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A modulator-demodulator or modem is a
computer hardware Computer hardware includes the physical parts of a computer, such as the computer case, case, central processing unit (CPU), Random-access memory, random access memory (RAM), Computer monitor, monitor, Computer mouse, mouse, Computer keyboard, ...
device that converts data from a digital format into a format suitable for an analog
transmission medium A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate ...
such as telephone or radio. A modem transmits data by
modulating In music, modulation is the change from one tonality ( tonic, or tonal center) to another. This may or may not be accompanied by a change in key signature (a key change). Modulations articulate or create the structure or form of many pieces, as ...
one or more
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
signals to encode
digital information Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digit (unit), digits. ...
, while the receiver demodulates the signal to recreate the original digital information. The goal is to produce a
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
that can be transmitted easily and decoded reliably. Modems can be used with almost any means of transmitting analog signals, from
light-emitting diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
s to
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
. Early modems were devices that used audible sounds suitable for transmission over traditional
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
systems and
leased line A leased line is a private telecommunications circuit between two or more locations provided according to a commercial contract. It is sometimes also known as a private circuit, and as a data line in the UK. Typically, leased lines are used by ...
s. These generally operated at 110 or 300 bits per second (bit/s), and the connection between devices was normally manual, using an attached
telephone handset A handset is a component of a telephone that a user holds to the ear and mouth to receive audio through the receiver and speak to the remote party using the built-in transmitter. In earlier telephones, the transmitter was mounted directly on ...
. By the 1970s, higher speeds of 1,200 and 2,400 bit/s for asynchronous dial connections, 4,800 bit/s for synchronous leased line connections and 35 kbit/s for synchronous conditioned leased lines were available. By the 1980s, less expensive 1,200 and 2,400 bit/s dialup modems were being released, and modems working on radio and other systems were available. As device sophistication grew rapidly in the late 1990s, telephone-based modems quickly exhausted the available
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, reaching 56 kbit/s. The rise of public use of the
internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
during the late 1990s led to demands for much higher performance, leading to the move away from audio-based systems to entirely new encodings on
cable television Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with broa ...
lines and short-range signals in
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
s on telephone lines. The move to
cellular telephone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...
s, especially in the late 1990s and the emergence of
smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whic ...
s in the 2000s led to the development of ever-faster radio-based systems. Today, modems are ubiquitous and largely invisible, included in almost every mobile computing device in one form or another, and generally capable of speeds on the order of tens or hundreds of megabytes per second.


Speeds

Modems are frequently classified by the maximum amount of data they can send in a given
unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI) and by extension most of the Western world, is the second, defined as a ...
, usually expressed in
bits per second In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
(symbol bit/s, sometimes abbreviated "bps") or rarely in
bytes per second In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
(symbol B/s). Modern broadband modem speeds are typically expressed in megabits per second (Mbit/s). Historically, modems were often classified by their
symbol rate In a digitally modulated signal or a line code, symbol rate, modulation rate or baud rate is the number of symbol changes, waveform changes, or signaling events across the transmission medium per unit of time. The symbol rate is measured in ''b ...
, measured in
baud In telecommunication and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modulatio ...
. 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 Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
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 could transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using
phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
. Many modems are variable-rate, permitting them to be used over a medium with less than ideal characteristics, such as a telephone line that is of poor quality or is too long. This capability is often adaptive so that a modem can discover the maximum practical transmission rate during the connect phase, or during operation.


Overall history

Modems grew out of the need to connect
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initia ...
s over ordinary phone lines instead of the more expensive leased lines which had previously been used for
current loop In electrical signalling an analog current loop is used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time. A major application of current loops is the industry de ...
–based teleprinters and automated
telegraph Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...
s. The earliest devices that satisfy the definition of a modem may be the multiplexers used by news wire services in the 1920s. In 1941, the
Allies An alliance is a relationship among people, groups, or states that have joined together for mutual benefit or to achieve some common purpose, whether or not explicit agreement has been worked out among them. Members of an alliance are called ...
developed a voice
encryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
system called
SIGSALY SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest-level Allied communications. It pioneered a number of digital communications concepts, including the ...
which used a
vocoder A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation. The vocoder was ...
to digitize speech, then encrypted the speech with one-time pad and encoded the digital data as tones using frequency shift keying. This was also a digital modulation technique, making this an early modem. Commercial modems largely did not become available until the late 1950s, when the rapid development of computer technology created demand for a method of connecting computers together over long distances, resulting in the Bell Company and then other businesses producing an increasing number of computer modems for use over both switched and leased telephone lines. Later developments would produce modems that operated over cable television lines,
power lines Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is ...
, and various radio technologies, as well as modems that achieved much higher speeds over telephone lines.


