeweilige Kanal wird mehrfach genutzt
für Sprache und Daten, z. B. bei Tonrufsystemen.
Insbesondere im Funk-Bereich finden sich zahlreiche Anwendungen, mit denen Fernwirk-
oder Fernsteuerungsaufgaben per Modulation gelöst werden, unter anderem bei
Funkfernsteuerungen im Modellbau. Es handelt sich hier in der Mehrzahl der Fälle nicht um
die Übertragung von Datenströmen, sondern vielmehr um die Übertragung von in Echtzeit
Datenübertragung per Funk findet sich z. B. im Richtfunk, aber auch im Packet Radio Netz
der Funkamateure oder auch im CB-Funk. Auch das GSM-Netz benutzt ebenso wie UMTS
Modulation, wobei hier oftmals von digitaler Modulation gesprochen wird, um von der
analogen Modulation im Vorgängersystem, dem C-Netz zu unterschieden. Der Unterschied
bewegt sich hierbei in der Beschaffenheit des Eingangssignals in den Modulator, bevor dieses
in ein Funk-Signal umgesetzt wird, während der Funk-Kanal der ganz normalen Wellen-
Auch die Modulation von Datensignalen auf Stromleitungen ist möglich. Der am weitesten
verbreitete Anwendungsfall ist die so genannte Rundsteuertechnik der
Energieversorgungsunternehmen, mit denen z. B. die Umschaltung der Stromzähler zwischen
Tag- und Nachtstrom bewerkstelligt wird.
In jüngerer Vergangenheit wurden auch Vermarktungsversuche für
Hochgeschwindigkeitsmodems (meistens bis etwa 1 MBit) unter dem Sammelbegriff PLC
unternommen, die aber über die Erprobungsphase nie hinauskamen und im Endeffekt nicht
an das Preis-/Leistungsverhältnis (höherer Aufwand bei niedrigerer Leistung) sowie die
Übertragungssicherheit der DSL-Technik heranreichen konnten. Die Technik nutzt dabei
typischerweise zahlreiche einzelne Trägerfrequenzen im Bereich zwischen 500 kHz und
10 MHz zur Modulation und Demodulation der Nutzdaten.
Verschiedene Bauformen des gleichen Modems
Das typische PC-Modem ist ein externes, serielles Gerät in flacher Bauform. Es wird
meistens per RS232 oder zunehmend mit USB mit einen Rechner verbunden. Es wird meist
durch ein Steckernetzteil versorgt und hat in manchen Fällen einen Netzschalter. Zur
Statusanzeige befindet sich an der Vorderseite oft eine Zeile mit Leuchtdioden, die den
Zustand der Schnittstellenleitungen anzeigen.
Im professionellen Bereich gibt es außer Tischgeräten manchmal auch eine Bauform, die den
Einbau in 19-Zoll-Gehäusen erlaubt. Im industriellen Bereich hat sich für Modems ein
Gehäuse für die DIN-Hutschienenmontage im Schaltschrank etabliert.
Eine alternative Bauform für Modems ist die Steckkartenform für einen standardisierten PC-
Steckplatz (i.a. PCI) oder einen proprietären Sockel. Hier ist meistens noch eine zusätzliche
Kapselung oder Schirmung vorhanden, um die Störstrahlung des PC-Inneren von der
Außenwelt und die Störungen der Telefonleitung vom PC-Inneren zu trennen. Die
Statusanzeige eines solchen Modems wird meistens durch Computersoftware am Bildschirm
des Rechners realisiert.
Überspannung und deren Folgen
Modems können auch in die Hauptplatine eines Rechners intergriert werden. Dieses ist
jedoch nur begrenzt empfehlenswert, da es je nach Region durchaus möglich und nicht
unwahrscheinlich ist, dass ein Modem durch Überspannungen auf der Telefonleitung
beschädigt wird und somit der gesamte Rechner schadhaft wäre und ersetzt werden müsste.
Modulare Systeme erlauben dagegen eine wesentlich differenziertere und kostengünstigere
Hersteller von Modem-Hardware [Bearbeiten]
Ein Stapel verschiedener Wählleitungsmodems
Folgende Hersteller und Marken haben bzw. haben früher Modems oder Komponenten dazu
power on the lower frequencies might cause Your continued donations keep Wikipedia running!
