Modem
Overview
 
A modem is a device that modulates an analog carrier signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 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 light emitting diodes to radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

.
The most familiar example is a voice band modem that turns the digital data of a personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

 into modulated electrical signals in the voice frequency
Voice frequency
A voice frequency or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech.In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz...

 range of a telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 channel.
Encyclopedia
A modem is a device that modulates an analog carrier signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 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 light emitting diodes to radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

.
The most familiar example is a voice band modem that turns the digital data of a personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

 into modulated electrical signals in the voice frequency
Voice frequency
A voice frequency or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech.In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz...

 range of a telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps). Modems can alternatively be classified by their symbol rate
Symbol rate
In digital communications, symbol rate is the number of symbol changes made to the transmission medium per second using a digitally modulated signal or a line code. The Symbol rate is measured in baud or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second...

, measured in baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...

. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency-shift keying, that is to say, tones of different frequencies, with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which was able to transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.

History

News wire services
News agency
A news agency is an organization of journalists established to supply news reports to news organizations: newspapers, magazines, and radio and television broadcasters. Such an agency may also be referred to as a wire service, newswire or news service.-History:The oldest news agency is Agence...

 in 1920s used multiplex
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

 equipment that met the definition, but the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems.

Modems grew out of the need to connect teletype machines over ordinary phone lines instead of more expensive leased lines which had previously been used for current loop
Current loop
A current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...

-based teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...

s and automated telegraphs. In 1943, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bits/second.
Mass-produced modems in the United States began as part of the SAGE
Semi Automatic Ground Environment
The Semi-Automatic Ground Environment was an automated control system for tracking and intercepting enemy bomber aircraft used by NORAD from the late 1950s into the 1980s...

 air-defense system in 1958, connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. SAGE modems were described by AT&T's Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 as conforming to their newly published Bell 101 dataset
Bell 101
The Bell 101 dataset or Bell 101 modem was the first commercial modem for computers, released by AT&T in 1958 for use by SAGE and in 1959 made commercial shortly after AT&T's Bell Labs announced their 110 baud modulation frequencies...

 standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.

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 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels running on their own set of two wires each.

The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...

 was used with the call originator transmitting at 1,070 or 1,270 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the KSR33, the ASR33
ASR33
The Teletype Model ASR-33 was a very popular model of teleprinter. Introduced about 1963 by Teletype Corporation and designed for light-duty office use, it was less rugged and less expensive than earlier Teletype machines or its heavy-duty cousin, the Model 35-ASR.The Model 33's printing mechanism...

, and the IBM 2741
IBM 2741
The IBM 2741 was a printing computer terminal introduced in 1965.It combined a ruggedized Selectric typewriter mechanism with IBM SLT electronics and an RS-232-C serial interface. It operated at about 14.1 characters per second with a data rate of 134.5 bits/second...

. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.

The Carterfone decision

For many years, the Bell System (AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...

) maintained a monopoly on the use of its phone lines, allowing only Bell-supplied devices to be attached to its network. 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
Acoustic coupler
In telecommunications, the term acoustic coupler has the following meanings:# An interface device for coupling electrical signals by acoustical means—usually into and out of a telephone instrument....

. Particularly common models from the 1970s were the Novation CAT
Novation CAT
Novation was an early modem manufacturer whose CAT series were popular in the early home computer market in the late 1970s and early 1980s, notably on the Apple II...

 and the Anderson-Jacobson, spun off from an in-house project at Stanford Research Institute (now SRI International). Hush-a-Phone v. FCC
Hush-a-Phone v. FCC
Hush-A-Phone v. United States, 238 F.2d 266 was a seminal ruling in United States telecommunications law decided by the D.C. Circuit Court of Appeals. Hush-A-Phone Corporation marketed a small, cup-like device which mounted on the speaking party's phone, reducing the risk of conversations being...

 was a seminal ruling in United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 telecommunications law decided by the DC Circuit Court of Appeals on November 8, 1956. The District Court found that it was within the FCC's authority to regulate the terms of use of AT&T's equipment. Subsequently, the FCC examiner found that as long as the device was not physically attached it would not threaten to degenerate the system. Later, in the Carterfone
Carterfone
The Carterfone is a device invented by Thomas Carter. It manually connects a two-way mobile radio system to the public switched telephone network , making it a direct predecessor to today's autopatch....

 decision of 1968, the FCC passed a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. AT&T's tests were complex, making electronically coupled modems 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 1,200 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 transmission. 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 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.