Dial-up

A dial-up modem transmits computer data over an ordinary
switched Switched may refer to: * Switched (band), an American music group * ''Switched'' (novel), first book in the young adult Trylle series by Amanda Hocking * ''Switched!'' (American TV series) * ''Switched!'' (Singaporean TV series) * "Switched" (' ...
telephone line that has not been designed for data use. This contrasts with
leased line A leased line is a private telecommunications circuit between two or more locations provided according to a commercial contract. It is sometimes also known as a private circuit, and as a data line in the UK. Typically, leased lines are used by ...
modems, which also operate over lines provided by a telephone company, but ones which are intended for data use and do not impose the same signaling constraints. The modulated data must fit the frequency constraints of a normal voice audio signal. Early modems, including acoustic coupled modems, relied on the communicating parties or an automatic calling unit to dial and establish a voice connection before switching their modems to line; more modern devices are able to perform the actions needed to connect a call through a telephone exchange, e.g., picking up the line, dialing, understanding signals sent back by phone company equipment (dialtone, ringing, busy signal) recognizing incoming ring signals and answering calls. Dial-up modems have been made in a wide variety of speeds and capabilities, with many capable of testing the line they are calling over and selecting the most advanced signaling mode that the line can support. Generally speaking, the fastest dialup modems ever available to consumers never exceeded 56 kbit/s and never achieved that speed in both directions. The dial-up modem was once a widely known technology, since it was mass-marketed to consumers in many countries for
dial-up internet access Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an Internet service provider (ISP) by dialing a telephone number on a conventional telepho ...
. In the 1990s, tens of millions of people in the United States used dial-up modems for internet access. Dial-up service has since been largely supplanted by
broadband internet In telecommunications, broadband is wide bandwidth data transmission which transports multiple signals at a wide range of frequencies and Internet traffic types, that enables messages to be sent simultaneously, used in fast internet connections. ...
, such as
DSL Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric di ...
, which typically still uses a modem, but of a very different type which may still operate over a normal phone line, but with substantially relaxed constraints.


History


1950s

Mass production of telephone line modems in the United States began as part of the
SAGE Sage or SAGE may refer to: Plants * ''Salvia officinalis'', common sage, a small evergreen subshrub used as a culinary herb ** Lamiaceae, a family of flowering plants commonly known as the mint or deadnettle or sage family ** ''Salvia'', a large ...
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 United States and
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
. Shortly afterwards in 1959, the technology in the SAGE modems was made available commercially as the
Bell 101 The Bell 101 dataset or Bell 101 modem was the first commercial modem for computers, released by AT&T Corporation in 1958 for use by SAGE and in 1959 made commercial shortly after AT&T's Bell Labs announced their 110 baud modulation frequencies. Th ...
, which provided 110 bit/s speeds. Bell called this and several other early modems "datasets".


1960s

Some early modems were based on
touch-tone Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
frequencies, such as Bell 400-style touch-tone modems. The Bell 103A standard was introduced by
AT&T AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile tel ...
in 1962. It provided full-duplex service at 300 bit/s over normal phone lines.
Frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
was used, with the call originator transmitting at 1,070 or 1,270  Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The 103 modem would eventually become a de facto standard once third-party (non-AT&T modems) reached the market, and throughout the 1970s, independently made modems compatible with the Bell 103 de facto standard were commonplace. Example models included the
Novation CAT Novation, in contract law and business law, is the act of – # replacing an obligation to perform with another obligation; or # adding an obligation to perform; or # replacing a party to an agreement with a new party. In international law, no ...
and the
Anderson-Jacobson Anderson Jacobson, also known for a time as CXR Anderson Jacobson and today as CXR Networks, is a vendor of communications equipment. Anderson Jacobson was an early manufacturer of acoustic modems and was spun off from SRI International (then the ...
. A lower-cost option was the
Pennywhistle modem The Pennywhistle was an early acoustic coupler modem originally designed and built by Lee Felsenstein in 1973, and later commercialized and offered for sale in 1976. It was one of the earliest modems available for hobbyist computer users. Like most ...
, designed to be built using readily available parts. Teletype machines were granted access to remote networks such as the
Teletypewriter Exchange The telex network is a station-to-station switched network of teleprinters similar to a telephone network, using telegraph-grade connecting circuits for two-way text-based messages. Telex was a major method of sending written messages electroni ...
using the Bell 103 modem. AT&T also produced reduced-cost units, the originate-only 113D and the answer-only 113B/C modems.


