Modem (from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Experiments have even been performed in the use of modems over the medium of two cans connected by a string.

The most familiar example is a voiceband modem that turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Serial, or Network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by Baud, the number of distinct symbols transmitted per second; these numbers are directly connected, but not necessarily in linear fashion (as discussed under Baud.)

Faster modems are used by Internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x10⁹) bits per second. Please note that one kilobit per second (kbit/s or kb/s or kbps) as used in this article means 1000 bits per second and not 1024 bits per second. For example, a 56k modem can transfer data at up to 56,000 bits per second over the phone line.

Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines.

People got along at 300 bps for quite a while. The reason this speed was tolerable was because 300 bps represents about 30 characters per second, which is a lot more characters per second than a person can type or read. Once people started transferring large programs and images to and from bulletin board systems, however, 300 bps became intolerable. Modem speeds went through a series of steps at approximately two-year intervals:

The common modem standards are:

300bps – 1960’s – 1983

1200bps – gained popularity in 1984 and 1985

2400bps

9600bps – first appeared in late 1990 and early 1991

14.4kbps

19.2kbps

28.8kbps – 1995 Quad speed Acer, Mad Dog Mcree, Nascar, V.34

36.6kbps – Us Robotics K56 flexation

56.6kbps – Standard in 1998. V.90. ITU (International Telecommunications Commission or International telegraphic Union) V.92 brought faster uploads, the modem could pause when the phone rang, and provided faster connections

ADSL – theoretically 8 megabits came in 1999

We'll use 300-bps modems as a starting point because they are extremely easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits.

When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare.

Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary. A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over the phone line.

Faster Modems

In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques.

International Telecommunication Union

The International Telecommunication Union (ITU) is an international organization established to standardize and regulate international radio and telecommunications. It was founded as the International Telegraph Union in Paris on May 17, 1865. Its main tasks include standardization, allocation of the radio spectrum, and organizing interconnection arrangements between different countries to allow international phone calls — in which regard it performs for telecommunications a similar function to what the UPU performs for postal services. It is one of the specialized agencies of the United Nations, and has its headquarters in Geneva, Switzerland, next to the main United Nations campus.

Voiceband modems generally remained at 300 and 1200 bit/s (V.21 and V.22) into the mid 1980s, although, over this period, the acoustic coupler disappeared, seemingly overnight, as Smartmodem-compatible modems flooded the market.

A V.22bis 2400-bit/s system similar in concept to the 1200-bit/s Bell 212 signalling was introduced in the U.S., and a slightly different, and incompatible, one in Europe. By the late 1980s, most modems could support all of these standards, and 2400-bit/s operation was becoming common.

Many other standards were also introduced for special purposes, commonly using a high-speed channel for receiving, and a lower-speed channel for sending. One typical example was used in the French Minitel system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1200 bit/s for reception, and 75 bit/s for sending commands back to the servers. Such solutions were useful in many circumstances in which one side would be sending more data than the other. In addition to a number of "medium-speed" standards, like Minitel, four U.S. companies became famous for high-speed versions of the same concept.

Data communication over the telephone network

V.1 Equivalence between binary notation symbols and the significant conditions of a two-condition code.

V.5 Standard that synchronous data signalling rates should be 600, 1200, 2400, 4800 and 9600 bit/s, to within better than 0.01%.

V.10 A recommendation, first agreed in 1976, for unbalanced electrical circuits for data communication at up to 100 kbit/s.

V.11 A recommendation, first agreed in 1976, for balanced electrical circuits for data communication at up to 10 Mbit/s.

V.17 A fax protocol that uses TCM modulation at 12 and 14.4 kbit/s.

V.21 A recommendation for full-duplex communication between two analogue dial-up modems using AFSK modulation at 300 baud to carry data at 300 bit/s.

V.22 A recommendation for full-duplex communication between two analogue dial-up modems using PSK modulation at 600 baud to carry data at 1200 or 600 bit/s.

V.22bis An extension of V.22 using QAM modulation at 600 baud to carry data at 2400 or 1200 bit/s, with fall-back to V.22 mode.