The Smartmodem and the rise of BBSes

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 and enable 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
Hayes command set
The Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...

 remains the basis for computer control of most modern modems.

Prior to the Hayes Smartmodem, dial-up modems almost universally required a two-step process to activate a connection: first, the user had to manually dial the remote number on a standard phone handset, and then secondly, plug the handset into an acoustic coupler. 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, thus eliminating the need for an associated phone instrument for dialing and the need for an acoustic coupler. The Smartmodem instead plugged directly into the phone line. This greatly simplified setup and operation. 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 system
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...

s (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 flourished.

Almost all modern modems can interoperate with fax
Fax
Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...

 machines. Digital faxes, introduced in the 1980s, are simply a particular image format sent over a high-speed (commonly 14.4 kbit/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.

Softmodem


A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled in hardware
Hardware
Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

 with software. In this case the modem is a simple interface designed to create voltage variations on the telephone line and to sample the line voltage levels (digital to analog
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

 and analog to digital
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

 converters). Softmodems are cheaper than traditional modems, since they have fewer hardware components. One downside is that the software generating and interpreting the modem tones is not simple (as most of the protocols are complex), 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 is lack of portability such that non-Windows operating systems (such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

) often do not have an equivalent driver to operate the modem.

Narrow-band/phone-line dialup modems

A standard modem of today contains 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 often incorporated into a single chip nowadays, 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
Escape sequence
An escape sequence is a series of characters used to change the state of computers and their attached peripheral devices. These are also known as control sequences, reflecting their use in device control. Some control sequences are special characters that always have the same meaning...

, "+++" followed by a pause of about a second, may be sent to the modem to return it to command mode, then a command (e.g. "ATH") to hang up the phone is sent. Note that on many modem controllers it is possible to issue commands to disable the escape sequence so that it is not possible for data being exchanged to trigger the mode change inadvertently.

The commands themselves are typically from the Hayes command set
Hayes command set
The Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...

, although that term is somewhat misleading. The original Hayes commands were useful for 300 bit/s operation only, and then extended for their 1,200 bit/s modems. Faster speeds required new commands, leading to a proliferation of command sets in the early 1990s. 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 chipsets. We call this the Hayes command set even today, although it has three or four times the numbers of commands as the actual standard.

Increasing speeds (V.21, V.22, V.22bis)

The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem putting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system, and also are not harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...

s of each other.

In the 1,200 bit/s and faster systems, phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....

 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, 1s and 0s could be pulled back out, Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid 1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signalling was introduced in the U.S., and a slightly different one in Europe. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common.

For more information on baud rates versus bit rates, see the companion article list of device bandwidths.

Increasing speeds (one-way proprietary standards)

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
Minitel
The Minitel is a Videotex online service accessible through the telephone lines, and is considered one of the world's most successful pre-World Wide Web online services. It was launched in France in 1982 by the PTT...

 system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for reception, and 75 bit/s for sending commands back to the server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

s.

Three U.S. companies became famous for high-speed versions of the same concept. Telebit
Telebit
Telebit was a US-based modem manufacturer, most notable for their TrailBlazer series of high-speed modems. One of the first modems to routinely exceed 9600 bit/s speeds, the TrailBlazer used a proprietary modulation scheme that proved highly resilient to interference, earning the product an almost...

 introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s channels to send data one-way at rates up to 18,432 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 change direction on the fly
On the fly
-Colloquial usage:In colloquial use, on the fly means something created when needed. The phrase is used to mean:# something that was not planned ahead# changes that are made during the execution of same activity: ex tempore, impromptu.-Automotive usage:...