1970s

The ''201A'' ''Data-Phone'' was a synchronous modem using two-bit-per-symbol
phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
(PSK) encoding, achieving 2,000 bit/s half-duplex over normal phone lines. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. In early 1973, Vadic introduced the ''VA3400'' which performed full-duplex at 1,200 bit/s over a normal phone line. In November 1976, AT&T introduced the 212A modem, similar in design, but using the lower frequency set for transmission. It was not compatible with the VA3400, but it would operate with 103A modem at 300 bit/s. 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.


1980s

A significant advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it introduced a command language which allowed the computer to make control requests, such as commands to dial or answer calls, over the same RS-232 interface used for the data connection. The command set used by this device became a de facto standard, the
Hayes command set The Hayes command set (also known as the AT command set) is a specific command language originally developed by Dennis Hayes for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which can b ...
, which was integrated into devices from many other manufacturers. Automatic dialing was not a new capability – it had been available via separate Automatic Calling Units, and via modems using the
X.21 X.21 (sometimes referred to as X21) is an interface specification for differential communications introduced in the mid-1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications bet ...
interface – but the Smartmodem made it available in a single device that could be used with even the most minimal implementations of the ubiquitous RS-232 interface, making this capability accessible from virtually any system or language. The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features. This eventually led to legal action over use of the patented Hayes command language. Dial modems generally remained at 300 and 1,200 bit/s (eventually becoming standards such as V.21 and V.22) into the mid-1980s. Commodore's 1982 ''VicModem'' for the
VIC-20 The VIC-20 (known as the VC-20 in Germany and the VIC-1001 in Japan) is an 8-bit home computer that was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PE ...
was the first modem to be sold under $100, and the first modem to sell a million units. In 1984,
V.22bis ''V.'' is the debut novel of Thomas Pynchon, published in 1963. It describes the exploits of a discharged United States Navy, U.S. Navy sailor named Benny Profane, his reconnection in New York City, New York with a group of pseudo-bohemianism, b ...
was created, a 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212. This bit rate increases was achieved by defining four or eight distinct symbols, which allowed the encoding of two or three bits per symbol instead of only one. By the late 1980s, many modems could support improved standards like this, and 2,400-bit/s operation was becoming common. Increasing modem speed greatly improved the responsiveness of online systems and made
file transfer File transfer is the transmission of a computer file through a communication channel from one computer system to another. Typically, file transfer is mediated by a communications protocol. In the history of computing, numerous file transfer protocol ...
practical. This led to rapid growth of
online service An online service provider (OSP) can, for example, be an Internet service provider, an email provider, a news provider (press), an entertainment provider (music, movies), a search engine, an e-commerce site, an online banking site, a health site, ...
s with large file libraries, which in turn gave more reason to own a modem. The rapid update of modems led to a similar rapid increase in BBS use. The introduction of
microcomputer A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (PC ...
systems with internal
expansion slot Expansion may refer to: Arts, entertainment and media * ''L'Expansion'', a French monthly business magazine * ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004 * ''Expansions'' (McCoy Tyner album), 1970 * ''Expansio ...
s made small internal modems practical. This led to a series of popular modems for the
S-100 bus The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus designed in 1974 as a part of the Altair 8800. The bus was the first industry standard expansion bus for the microcomputer industry. computers, consisting of ...
and
Apple II The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...
computers that could directly dial out, answer incoming calls, and hang up entirely from software, the basic requirements of a
bulletin board system A bulletin board system (BBS), also called computer bulletin board service (CBBS), is a computer server running software that allows users to connect to the system using a terminal program. Once logged in, the user can perform functions such as ...
(BBS). The seminal
CBBS CBBS ("Computerized Bulletin Board System") was a computer program created by Ward Christensen and Randy Suess to allow them and other computer hobbyists to exchange information between each other. Jason Scott: , 2005, Episode 1 In Janua ...
for instance was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed.
Echo cancellation Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression i ...
became a feature of modems in this period, which improved the bandwidth available to both modems by allowing them to ignore their own reflected signals. Additional improvements were introduced by
quadrature amplitude modulation Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signal ...
(QAM) encoding, which increased the number of bits per symbol to four through a combination of phase shift and amplitude. Transmitting at 1,200 baud produced the 4,800 bit/s V.27ter standard, and at 2,400 baud the 9,600 bit/s V.32. The
carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a m ...
was 1,650 Hz in both systems. The introduction of these higher-speed systems also led to the development of the digital
fax Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...
machine during the 1980s. While early fax technology also used modulated signals on a phone line, digital fax used the now-standard digital encoding used by computer modems. This eventually allowed computers to send and receive fax images.


