Modem - Wikipedia, the free encyclopedia

Modem

From Wikipedia, the free encyclopedia

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 string.

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.

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Contents

● 1 History ❍ 1.1 AT&T monopoly in the United States ❍ 1.2 The Carterfone decision ❍ 1.3 The Smartmodem ❍ 1.4 Increasing speeds ❍ 1.5 v.32 ❍ 1.6 v.34 ❍ 1.7 v.90 ❍ 1.8 V.92

● 2 Long haul modems ● 3 Narrowband

❍ 3.1 Winmodem ● 4 Radio modems ● 5 Broadband ● 6 Voice modem ● 7 Popularity ● 8 See also ● 9 References ● 10 External links

History

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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 located at various airbases, radar sites and command-and-control

centers to the SAGE director centers scattered around the US and Canada. SAGE ran on dedicated

communications lines, but the devices at either 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 a "mini-SAGE" being developed as an automated airline ticketing system.

In this case the terminals were located 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

system.

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 their network. For the growing group of

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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 1070/1270 Hz and the answering modem transmitting at 2025/2225 Hz. The

readily available 103A2 gave an important boost to the use of remote low-speed terminals such as

the KSR33, 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

Prior to 1968, AT&T maintained a monopoly on what devices could be electrically connected to

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their 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 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. Of course 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 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 deliberately 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 103A

operation.

The Smartmodem

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.

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Prior to 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. Since the modem could not dial the phone, the acoustic coupler

remained common because you needed to dial the phone separately anyway. 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 with, and in turn, 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 would send the

dialing commands became common.

Although it was not recognized at the time, the Smartmodem also allowed the creation of the first

bulletin board systems (BBSes). In the past modems had typically been call-only (the acoustic

coupled variety) used on the client-side, or much more expensive (although for no technical

reason) answer-only systems used on the server-side. The Smartmodem, on the other hand, could

operate in either mode by sending the appropriate commands from the computer. Suddenly there

was a low-cost server-side modem on the market, and BBSes soon followed.

Increasing speeds

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 into the market.

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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

US, 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 common.

A huge number of other standards were also introduced for special-purpose situations, 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, where 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.

These sorts of solutions were useful in a number of situations where one side would be sending

more data than the other. In addition to a number of "medium-speed" standards like Minitel, four

US companies became famous for high-speed versions of the same concept.

Telebit introduced their 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 clever feature that allowed them to "spoof" the UUCP

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"g" protocol, commonly used on Unix systems to send e-mail, and thereby speed UUCP up by a

tremendous amount. Trailblazers became extremely common on Unix systems as a result, and

maintained their stranglehold on 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. Although they did not

offer spoofing, USR instead created a large market among Fidonet users by offering their modems

to BBS sysops at a greatly discounted price. This translated to some extent into sales to end-users

who wanted faster file transfers.

Hayes was forced to compete, and introduced their 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 their high-speed modems remained rare.

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 – in general use 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 another 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 use.

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v.32

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 standard.

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 started falling.

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, not

terribly reliable, was extremely common, but Texas Instruments and AT&T Paragon quickly

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responded with similar chip sets 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.

v.34

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".

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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.

v.90

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

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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

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

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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 v.92.

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.

Narrowband

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

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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

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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, 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 Codex Co. 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 telephone networks.

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

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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 and 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

(E.G. 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

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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.

Winmodem

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

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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 versions.

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).

Radio modems

Direct broadcast satellite, GPS, 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

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Modem - Wikipedia, the free encyclopedia

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 are half duplex, meaning that they cannot 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.

Wireless data modems are used in the WiFi and WiMax standards, operating at microwave

frequencies.

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Modem - Wikipedia, the free encyclopedia

WiFi could be used in laptops for Internet connections (wireless access point and wireless

application protocol/WAP)

Broadband

DSL Modem

ADSL modems, a more recent development, are not limited to the telephone's "voiceband" audio

frequencies. Some ADSL modems use coded orthogonal frequency division modulation.

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

powerline modems.

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.

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Modem - Wikipedia, the free encyclopedia

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 modem

Voice modems are regular modems that are capable of playing audio over the telephone line. They

are used for telephony applications.

Popularity

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.

Internet access methods

Dial-up • ISDN • DSL • Cable • Wi-Fi • WiMAX • Satellite • Fiber Optic • Power-line Internet

See also

● 56 kbit/s line ● dial up ● Digital-to-analog converter ● Flat rate ● K56flex

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Modem - Wikipedia, the free encyclopedia

● Modulation (for a fuller list of modulation techniques) ● TCP/IP ● v.34 ● v.90 ● v.92 ● X2 (Chipset) ● Broadband: satellite modem, ADSL, cablemodem, PLC. ● Chapter Hayes-compatible Modems and AT Commands of the Serial Data

Communications Programming Wikibook

References

● Upgrading and Repairing PCs - 16th Edition - Scott Mueller

External links

Wikimedia Commons has media related to: Modems

● Installing, testing, troubleshooting & tweaking modems ● 56k ● v.92 ● Columbia University - Protocols Explained ● Costmo Modem Site ● Data Modem Solutions Site ● How Stuff Works - Modems ● International Telecommunications Union ITU: Data communication over the telephone

network ● ModemHelp.org ● Modem-Help.co.uk ● Modems.com ● ModemSite.com ● Basic handshakes & modulations - V.22, V.22bis & V.32 handshakes ● Federal Communications Commission TELECOMMUNICATION: PART

64_MISCELLANEOUS RULES RELATING TO COMMON CARRIERS

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Categories: Bulletin board systems | Computing portmanteaus | Modems

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