. The Trailblazer modems also supported a feature that allowed them to spoof the UUCP
UUCP
UUCP is an abbreviation for Unix-to-Unix Copy. The term generally refers to a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers. Specifically, a command named uucp is one of the programs in the suite; it...

 g protocol, commonly used on Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 systems to send e-mail
E-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

, 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
U.S. Robotics
USRobotics Corporation is a company that makes computer modems and related products. It sold high-speed modems in the 1980s, and had a reputation for high quality and compatibility. With the reduced usage of voiceband modems in North America in the early 21st century, USR is now one of the few...

 (USR) introduced a similar system, known as HST, although this supplied only 9,600 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
FidoNet
FidoNet is a worldwide computer network that is used for communication between bulletin board systems. It was most popular in the early to mid 1990s, prior to the introduction of easy and affordable access to the Internet...

 users by offering its modems to BBS sysop
SysOp
A sysop is an administrator of a multi-user computer system, such as a bulletin board system or an online service virtual community. It may also be used to refer to administrators of other Internet-based network services....

s 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 9,600-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 rare.

4,800 and 9,600 bit/s (V.27ter, V.32)

Echo cancellation was the next major advance in modem design. Local telephone lines use the same wires to send and receive, which results in a small amount of the outgoing signal bouncing back. This signal can confuse the modem, which was unable to distinguish between the echo and the signal from the remote modem. This was why earlier modems split the signal frequencies into 'answer' and 'originate'; the modem could then ignore its own transmitting frequencies. Even with improvements to the phone system allowing higher speeds, this splitting of available phone signal bandwidth still imposed a half-speed limit on modems.

Echo cancellation got around this problem. Measuring the echo delays and magnitudes allowed the modem to tell if the received signal was from itself or the remote modem, and create an equal and opposite signal to cancel its own. Modems were then able to send over the whole frequency spectrum in both directions at the same time, leading to the development of 4,800 and 9,600 bit/s modems.

Increases in speed have used increasingly complicated communications theory. 1,200 and 2,400 bit/s modems used the phase shift key (PSK) concept. This could transmit two or three bits per symbol. The next major advance encoded four bits into a combination of amplitude and phase, known as Quadrature Amplitude Modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...

 (QAM).

The new V.27ter and V.32 standards were able to transmit 4 bits per symbol, at a rate of 1,200 or 2,400 baud, giving an effective bit rate of 4,800 or 9,600 bit/s. The carrier frequency was 1,650 Hz. For many years, most engineers considered this rate to be the limit of data communications over telephone networks.

Error correction and compression

Operations at these speeds pushed the limits of the phone lines, resulting in high error rates. This led to the introduction of error-correction systems built into the modems, made most famous with Microcom
Microcom
Microcom, Inc. was a major modem vendor during the 1980s, although they were never as popular as the "big three", Hayes, U.S. Robotics and Telebit. Nevertheless they hold an important place in modem history due to their introduction of the MNP error-correction and compression protocols, which were...

's MNP
Microcom Networking Protocol
The MNP family of error-correcting protocols were commonly used on early high-speed modems. Originally developed for use on Microcom's own family of modems, the protocol was later openly licensed and used by most of the modem industry, notably the "big three", Telebit, USRobotics and Hayes...

 systems. A string of MNP standards came out in the 1980s, each increasing the effective data rate by minimizing overhead, from about 75% theoretical maximum in MNP 1, to 95% in MNP 4. The new method called MNP 5 took this a step further, adding data compression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 to the system, thereby increasing the data rate above the modem's rating. Generally the user could expect an MNP5 modem to transfer at about 130% the normal data rate of the modem. Details of MNP were later released and became popular on a series of 2,400-bit/s modems, and ultimately led to the development of V.42 and V.42bis ITU standards. V.42 and V.42bis were non-compatible with MNP but were similar in concept: Error correction and compression.