1990s

In the early 1990s, V.32 modems operating at 9,600 bit/s were introduced, but were expensive and were only starting to enter the market when V.32bis was standardized, which operated at 14,400 bit/s.
Rockwell International Rockwell International was a major American manufacturing conglomerate involved in aircraft, the space industry, defense and commercial electronics, components in the automotive industry, printing presses, avionics and industrial products. R ...
's chip division developed a new driver chip set incorporating the V.32bis standard and aggressively priced it.
Supra, Inc. Supra Corporation was best known as a manufacturer of modems for personal computers, but also produced a range of hardware for the Amiga and Atari ST, including hard drives, SCSI controllers, memory boards, and processor accelerators. They were ...
arranged a short-term exclusivity arrangement with Rockwell, and developed the
SupraFAXModem 14400 The SupraFAXModem 14400 was a v.32bis modem. When it was launched by Supra, Inc. in January 1992 at a $399 price point,Difference sources list the price between $349 and $399 the 14,400 bit/s model was less expensive than most existing 9600 b ...
based on it. Introduced in January 1992 at (or less), it was half the price of the slower V.32 modems already on the market. This led to a price war, and by the end of the year V.32 was dead, never having been really established, and V.32bis modems were widely available for . V.32bis was so successful that the older high-speed standards had little advantages. USRobotics (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'', but neither non-standard modem sold well. Consumer interest in these proprietary improvements waned during the lengthy introduction of the 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 a standard was ratified (1994), manufacturers used more flexible components, generally a
DSP DSP may refer to: Computing * Digital signal processing, the mathematical manipulation of an information signal * Digital signal processor, a microprocessor designed for digital signal processing * Yamaha DSP-1, a proprietary digital signal ...
and
microcontroller A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable i ...
, as opposed to purpose-designed
ASIC An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficien ...
modem chips. This would allow later firmware updates to conform with the standards once ratified. The ITU standard V.34 represents the culmination of these joint efforts. It employed the most powerful coding techniques available at the time, including channel encoding and shape encoding. From the mere four bits per symbol (), 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 modems. This rate is near the theoretical
Shannon limit In information theory, the noisy-channel coding theorem (sometimes Shannon's theorem or Shannon's limit), establishes that for any given degree of noise contamination of a communication channel, it is possible to communicate discrete data (dig ...
of a phone line.


= technologies

= While speeds had been available for leased-line modems for some time, they did not become available for dial up modems until the late 1990s. In the late 1990s, technologies to achieve speeds above began to be introduced. Several approaches were used, but all of them began as solutions to a single fundamental problem with phone lines. By the time technology companies began to investigate speeds above , telephone companies had switched almost entirely to all-digital networks. As soon as a phone line reached a local central office, a ''line card'' converted the analog signal from the subscriber to a digital one and conversely. While digitally encoded telephone lines notionally provide the same bandwidth as the analog systems they replaced, the digitization itself placed constraints on the ''types'' of waveforms that could be reliably encoded. The first problem was that the process of analog-to-digital conversion is intrinsically lossy, but second, and more importantly, the digital signals used by the telcos were not "linear": they did not encode all frequencies the same way, instead utilizing a nonlinear encoding ( μ-law and
a-law An A-law algorithm is a standard companding algorithm, used in European 8-bit PCM digital communications systems to optimize, i.e. modify, the dynamic range of an analog signal for digitizing. It is one of two versions of the G.711 standard ...
) meant to favor the nonlinear response of the human ear to voice signals. This made it very difficult to find a encoding that could survive the digitizing process. Modem manufacturers discovered that, while the analog to digital conversion could not preserve higher speeds, ''digital-to-analog'' conversions could. Because it was possible for an ISP to obtain a direct digital connection to a telco, a ''digital modem'' one that connects directly to a digital telephone network interface, such as T1 or PRI could send a signal that utilized every bit of bandwidth available in the system. While that signal still had to be converted back to analog at the subscriber end, that conversion would not distort the signal in the same way that the opposite direction did.


Early 56k dial-up products

The first 56k dial-up option was a proprietary design from
USRobotics U.S. Robotics Corporation, often called USR, is a company that produces USRobotics computer modems and related products. Its initial marketing was aimed at bulletin board systems, where its high-speed HST protocol made FidoNet transfers much fas ...
, which they called "X2" because 56k was twice the speed (×2) of 28k modems. At that time, USRobotics held a 40% share of the retail modem market, while Rockwell International held an 80% share of the modem
chipset In a computer system, a chipset is a set of electronic components An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are ...
market. Concerned with being shut out, Rockwell began work on a rival 56k technology. They joined with
Lucent Lucent Technologies, Inc. was an American Multinational corporation, multinational telecommunications equipment company headquartered in Murray Hill, New Jersey, Murray Hill, New Jersey. It was established on September 30, 1996, through the dives ...
and
Motorola Motorola, Inc. () was an American Multinational corporation, multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent p ...
to develop what they called "K56Flex" or just "Flex". Both technologies reached the market around February 1997; although problems with K56Flex modems were noted in product reviews through July, within six months the two technologies worked equally well, with variations dependent largely on local connection characteristics. The retail price of these early 56K modems was about , compared to for standard 33k modems. Compatible equipment was also required at the
Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise private ...
s (ISPs) end, with costs varying depending on whether their current equipment could be upgraded. About half of all ISPs offered 56k support by October 1997. Consumer sales were relatively low, which USRobotics and Rockwell attributed to conflicting standards.