Asymmetric Digital Subscriber Line
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ANSI T1.413-1998 Issue 2
ITU G.992.3/4 Annex J
ITU G.992.3/4 Annex L
Asymmetric Digital Subscriber Line (ADSL) is a ITU G.992.5 Annex L
form of DSL, a data communications technology ITU G.992.5 Annex M
that enables faster data transmission over copper
telephone lines than a conventional modem can
provide. It does this by utilizing frequencies that are normally not used by a voice telephone
call, in particular, frequencies higher than normal human hearing. This signal will not travel
very far over normal telephone cables, so ADSL can only be used over short distances,
typically less than 2 km. Once the signal reaches the telephone company's local office, the
ADSL signal is stripped off and immediately routed onto a conventional internet network,
while any voice-frequency signal is switched into the conventional phone network. This
allows a single telephone connection to be used for both ADSL and voice calls at the same
The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data
flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market
ADSL as a service for people to connect to the Internet in a relatively passive mode: able to
use the higher speed direction for the "download" from the Internet but not needing to run
servers that would require bandwidth in the other direction.
There are both technical and marketing reasons why ADSL is in many places the most
common type offered to home users. On the technical side, there is likely to be more crosstalk
from other circuits at the DSLAM end (where the wires from many local loops are close
together) than at the customer premises. Thus the upload signal is weakest at the noisiest part
of the local loop, while the download signal is strongest at the noisiest part of the local loop.
It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does
the modem on the customer end. Since the typical home user in fact does prefer a higher
download speed, the telephone companies chose to make a virtue out of necessity, hence
For conventional ADSL, downstream rates start at 128 kbit/s (though a minimum offering of
512 kbit/s is more common) and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the
DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and
typically reach 128 kbit/s or 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL
Lite is sometimes used for the slower versions.
Note that distances are only approximations aimed at consumers of ADSL services. Signal
attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely
independently of distance (e.g., non-copper cabling, cable diameter). Real world performance
is also dependent on the line impedance, which can change dynamically either dependent on
weather conditions (very common for old overhead lines) or on the number and quality of
joints or junctions in a particular cable length.
A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans
of less than 2.5 km (8000 ft). More flexible framing and error correction configurations are
responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts
these rates to up to 24 Mbit/s for spans of less than 1.5 km (5000 feet) by doubling the
downstream spectrum upper limit to 2.2MHz. ADSL2/2+ also offer seamless bonding
options, allowing lines with higher attenuation or lower Signal to Noise Ratio(SNR) to be
bonded together to achieve theoretically the sum total of the number of lines (i.e., up to
50 Mbit/s for two lines, etc.), as well as options in power management and seamless rate
adaptation — changing the data rate used without requiring to resynchronize.
Because of the relatively low data-rate (compared to optical backbone networks), ATM is an
appropriate technology for multiplexing time-critical data such as digital voice with less time-
critical data such as web traffic; ADSL is commonly deployed with ATM to ensure that this
remains a possibility. In a triple play scenario, different ATM virtual circuits (VCs) may be
allocated for different services.
More recently, network operators are increasingly moving away from ATM, and towards
Ethernet-based solutions, where 802.1Q and/or VPLS offer multiplexing solutions. The main
reason for this switch is cost savings and the possibility of removing the older and more
expensive ATM network.
ADSL service providers may offer either dynamic or static IP addressing. Static addressing is
preferable for people who may wish to connect to their office via a virtual private network,
for some Internet gaming, and for those wishing to use ADSL to host a Web server.
1 How ADSL works
o 1.1 On the wire
o 1.2 Modulation
2 ADSL standards
o 2.1 Footnotes
3 See also
4 External links
How ADSL works
On the wire
ADSL uses two separate frequency bands, referred to as the upstream and downstream bands.
The upstream band is used for communication from the end user to the telephone central
office. The downstream band is used for communicating from the central office to the end
user. With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for
upstream communication, while 138 kHz – 1104 kHz is used for downstream
Frequency plan for ADSL. The red area is the frequency range used by normal voice
telephony, the green and blue areas are used for ADSL.
Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial
training, the ADSL modem tests which of the available channels have an acceptable signal-
to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may
introduce errors on some frequencies. By keeping the channels small, an error on one
frequency thus need not render the line unusable: the channel will not be used, merely
resulting in reduced throughput on an otherwise functional ADSL connection.
Vendors may support usage of higher frequencies as a proprietary extension to the standard.
However, this requires matching vendor-supplied equipment on both ends of the line, and
will likely result in crosstalk issues that affect other lines in the same bundle.
There is a direct relationship between the number of channels available and the throughput
capacity of the ADSL connection. The exact data capacity per channel depends on the
modulation method used.
A common error is to attribute the A in ADSL to the word asynchronous. ADSL technologies
use a synchronous framed protocol for data transmission on the wire.
ADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT. CAP was
the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL
installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1
and G.992.2 (also called G.dmt and G.lite respectively). Therefore, all modern installations of
ADSL are based on the DMT modulation scheme.
Standard name Common name Downstream rate Upstream rate
ANSI T1.413-1998 Issue 2 ADSL 8 Mbit/s 1.0 Mbit/s
ITU G.992.1 ADSL (G.DMT) 8 Mbit/s 1.0 Mbit/s
ITU G.992.2 ADSL Lite (G.Lite) 1.5 Mbit/s 0.5 Mbit/s
ITU G.992.3/4 ADSL2 12 Mbit/s 1.0 Mbit/s
ITU G.992.3/4 Annex J ADSL2 12 Mbit/s 3.5 Mbit/s
ITU G.992.3/4 Annex L RE-ADSL2 5 Mbit/s 0.8 Mbit/s
ITU G.992.5 ADSL2+ 24 Mbit/s 1.0 Mbit/s
ITU G.992.5 Annex L RE-ADSL2+ 24 Mbit/s 1.0 Mbit/s
ITU G.992.5 Annex M ADSL2+ 24 Mbit/s 3.5 Mbit/s
Annexes J and M shift the upstream/downstream frequency split up to 276kHz (from 138kHz
used in the commonly deployed annex A) in order to boost upstream rates. Additionally, the
"all-digital-loop" variants of ADSL2 and ADSL2+ (annexes I and J) support an extra 256
kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for
While the ADSL access utilizes the 1.1 MHz band, ADSL2+ utilizes the 2.2 MHz band.
The downstream and upstream rates displayed are theoretical maximums. Note also that
because DSLAM and ADSL modems may have been implemented based on differing or
incomplete standards some manufacturers may advertise different speeds. For example,
Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in
ADSL2 and ADSL2+.
1. ^ a b ADSL2 Annex L is also known as RE-ADSL2, where
'RE' stands for 'Reach Extended.' With this ADSL standard,
the power of the lower frequencies used for transmitting data
is boosted up to increase the reach of this signal up to 7
kilometers (23,000 ft). The upper frequency limit for RE-
ADSL2 is reduced to 552kHz to keep the total power
roughly the same as annex A. Since RE-ADSL2 is intended
for use on long loops there isn't much (any) usable
bandwidth above 552kHz anyway. Although this standard
has been ratified by the ITU, not all local loop network
maintainers allow this protocol to be used on their network,
simply because the extra problems for existing services due
Your continued donations keep Wikipedia running!
Integrated Services Digital Network
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Please wikify (format) this article or section as suggested in the Guide to layout and the
Manual of Style.
Remove this template after wikifying. This article has been tagged since August 2006.
For the album by The Future Sound of London, see
Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network
system, designed to allow digital transmission of voice and data over ordinary telephone
copper wires, resulting in better quality and higher speeds than available with analog systems.
More broadly, ISDN is a set of protocols for establishing and breaking circuit switched
connections, and for advanced call features for the user. The English term is a "backronym",
thought better for English-language advertisements than the original, "Integriertes Sprach-
und Datennetz" (German for "Integrated Speech and Data Net").
In a videoconference, ISDN provides simultaneous voice, video, and text transmission
between individual desktop videoconferencing systems and group (room) videoconferencing
In the phrase "Integrated Services Digital Network",
Integrated Services refers to ISDN's ability to deliver at
minimum two simultaneous connections, in any
combination of data, voice, video, and fax, over a
single line. Multiple devices can be attached to the line,
and used as needed. That means an ISDN line can take
care of most people's complete communications needs,
without forcing the purchase of multiple analog phone
lines at a much higher transmission rate.