Another common feature of these high-speed modems was the concept of fallback
Fall back and forward
Fall back is a feature of a modem protocol in data communication whereby two communicating modems which experience data corruption can renegotiate with each other to use a lower-speed connection...

, or speed hunting, 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 respond to 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 9,600 bit/s, but, when a user with a 2,400-bit/s modem called in, the USR would fallback to the common 2,400-bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at 2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able to communicate with each other at only 2,400 bit/s.

Breaking the 9.6k barrier

In 1980, Gottfried Ungerboeck
Gottfried Ungerboeck
Gottfried Ungerboeck is an Austrian communications engineer.Ungerboeck received an electrical engineering degree from Vienna University of Technology in 1964, and a Ph.D. from the Swiss Federal Institute of Technology, Zurich, in 1970...

 from IBM Zurich Research Laboratory
IBM Research
IBM Research, a division of IBM, is a research and advanced development organization and currently consists of eight locations throughout the world and hundreds of projects....

 applied 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 parity check coding to the bits in each symbol, and mapping the encoded bits into a two-dimensional diamond pattern, Ungerboeck 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
Trellis modulation
In telecommunication, trellis modulation is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines...

).

Error correcting codes, which encode code words (sets of bits) in such a way that they are far from each other, so that in case of error they are still closest to the original word (and not confused with another) can be thought of as analogous to sphere packing
Sphere packing
In geometry, a sphere packing is an arrangement of non-overlapping spheres within a containing space. The spheres considered are usually all of identical size, and the space is usually three-dimensional Euclidean space...

 or packing pennies on a surface: the further two bit sequences are from one another, the easier it is to correct minor errors.

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, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter (also known as V.32 terbo or tertiary), but neither non-standard modem sold well.

V.34/28.8k and 33.6k

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 release hardware and introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally a DSP
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...

 and microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

, as opposed to purpose-designed ASIC
ASIC
ASIC may refer to:* Application-specific integrated circuit, an integrated circuit developed for a particular use, as opposed to a customised general-purpose device.* ASIC programming language, a dialect of BASIC...

 modem chips.

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 4 bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity of a narrowband line is , with the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth from 300–4000 Hz, so using (SNR = 30dB): capacity is approximately 35 kbit/s.

Without the discovery and eventual application of trellis modulation, maximum telephone rates using voice-bandwidth channels would have been limited to 3,429 baud * 4 bit/symbol approximately 14 kbit/s using traditional QAM. (DSL makes use of the bandwidth of traditional copper-wire twisted pairs between subscriber and the central office, which far exceeds that of analog voice circuitry.)

V.61/V.70 Analog/Digital Simultaneous Voice and Data

The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This technology allowed users of v.61 modems to engage in point-to-point voice conversations with each other while their respective modems communicated.

In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became available to consumers, and the standard was ratified as v.70 by the International Telecommunication Union (ITU) in 1996.

Two DSVD modems can establish a completely digital link between each other over standard phone lines. Sometimes referred to as "the poor man's ISDN," and employing a similar technology, v.70 compatible modems allow for a maximum speed of 33.6 kbit/s between peers. By using a majority of the bandwidth for data and reserving part for voice transmission, DSVD modems allow users to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer.

One practical use for this technology was realized by early two player video gamers, who could hold voice communication with each other while in game over the PSTN.

Advocates of DSVD envisioned whiteboard sharing and other practical applications for the standard, however, with advent of cheaper 56kbit/s analog modems intended for Internet connectivity, peer-to-peer data transmission over the PSTN became quickly irrelevant. Also, the standard was never expanded to allow for the making or receiving of arbitrary phone calls while the modem was in use, due to the cost of infrastructure upgrades to telephone companies, and the advent of ISDN and DSL technologies which effectively accomplished the same goal.

Today, Multi-Tech is the only known company to continue to support a v.70 compatible modem. While their device also offers v.92 at 56kbit/s, it remains significantly more expensive than comparable modems sans v.70 support.