Standardized 56k (V.90/V.92)

In February 1998, The
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
(ITU) announced the draft of a new standard V.90 with strong industry support. Incompatible with either existing standard, it was an amalgam of both, but was designed to allow both types of modem by a firmware upgrade. The V.90 standard was approved in September 1998 and widely adopted by ISPs and consumers. The
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; prev ...
standard was approved by ITU in November 2000 and utilized digital
PCM Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amp ...
technology to increase the upload speed to a maximum of . The high upload speed was a tradeoff. upstream rate would reduce the downstream as low as due to echo effects on the line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a plain 33. analog connection in order to maintain a high digital downstream of or higher. V.92 also added two other features. The first is the ability for users who have call waiting to put their
dial-up Internet Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an Internet service provider (ISP) by dialing a telephone number on a conventional telepho ...
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, achieved by remembering the analog and digital characteristics of the telephone line and using this saved information when reconnecting.


Evolution of dial-up speeds

These values 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 This is a list of interface bit rates, is a measure of information transfer rates, or digital bandwidth capacity, at which digital interfaces in a computer or network can communicate over various kinds of buses and channels. The distinction can ...
''. A
baud In telecommunication and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modulatio ...
is one symbol per second; each symbol may encode one or more data bits.


Compression

Many dial-up modems implement standards for
data compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
to achieve higher effective throughput for the same bitrate.
V.44 The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces. ''Note:'' the '' bis'' and ''ter'' suffixes are ITU-T standard designato ...
is an example used in conjunction with
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; prev ...
to achieve speeds greater than 56k over ordinary phone lines. As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase apparent throughput. This server-side compression can operate much more efficiently than the on-the-fly compression performed within modems, because the compression techniques are content-specific (JPEG, text, EXE, etc.). Website text, images, and Flash media are typically compacted to approximately 4%, 12%, and 30%, respectively. The drawback is a loss in quality, as they use
lossy compression In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
which causes images to become pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up". These accelerated downloads are integrated into the
Opera Opera is a form of theatre in which music is a fundamental component and dramatic roles are taken by singers. Such a "work" (the literal translation of the Italian word "opera") is typically a collaboration between a composer and a librett ...
and Amazon Silk web browsers, using their own server-side text and image compression.


Methods of attachment

Dial-up modems can attach in two different ways: with an acoustic coupler, or with a direct electrical connection.


Directly connected modems

The case ''Hush-A-Phone Corp. v. United States'', which legalized acoustic couplers, applied only to mechanical connections to a telephone set, not electrical connections to the telephone line. The ''Carterfone'' decision of 1968, however, permitted customers to attach devices directly to a telephone line as long as they followed stringent Bell-defined standards for non-interference with the phone network. This opened the door to independent (non-AT&T) manufacture of direct-connect modems, that plugged directly into the phone line rather than via an acoustic coupler. While ''Carterfone'' required AT&T to permit connection of devices, AT&T successfully argued that they should be allowed to require the use of a special device to protect their network, placed in between the third-party modem and the line, called a Data Access Arrangement or DAA. The use of DAAs was mandatory from 1969 to 1975 when the new FCC Part 68 rules allowed the use of devices without a Bell-provided DAA, subject to equivalent circuitry being included in the third-party device. Virtually all modems produced after the 1980s are direct-connect.


Acoustic couplers

While Bell (AT&T) provided modems that attached via direct wire connection to the phone network as early as 1958, their regulations at the time did not permit the direct electrical connection of any non-Bell device to a telephone line. However, the Hush-A-Phone Corp. v. United States, Hush-a-Phone ruling allowed customers to attach any device ''to a telephone set'' as long as it did not interfere with its functionality. This allowed third-party (non-Bell) manufacturers to sell modems utilizing an ''acoustic coupler''. With an acoustic coupler, an ordinary telephone handset was placed in a cradle containing a speaker and microphone positioned to match up with those on the handset. The tones used by the modem were transmitted and received into the handset, which then relayed them to the phone line. Because the modem was not electrically connected, it was incapable of picking up, hanging up or dialing, all of which required direct control of the line. Touch-tone dialing would have been possible, but touch-tone was not universally available at this time. Consequently, the dialing process was executed by the user lifting the handset, dialing, then placing the handset on the coupler. To accelerate this process, a user could purchase a ''dialer'' or ''Automatic Calling Unit''.