Digital refers to its purely digital transmission, as
opposed to the analog transmission of plain old
telephone service. If you're using an analog telephone
modem for Internet access at this moment, your
Internet service provider's modem has converted this
site's digital content to analog signals before sending it
to you, and your modem converts those signals back to
digital when receiving (the same thing happens with
every keystroke and mouse click you transmit). When
you connect with ISDN, there is no analog conversion.
ISDN transmits data digitally, resulting in a very clear
transmission quality. There is none of the static and
noise of analog transmissions that can slow
Network refers to the fact that ISDN is not simply a
point-to-point solution like a leased line. ISDN
networks extend from the local telephone exchange to
the remote user and include all of the
telecommunications and switching equipment in
Consumer and industry perspectives
There are two points of view into the ISDN world. The most common viewpoint is that of the
end user who wants to get a digital connection into the telephone/data network from home,
whose performance would be better than an ordinary analog modem connection. The typical
end-user's connection to the Internet is related to this point of view, and talk about the merits
of various ISDN modems, carriers' offerings and tarriffing (features, pricing) are from this
perspective. Much of the following discussion is from this point of view, but it should be
noted that as a data connection service, ISDN has been mostly superseded by DSL.
There is however a second viewpoint: that of the telephone industry, where ISDN is not a
dead issue. A telephone network can be thought of as a collection of wires strung between
switching systems. The common electrical specification for the signals on these wires is T1
or E1. On a normal T1, the signalling is done with A&B bits to indicate on or off hook
conditions and MF and DTMF tones to encode the destination number. ISDN is much better
than this as messages can be sent much more quickly than by trying to encode numbers as
long (100 ms per digit) tone sequences. This translated to much faster call setup times which
is greatly desired by carriers who have to pay for line time and also by callers who become
impatient while their call hops from switch to switch.
It is also used as a smart network technology intended to add new services to the public
switched telephone network (the PSTN) by giving users direct access to end-to-end circuit-
switched digital services.
ISDN has never gained popularity as a telephone network in the United States and today
remains a niche product. It is still commonly used in recording studios, when a voice-over
actor is in one studio, but the director and producer are in a studio at another location. ISDN
is used because of its "guaranteed" real-time, not-over-the-Internet service, and its superior
audio fidelity as compared to POTS service.
In Japan, it became popular to some extent from around 1999 to 2001, but now that ADSL
has been introduced, the number of subscribers is in decline. NTT, a dominant Japanese
telephone company, provides an ISDN service with the names INS64 and INS1500, which
are much less recognized than ISDN.
In the UK, British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until
April 2006 they also offered Home Highway and Business Highway, which are BRI ISDN
based services which offer integrated analogue connectivity as well as ISDN. Later versions
of the Highway products also included built in USB sockets for direct computer access.
Home Highway has been bought by many home users, usually for Internet connection as,
although not as fast as ADSL, it was available before ADSL and in places where ADSL does
In France, France Télécom offers ISDN services under their product name Numeris (2 B+D)
of which a profesional Duo and home Itoo version is available. ISDN is generally known as
RNIS in France and has widespread availability. The introduction of ADSL is reducing ISDN
use for data transfer and internet access, although it is still common in more rural and
In Germany, ISDN is very popular with an installed base of 25 mio. channels (29% of all
subscriber lines in Germany as of 2003 and 20% of all ISDN channels worldwide). Due to
the success of ISDN, the number of installed analog lines is decreasing. Deutsche Telekom
(DTAG) offers both BRI and PRI. Competing phone companies often offer ISDN only and
no analog lines.
In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). B channels
are used for data (which may include voice), and D channels are intended for signalling and
control (but can also be used for data).
There are three ISDN implementations. Basic rate interface (BRI) — also Basic rate access
(BRA) — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel
with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D.
Primary rate interface (PRI) — also Primary rate access (PRA) — contains a greater
number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B
channels for PRI varies according to the nation: in North America and Japan it is 23B+1D,
with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe and Australia it is 30B+1D, with an
aggregate bit rate of 2.048 Mbit/s (E1). Broadband Integrated Services Digital Network
(BISDN) is another ISDN implementation and it is able to manage different types of services
at the same time. It is primarily used within network backbones and employs ATM.