Using digital lines and PCM (V.90/92)

In the late 1990s Rockwell/Lucent and U.S. Robotics introduced new competing technologies 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 signaling (e.g., 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). In the United States, government regulation limits the maximum power output, resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6 kbit/s.

Later in V.92
V.92
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. For example a 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s, due to echo on the telephone line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher. V.92 also adds two other 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 quickly connect to one's ISP. This is achieved by remembering the analog and digital characteristics of the telephone line, and using this saved information to reconnect at a fast pace.

Using compression to exceed 56k

Today's V.42, V.42bis and V.44 standards allow the modem to transmit data faster than its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 320 kbit/s using pure text. However, the compression ratio tends to vary due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). At some points the modem will be sending compressed files at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.

In such situations a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream. This is accomplished through hardware flow control using extra lines on the modem–computer connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s, and the modem will tell the computer when to start or stop sending data.

Compression by the ISP

As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero and Juno started using pre-compression to increase the throughput and maintain their customer base. As example, the Netscape
Netscape
Netscape Communications is a US computer services company, best known for Netscape Navigator, its web browser. When it was an independent company, its headquarters were in Mountain View, California...

 ISP used a compression program that compressed images, text, and other objects at the modem server, just prior to sending them across the phone line. Certain content using lossy compression (e.g., images) may be recompressed (transcoded) using different parameters to the compression algorithm, making the transmitted content smaller but of lower quality. The server-side compression operates much more efficiently than the on-the-fly compression of V.44-enabled modems due to the fact that V.44 is a generalized compression algorithm whereas other compression techniques are application-specific (JPEG, MPEG, Vorbis, etc.). Typically Website text is compacted to 4%, thus increasing effective throughput to approximately 1,300 kbit/s. The accelerator also pre-compresses Flash executables and images to approximately 30% and 12%, respectively.

The drawback of this approach is a loss in quality, where the GIF and JPEG images are lossy compressed, which causes the content to become pixelated and smeared. However the speed is dramatically improved such that Web pages load in less than 5 seconds, and the user can manually choose to view the uncompressed images at any time. The ISPs employing this approach advertise it as "surf 5× faster" or simply "accelerated dial-up".

These accelerated downloads are now integrated into the Opera
Opera (web browser)
Opera is a web browser and Internet suite developed by Opera Software with over 200 million users worldwide. The browser handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, chatting on IRC, downloading files via BitTorrent,...

 web browser.

List of dialup speeds

Note that the values given are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines). For a complete list see the companion article list of device bandwidths. A baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...

 is one symbol per second; each symbol may encode one or more data bits.
Connection Modulation Bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....

 [kbit/s]
Year Released
110 baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...

 Bell 101
Bell 101
The Bell 101 dataset or Bell 101 modem was the first commercial modem for computers, released by AT&T in 1958 for use by SAGE and in 1959 made commercial shortly after AT&T's Bell Labs announced their 110 baud modulation frequencies...

 modem
FSK 0.1 1958
300 baud (Bell 103 or V.21) FSK 0.3 1962
1200 modem (1200 baud) (Bell 202) FSK 1.2
1200 Modem (600 baud) (Bell 212A
Bell 212A
The Bell 212A modulation scheme defined a standard method of transmitting full-duplex asynchronous serial data at 1.2 kbit/s over analogue transmission lines. The equivalent, but incompatible ITU-T standard is V.22....

 or V.22)
QPSK 1.2 1980?
2400 Modem (600 baud) (V.22bis) QAM 2.4 1984
2400 Modem (1200 baud) (V.26bis) PSK 2.4
4800 Modem (1600 baud) (V.27ter) PSK 4.8
9600 Modem (2400 baud) (V.32) QAM 9.6 1984
14.4k Modem (2400 baud) (V.32bis) QAM 14.4 1991
28.8k Modem (3200 baud) (V.34) QAM 28.8 1994
33.6k Modem (3429 baud) (V.34) QAM 33.6
56k Modem (8000/3429 baud) (V.90) 56.0/33.6 1998
56k Modem (8000/8000 baud) (V.92
V.92
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