Automatic Calling Units / Dialers

Early modems – could not place or receive calls on their own, but required human intervention for these steps. As early as 1964, Bell provided ''Automatic Calling Units'' that connected separately to a second serial port on a host machine and could be commanded to open the line, dial a number, and even ensure the far end had successfully connected before transferring control to the modem. Later on, third-party models would become available, sometimes known simply as ''dialers'', and features such as the ability to automatically sign in to time-sharing systems. Eventually this capability would be built into modems and no longer require a separate device.


Controller-based modems vs. soft modems

Prior to the 1990s, modems contained all the electronics and intelligence to convert data in discrete form to an analog (modulated) signal and back again, and to handle the dialing process, as a mix of discrete logic and special-purpose chips. This type of modem is sometimes referred to as ''controller-based''. In 1993, Digicom introduced the ''Connection 96 Plus'', a modem which replaced the discrete and custom components with a general purpose digital signal processor, which could be reprogrammed to upgrade to newer standards. Subsequently, USRobotics released the ''Sportster Winmodem'', a similarly upgradable DSP-based design. As this design trend spread, both terms – ''soft modem'' and ''Winmodem'' – obtained a negative connotation in non-Windows-based computing circles because the drivers were either unavailable for non-Windows platforms, or were only available as unmaintainable closed-source binaries, a particular problem for Linux users. Later in the 1990s, software-based modems became available. These are essentially sound cards, and in fact a common design uses the AC'97 audio codec, which provides multichannel audio to a PC and includes three audio channels for modem signals. The audio sent and received on the line by a modem of this type is generated and processed entirely in software, often in a device driver. There is little functional difference from the user's perspective, but this design reduces the cost of a modem by moving most of the processing power into inexpensive software instead of expensive hardware Digital signal processor, DSPs or discrete components. Soft modems of both types either are internal cards or connect over external buses such as USB. They never utilize RS-232 because they require high bandwidth channels to the host computers to carry the raw audio signals generated (sent) or analyzed (received) by software. Since the interface is not RS-232, there is no standard for communication with the device directly. Instead, soft modems come with drivers which create an emulated RS-232 port, which standard modem software (such as an operating system dialer application) can communicate with.


Voice/fax modems

"Voice" and "fax" are terms added to describe any dial modem that is capable of recording/playing audio or transmitting/receiving faxes. Some modems are capable of all three functions. Voice modem command set, Voice modems are used for computer telephony integration applications as simple as placing/receiving calls directly through a computer with a headset, and as complex as fully automated robocalling systems. Fax modems can be used for computer-based faxing, in which faxes are sent and received without inbound or outbound faxes ever needing to ever be printed on paper. This differs from Internet fax, efax, in which faxing occurs over the internet, in some cases involving no phone lines whatsoever.


Modem Over IP (Modem Relay)

The ITU-T V.150.1 Recommendation defines procedures for the inter-operation of PSTN to IP gateways. In a classic example of this setup, each dial-up modem would connect to a modem relay gateway. The gateways are then connected to an IP network (such as the Internet). The analog connection from the modem is terminated at the gateway and the signal is demodulated. The demodulated control signals are transported over the IP network in an Real-time Transport Protocol, RTP packet type defined as State Signaling Events (SSEs). The data from the demodulated signal is sent over the IP network via a transport protocol (also defined as an RTP payload) called Simple Packet Relay Transport (SPRT). Both the SSE and SPRT packet formats are defined in the V.150.1 Recommendation (Annex C and Annex B respectively). The gateway at the remote end that receives the packets uses the information to re-modulate the signal for the modem connected at that end. While the V.150.1 Recommendation is not widely deployed, a pared down version of the recommendation called "Minimum Essential Requirements (MER) for V.150.1 Gateways" (SCIP-216) is used in Secure telephone, Secure Telephony applications.


Cloud-based Modems

While traditionally a hardware device, fully software-based modems with the ability to be deployed in a cloud environment (such as Microsoft Azure or Amazon Web Services, AWS) do exist. Leveraging a Voice over IP, Voice-over-IP (VoIP) connection through a SIP trunking, SIP Trunk, the modulated audio samples are generated and sent over an IP network via Real-time Transport Protocol, RTP and an uncompressed audio codec (such as G.711 μ-law or a-law).