Another alternative ISDN configuration can be used in which the B channels of an ISDN
basic rate interface are bonded to provide a total duplex bandwidth of 128 kbit/s. This
precludes use of the line for voice calls while the internet connection is in use.
Using bipolar with eight-zero substitution encoding technique, call data is transmitted over
the data (B) channels, with the signalling (D) channels used for call setup and management.
Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel
between the end parties, lasting until the call is terminated. There can be as many calls as
there are data channels, to the same or different end-points. Bearer channels may also be
multiplexed into what may be considered single, higher-bandwidth channels via a process
called B channel bonding.
The D channel can also be used for sending and receiving X.25 data packets, and connection
to X.25 packet network, this is specified in X.31. In practice, X.31 was only commercially
implemented in France and Japan.
A set of reference points are defined in the ISDN standard to refer to certain points between
the telco and the end user ISDN equipment.
R - defines the point between a non-ISDN device and a
terminal adapter (TA) which provides translation to
and from such a device
S - defines the point between the ISDN equipment (or
TA) and a Network Termination Type 2 (NT-2) device
T - defines the point between the NT-2 and NT-1
U - defines the point between the NT-1 and the telco
Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T
reference points are generally collapsed into the S/T reference point.
Inside North America, the NT-1 device is considered customer premises equipment and
must be maintained by the customer, thus, the U interface is provided to the customer. In
other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided
to the customer.
Types of communications handled
Among the kinds of data that can be moved over the 64 kbit/s channels are pulse-code
modulated voice calls, providing access to the traditional voice PSTN. This information can
be passed between the network and the user end-point at call set-up time. In North America,
ISDN is now used mostly as an alternative to analog connections, most commonly for
Internet access. Some of the services envisioned as being delivered over ISDN are now
delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been
successfully marketed as a phone with features, as opposed to a POTS phone (Plain Old
Telephone Service) with few or no features. Meanwhile, features that were first available
with ISDN (such as Three-Way Call, Call Forwarding, Caller ID, etc.) are now commonly
available for ordinary analog phones as well, eliminating this advantage of ISDN. Another
advantage of ISDN was the possibility of multiple simultaneous calls (one call per B
channel), e.g. for big families, but with the increased popularity and reduced prices of mobile
telephony this has become less interesting as well, making ISDN unappealing to the private
customer. However, ISDN is typically more reliable than POTS, and has a significantly faster
call setup time compared with POTS, and IP connections over ISDN typically have some 30-
35ms round trip time, as opposed to 120-180ms (both measured with otherwise unused lines)
over 56k or V.34 modems, making ISDN more pleasant for telecommuters.
Where an analog connection requires a modem, an ISDN connection requires a terminal
adapter (TA). The function of an ISDN terminal adapter is often delivered in the form of a PC
card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of
combined ISDN- and ADSL-routers.
ISDN is commonly used in Radio Broadcasting. Since ISDN provides a high quality
connection this assists in delivering good quality audio for transmission in radio. Most radio
studios are equiped with ISDN lines as their main form of communication with other studios
or standard phone lines.
A sample ISDN call
The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD
and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-
channel). The call is originating from the switch where the trace was taken and goes out to
some other switch, possibly an end-office LEC, who terminates the call.
The first line format is <time> <D-channel> <Transmitted/Received> <LAPD/ISDN message
ID>. If the message is an ISDN level message, then a decoding of the message is attempted
showing the various Information Elements that make up the message. All ISDN messages are
tagged with an ID number relative to the switch that started the call (local/remote). Following
this optional decoding is a dump of the bytes of the message in <offset> <hex> ... <hex>
<ascii> ... <ascii> format.
The RR messages at the beginning prior to the call are the keep alive messages. Then you
will see a SETUP message that starts the call. Each message is acknowledged by the other
side with a RR.
10:49:47.33 21/1/24 R RR
0000 02 01 01 a5 ....
10:49:47.34 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:17.57 21/1/24 R RR
0000 02 01 01 a5 ....
10:50:17.58 21/1/24 T RR
0000 02 01 01 b9 ....
10:50:24.37 21/1/24 T SETUP
Call Reference : 000062-local
Bearer Capability : CCITT, Speech, Circuit mode, 64 kbit/s