)
56.0/48.0 2000
Bonding modem (two 56k modems)) (V.92
V.92
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

)
112.0/96.0
Hardware compression (variable) (V.90/V.42bis) 56.0-220.0
Hardware compression (variable) (V.92
V.92
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

/V.44)
56.0-320.0
Server-side web compression (variable) (Netscape ISP) 100.0-1,000.0

Radio modems

Direct broadcast satellite
Direct broadcast satellite
Direct broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception.A designation broader than DBS would be direct-to-home signals, or DTH. This has initially distinguished the transmissions directly intended for home viewers from cable television...

, WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

, and mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modem
Radio modem
Radio modems transfer data wirelessly across a range of up to tens of kilometres.Using radio modems is a modern way to create Private Radio Networks . Private radio networks are used in industrial critical applications, when real-time data communication is needed. Also Radio Data Modem enables user...

s 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
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

 has a very large bandwidth, however signal attenuation becomes a major problem at high data rates if a baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...

 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
Frequency-division multiple access
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like...

 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 modem
Wireless modem
A wireless modem is a type of modulator-demodulator which connects to a wireless network instead of using telephone or cable television lines...

s are used in the WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

 and WiMax
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...

 standards, operating at microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 frequencies.

WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

 is principally used in laptop
Laptop
A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...

s for Internet connections (wireless access point
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...

) and wireless application protocol
Wireless Application Protocol
Wireless Application Protocol is a technical standard for accessing information over a mobile wireless network.A WAP browser is a web browser for mobile devices such as mobile phones that uses the protocol.Before the introduction of WAP, mobile service providers had limited opportunities to offer...

 (WAP).

Mobile modems and routers

Modems which use a mobile telephone system (GPRS, UMTS, HSPA
High Speed Packet Access
High Speed Packet Access is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access and High Speed Uplink Packet Access , that extends and improves the performance of existing WCDMA protocols...

, EVDO, WiMax
WiMAX
WiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...

, etc.), are known as wireless modem
Wireless modem
A wireless modem is a type of modulator-demodulator which connects to a wireless network instead of using telephone or cable television lines...

s (sometimes also called cellular modems). Wireless modems can be embedded inside a laptop or appliance or external to it. External wireless modems are connect card
Connect card
A mobile broadband modem, also known as a connect card or data card, allows a laptop, a personal computer or a router to connect to receive Internet access via mobile broadband.-History:...

s, usb modems for mobile broadband
Mobile Broadband
Mobile broadband is the marketing term for wireless Internet access through a portable modem, mobile phone or other mobile device.-Description:...

 and cellular router
Cellular router
Cellular routers are routers that provide shared Internet access by incorporating a cellular data modem and providing traditional interfaces like Ethernet and WiFi....

s. A connect card
Connect card
A mobile broadband modem, also known as a connect card or data card, allows a laptop, a personal computer or a router to connect to receive Internet access via mobile broadband.-History:...

 is a PC card
PC card
In computing, PC Card is the form factor of a peripheral interface designed for laptop computers. The PC Card standard was defined and developed by the Personal Computer Memory Card International Association which itself was created by a number of computer industry companies in the United States...

 or ExpressCard
ExpressCard
ExpressCard is an interface to allow peripheral devices to be connected to a computer, usually a laptop computer. Formerly called NEWCARD, the ExpressCard standard specifies the design of slots built into the computer and of cards which can be inserted into ExpressCard slots. The cards contain...

 which slides into a PCMCIA/PC card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot. A cellular router may have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular Routers may not be modems per se, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it (since it can route, or support multipoint to multipoint connections), while the modem is made for one connection.

Most of the GSM wireless modems come with an integrated SIM cardholder (i.e., Huawei E220
Huawei E220
The Huawei E220 is a Huawei HSDPA access device manufactured by Huawei and notable for using the USB interface .Technically it is a modem, USB and virtual CD-ROM device....