Popularity

A 1994 Software Publishers Association found that although 60% of computers in US households had a modem, only 7% of households went online. A Consumer Electronics Association, CEA study in 2006 found that dial-up Internet access was declining in the US. In 2000, dial-up Internet connections accounted for 74% of all US residential Internet connections. The United States 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 stood at 36% in 2006. Voiceband modems were once the most popular means of Internet access in the US, but with the advent of new ways of accessing the Internet, the traditional 56K modem was losing popularity. The dial-up modem is still widely used by customers in rural areas where DSL, cable, wireless broadband, satellite, or fiber optic service are either not available or they are unwilling to pay what the available broadband companies charge. In its 2012 annual report, AOL showed it still collected around $700 million in fees from about three million dial-up users.


TTY/TDD

Telecommunications device for the deaf, TDD devices are a subset of the
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initia ...
intended for use by the deaf or hard of hearing, essentially a small teletype with a built-in dial-up modem and acoustic coupler. The first models produced in 1964 utilized Frequency-shift keying, FSK modulation much like early computer modems.


Leased-line modems

A
leased line A leased line is a private telecommunications circuit between two or more locations provided according to a commercial contract. It is sometimes also known as a private circuit, and as a data line in the UK. Typically, leased lines are used by ...
modem also uses ordinary phone wiring, like dial-up and DSL, but does not use the same network topology. While dial-up uses a normal phone line and connects through the telephone switching system, and DSL uses a normal phone line but connects to equipment at the telco central office, leased lines do not terminate at the telco. Leased lines are pairs of telephone wire that have been connected together at one or more telco central offices so that they form a continuous circuit between two subscriber locations, such as a business' headquarters and a satellite office. They provide no power or dialtone - they are simply a pair of wires connected at two distant locations. A dialup modem will not function across this type of line, because it does not provide the power, dialtone and switching that those modems require. However, a modem with leased-line capability can operate over such a line, and in fact can have greater performance because the line is not passing through the telco switching equipment, the signal is not filtered, and therefore greater bandwidth is available. Leased-line modems can operate in 2-wire or 4-wire mode. The former uses a single pair of wires and can only transmit in one direction at a time, while the latter uses two pairs of wires and can transmit in both directions simultaneously. When two pairs are available, bandwidth can be as high as 1.5 Mbit/s, a full data T1 line, T1 circuit.


Broadband

The term ''broadband'' was previously used to describe communications faster than what was available on voice grade channels. The term ''broadband'' gained widespread adoption in the late 1990s to describe internet access technology exceeding the 56 kilobit/s maximum of dialup. There are many broadband technologies, such as various DSL (digital subscriber line) technologies and cable broadband. DSL technologies such as Asymmetric digital subscriber line, ADSL, High-bit-rate digital subscriber line, HDSL, and VDSL use telephone lines (wires that were installed by a telephone company and originally intended for use by a telephone subscriber) but do not utilize most of the rest of the telephone system. Their signals are not sent through ordinary phone exchanges, but are instead received by special equipment (a Digital subscriber line access multiplexer, DSLAM) at the telephone company central office. Because the signal does not pass through the telephone exchange, no "dialing" is required, and the bandwidth constraints of an ordinary voice call are not imposed. This allows much higher frequencies, and therefore much faster speeds. ADSL in particular is designed to permit voice calls and data usage over the same line simultaneously. Similarly, cable modems use infrastructure originally intended to carry television signals, and like DSL, typically permit receiving television signals at the same time as broadband internet service. Other broadband modems include FTTx modems, satellite modems, and Power line communication, power line modems.


Terminology

Different terms are used for broadband modems, because they frequently contain more than just a modulation/demodulation component. Because high-speed connections are frequently used by multiple computers at once, many broadband modems do not have direct (e.g. USB) PC connections, but connect over a network such as Ethernet or Wi-Fi. Early broadband modems offered Ethernet handoff allowing the use of one or more public IP addresses, but no other services such as NAT and DHCP that would allow multiple computers to share one connection. This led to many consumers purchasing separate "broadband routers," placed between the modem and their network, to perform these functions. Eventually, ISPs began providing residential gateways which combined the modem and broadband router into a single package that provided routing, Network address translation, NAT, security features, and even Wi-Fi access in addition to modem functionality, so that subscribers could connect their entire household without purchasing any extra equipment. Even later, these devices were extended to provide "Triple play (telecommunications), triple play" features such as telephony and television service. Nonetheless, these devices are still often referred to simply as "modems" by service providers and manufacturers. Consequently, the terms "modem", "router", and "gateway" are now used interchangeably in casual speech, but in a technical context "modem" may carry a specific connotation of basic functionality with no routing or other features, while the others describe a device with features such as NAT. Broadband modems may also handle authentication such as Point-to-Point Protocol over Ethernet, PPPoE. While it is often possible to authenticate a broadband connection from a users PC, as was the case with dial-up internet service, moving this task to the broadband modem allows it to establish and maintain the connection itself, which makes sharing access between PCs easier since each one does not have to authenticate separately. Broadband modems typically remain authenticated to the ISP as long as they are powered on.