, Sierra 881, etc.) and some models are also provided with a microSD memory slot and/or jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885. The CDMA (EVDO) versions do not use R-UIM cards, but use Electronic Serial Number
Electronic Serial Number
Electronic serial numbers were created by the U.S. Federal Communications Commission to uniquely identify mobile devices, from the days of AMPS in the United States from the early 1980s. The administrative role was taken over by the Telecommunications Industry Association in 1997 and is still...

 (ESN) instead.

The cost of using a wireless modem varies from country to country. Some carriers implement flat rate plans for unlimited data transfers. Some have caps (or maximum limits) on the amount of data that can be transferred per month. Other countries have plans that charge a fixed rate per data transferred—per megabyte or even kilobyte of data downloaded; this tends to add up quickly in today's content-filled world, which is why many people are pushing for flat data rates
Flat rate
A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage. Rarely, it may refer to a rate that does not vary with usage or time of use...

.

The faster data rates of the newest wireless modem technologies (UMTS, HSPA, EVDO, WiMax) are also considered to be broadband wireless modems and compete with other broadband modems below.

Until end of April 2011, worldwide shipments of USB modems still surpass embedded 3G and 4G modules by 3:1 due to USB modems can be easily discarded, but embedded modems could start to gain popularity as tablet sales grow and as the incremental cost of the modems shrinks, so by 2016 the ratio may change to 1:1.

Broadband

ADSL
Asymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...

 modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Some ADSL modems use coded orthogonal frequency division modulation (DMT, for Discrete MultiTone; also called COFDM, for digital TV in much of the world).

Cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...

s 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 multiple access.

New types of broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...

 modems are beginning to appear, such as doubleway satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 and power line
Power line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

 modems.

Broadband modems should still be classed as modems, since they use complex waveforms to carry digital data. They are more advanced devices than traditional dial-up modems as they are capable of modulating/demodulating hundreds of channels simultaneously.

Many broadband modems include the functions of a router (with Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 and WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

 ports) and other features such as DHCP, NAT
Network address translation
In computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device....

 and firewall features.

When broadband technology was introduced, networking and routers were unfamiliar to consumers. However, many people knew what a modem was as most internet access was through dial-up. Due to this familiarity, companies started selling broadband modems using the familiar term modem rather than vaguer ones like adapter or transceiver, or even "bridge".

Home networking

Although the name modem is seldom used in this case, modems are also used for high-speed home networking applications, especially those using existing home wiring. One example is the G.hn
G.hn
G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...

 standard, developed by ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

, which provides a high-speed (up to 1 Gbit/s) Local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

 using existing home wiring (power lines
Power line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

, phone lines and coaxial cables
Ethernet over coax
Ethernet over Coax is a family of technologies that supports the transmission of Ethernet frames over coaxial cable.- History :The first Ethernet standard, known as 10BASE5 in the family of IEEE 802.3, specified baseband operation over coaxial cable...

). G.hn devices use orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...

 (OFDM) to modulate a digital signal for transmission over the wire.

The phrase "null modem
Null modem
Null modem is a communication method to connect two DTEs directly using an RS-232 serial cable. The name stems from the historical use of the RS-232 cable to connect two teleprinter devices to modems in order to communicate with one another; null modem communication was possible by instead using...

" was used to describe attaching a specially wired cable between the serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...

s of two personal computers. Basically, the transmit output of one computer was wired to the receive input of the other; this was true for both computers. The same software used with modems (such as Procomm or Minicom) could be used with the null modem connection.

Deep-space telecommunications

Many modern modems have their origin in deep space
Deep Space
- Star Trek :* Battle of Deep Space Nine, the first major battle of the Dominion War* Deep Space Nine , space station in the fictional Star Trek universe* Deep Space Nine relaunch, number of novels released since 2000...

 telecommunications systems of the 1960s.