Radio

Any communication technology sending digital data wirelessly involves a modem. This includes direct broadcast satellite, WiFi, WiMAX, WiMax, mobile phones, Global Positioning System, GPS, Bluetooth and Near-field communication, NFC. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer networking, computer network links to outlying areas where Fiber-optic communication, fiber optic is not economical. 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 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 media access controllers 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.


Mobile broadband

Modems which use a mobile telephone system (GPRS, UMTS, High Speed Packet Access, HSPA, Evolution-Data Optimized, EVDO, WiMAX, WiMax, 5G etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a laptop, mobile phone or other device, or be connected externally. External wireless modems include connect cards, USB modems, and cellular routers. Most GSM wireless modems come with an integrated SIM cardholder (i.e. Huawei E220, Sierra 881.) Some models are also provided with a microSD memory slot and/or jack for additional external antenna, (Huawei E1762, Sierra Compass 885.) The CDMA (EVDO) versions do not typically use Removable User Identity Module, R-UIM cards, but use Electronic Serial Number (ESN) instead. Until the end of April 2011, worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3:1 because USB modems can be easily discarded. Embedded modems may overtake separate modems as tablet sales grow and the incremental cost of the modems shrinks, so by 2016, the ratio may change to 1:1. Like mobile phones, mobile broadband modems can be SIM locked to a particular network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using an Subsidy Password, 'unlock code'.


Optical modem

A modem that connects to a fiber optic network is known as an Network interface device#Optical network terminals, optical network terminal (ONT) or optical network unit (ONU). These are commonly used in Fiber-to-the home, fiber to the home installations, installed inside or outside a house to convert the optical medium to a copper Ethernet interface, after which a router or gateway is often installed to perform authentication, routing, NAT, and other typical consumer internet functions, in addition to "Triple play (telecommunications), triple play" features such as telephony and television service. Fiber optic systems can use quadrature amplitude modulation to maximize throughput. 16QAM uses a 16-point constellation to send four bits per symbol, with speeds on the order of 200 or 400 gigabits per second. 64QAM uses a 64-point constellation to send six bits per symbol, with speeds up to 65 terabits per second. Although this technology has been announced, it may not yet be commonly used.


Home networking

Although the name ''modem'' is seldom used, some high-speed home networking applications do use modems, such as Powerline Ethernet, powerline ethernet. The G.hn standard for instance, developed by ITU-T, provides a high-speed (up to 1 Gbit/s) local area network using existing home wiring (power lines, phone lines, and ethernet over coax, coaxial cables). G.hn devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal for transmission over the wire. As described above, technologies like Wi-Fi and Bluetooth also use modems to communicate over radio at short distances.


Null modem

A null modem cable is a specially wired cable connected between the serial ports of two devices, with the transmit and receive lines reversed. It is used to connect two devices directly without a modem. The same software or hardware typically used with modems (such as Procomm or Minicom) could be used with this type of connection. A null modem adapter is a small device with plugs on both end which is placed on the end of a normal "straight-through" serial cable to convert it into a null-modem cable.


Short-haul modem

A "short haul modem" is a device that bridges the gap between leased-line and dial-up modems. Like a leased-line modem, they transmit over "bare" lines with no power or telco switching equipment, but are not intended for the same distances that leased lines can achieve. Ranges up to several miles are possible, but significantly, short-haul modems can be used for ''medium'' distances, greater than the maximum length of a basic serial cable but still relatively short, such as within a single building or campus. This allows a serial connection to be extended for perhaps only several hundred to several thousand feet, a case where obtaining an entire telephone or leased line would be overkill. While some short-haul modems do in fact use modulation, low-end devices (for reasons of cost or power consumption) are simple "line drivers" that increase the level of the digital signal but do not modulate it. These are not technically modems, but the same terminology is used for them.


See also


References


External links

* b:Serial Programming/Modems and AT Commands, Hayes-compatible Modems and AT Commands from the b:Serial Programming, Serial Data Communications Programming ''Wikibook''
International Telecommunication Union ITU
Data communication over the telephone network

– V.22, V.22bis, V.32 and V.34 handshakes
Getting connected: a history of modems
– techradar
Difference between Modems and Routers
– Bugswave
Telecommunications Transmission_Engineering Volume 2 Facilities
- AT&T {{Authority control Modems, American inventions Bulletin board systems Computer-related introductions in 1958 Logical link control Physical layer protocols