Differences between deep space telecom modems and landline modems:
  • digital modulation formats that have high doppler immunity are typically used
  • waveform complexity tends to be low, typically binary phase shift keying
  • error correction varies mission to mission, but is typically much stronger than most landline modems

Voice modem

Voice modems are regular modems that are capable of recording or playing audio over the telephone line. They are used for telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

 applications. See Voice modem command set
Voice modem command set
Voice modem is a term commonly used to describe an analog telephone data modem with a built-in capability of transmitting and receiving voice recordings over the phone line. Voice modems are used for telephony and answering machine applications...

 for more details on voice modems. This type of modem can be used as an FXO card for Private branch exchange systems (compare V.92
V.92
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

).

Popularity

A CEA
Consumer Electronics Association
The Consumer Electronics Association is a standards and trade organization for the consumer electronics industry in the United States. The Consumer Electronics Association is the preeminent trade association promoting growth in the $173 billion U.S...

 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. The US demographic pattern for (dial-up modem users per capita) has been more or less mirrored in Canada and Australia for the past 20 years.

Dial-up modem use in the US had dropped to 60% by 2003, and in 2006 stood at 36%. Voiceband modems were once the most popular means of Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 access in the U.S., but with the advent of new ways of accessing the Internet, the traditional 56K modem is losing popularity.

See also

  • Chapter Hayes-compatible Modems and AT Commands of the Serial Data Communications Programming Wikibook
  • 56 kbit/s line
    56 kbit/s line
    A 56 kbit/s line is a digital connection capable of carrying 56 kilobits per second , or 56,000 bit/s, the data rate of a classical single channel digital telephone line in North America...

  • Automatic negotiation (or handshake)
  • BBN Technologies
    BBN Technologies
    BBN Technologies is a high-technology company which provides research and development services. BBN is based next to Fresh Pond in Cambridge, Massachusetts, USA...

     (developed the first model in 1963)
  • Broadband
    Broadband
    The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...

    : satellite modem
    Satellite modem
    A satellite modem or sat modem is a modem used to establish data transfers using a communications satellite as a relay.There is a wide range of satellite modems from cheap devices for home internet access to expensive multifunctional equipment for enterprise use.A "modem" stands for...

    , ADSL, cablemodem, PLC
    Power line communication
    Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...

    .
  • Command and Data modes (modem)
    Command and Data modes (modem)
    Command and Data modes refer to the two modes in which a computer modem may operate. These modes are defined in the Hayes command set, which is the de-facto standard for all modems...

  • Device driver
    Device driver
    In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

  • DHCP
  • Ethernet
    Ethernet
    Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

  • Fax demodulator
    Fax demodulator
    A fax demodulator is a device used to intercept fax messages by listening in on a telephone line or radio signal.A typical fax transmission requires a two-way conversation between two modems . Each modem may be part of a “fax machine” incorporating an image scanner and a printer...

  • Internet outdial
    Internet outdial
    An Internet outdial is a modem that can be connected to through the Internet and then used to dial a telephone number. Normal outdials will only call local numbers, but a global outdial, or GOD, is capable of calling long distance...

  • TCP/IP
    Internet protocol suite
    The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...

  • ITU V-series
    ITU V-Series
    The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces....

     telephone network modem standards, including V.92
    V.92
    V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...

  • K56flex
  • Modulation
    Modulation
    In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

     (for a fuller list of modulation techniques)
  • Plug-and-Play
    Plug-and-play
    In computing, plug and play is a term used to describe the characteristic of a computer bus, or device specification, which facilitates the discovery of a hardware component in a system, without the need for physical device configuration, or user intervention in resolving resource conflicts.Plug...

  • RJ-11
  • Wake-Up on Ring (WOR)
  • X2 (Chipset)
  • Zeroconf
    Zeroconf
    Zero configuration networking , is a set of techniques that automatically creates a usable Internet Protocol network without manual operator intervention or special configuration servers....


External links

Standards organizations and modem protocols


General modem info (drivers, chipsets, etc.)


Other


The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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