V.23 A recommendation for half-duplex communication between two analogue dial-up modems using FSK modulation at up to 600 or 1200 baud to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 baud reverse channel carries 75 bit/s.

V.24 A list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit terminating equipment (DCE), first agreed in 1964. This is equivalent to a subset of EIA RS 232: for the electrical and physical details, see V.28 and others.

V.25 A recommendation, first agreed in 1968, for automatic calling and/or answering equipment on dial-up lines, using interchange circuits defined in V.24 specifically for parallel automatic calling. It includes procedures for disabling network echo cancellers and suppressors.

V.25bis An extension of V.25 using the serial interchange circuits defined in V.24 for normal data transfer. Command formats are defined for asynchronous, and synchronous character- or bit-oriented (HDLC) operation.

V.26 A recommendation, first agreed in 1968, for full-duplex communication between two analogue fixed-line modems using PSK modulation at 1,200 baud to carry synchronous data at 2400 bit/s. An optional 75 baud reverse channel carries 75 bit/s in either direction.

V.26bis An extension of V.26, first agreed in 1972, for half-duplex operation of dial-up modems, adding a fall-back rate of 1200 bit/s (still at 1200 baud).

V.26ter An extension of V.26, first agreed in 1984, for full-duplex fixed-line or dial-up operation to carry synchronous or asynchronous data with a fall-back rate of 1200 bit/s (at 1200 baud), separating channels by echo cancellation.

V.27 A recommendation, first agreed in 1972, for full-duplex or half-duplex communication between two analogue fixed-line modems using PSK modulation at 1600 baud to carry synchronous data at 4800 bit/s. An optional 75 baud reverse channel carries 75 bit/s.

V.27bis An extension of V.27, first agreed in 1976, adding a fall-back modulation rate, compatible with V.26, of 1200 bauds to carry data at 2400 bit/s. An adaptive equalizer is included to handle lower grade lines.

V.27ter An extension of V.27bis for use on dial-up lines.

V.28 A recommendation, first agreed in 1972, for unbalanced electrical circuits for data communication. Together with the circuit specifications of V.24 and the 25-pin connector and pin assignments of IS 2110, this is compatible with EIA RS 232.

V.29 A recommendation, first agreed in 1976, for full-duplex communication between two analogue fixed-line modems using QAM modulation at 2400 baud to carry synchronous data at 9600 bit/s. Fallback rates of 7200 and 4800 bit/s use 2400 baud at reduced modulations. Multiplexing of 7200, 4800 and 2400 bit/s subchannels up to an aggregate rate of 9600 bit/s is optional. An adaptation of this standard is used for facsimile (fax) transmission.

V.32 A recommendation, first agreed in 1984, for a family of duplex analogue dial-up or fixed-line modems using QAM modulation at 2400 baud to carry data at 9600/4800/2400 bit/s.

V.32bis A duplex modem operating at data rates of up to 14.4 kbit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits, with fallback to 12 kbit/s. This standard was improved on by modem manufacturers to create the V.terbo adhoc standard, signalling at 19.2 kbit/s, as suggestive of a V.32ter standard that never materialized.

V.34 is the ITU-T standard for full-duplex data communications up to 28.8 kbit/s with fallback to lower speeds depending on the remote modem and the conditions of the phone line. This standard was known informally as V.Fast, hence pre-standard modems called V.FC (V.FastClass).

V.34bis A communications protocol for full-duplex datacommunications up to 33.6 kbit/s between two analog modems on dial-up lines.

V.42 Error correction protocol

V.42bis Data compression procedures for data circuit terminating equipment (DCE) using error correction procedures to try to ensure the transfer of error-free data, even over the noisiest telephone lines. Ratified by CCITT in January 1990.

V.44 Data compression protocol

V.90 A digital modem and analogue modem pair for use on the Public Switched Telephone Network (PSTN) at data rates of up to 56 kbit/s downstream and up to 33.6 kbit/s upstream, using PCM encoding downstream, and QAM encoding upstream. V.90 Mode 2 used PCM upstream as well. This standard was known informally as V.Last and some pre-standard modems indicate V.Last compatibility or upgradeability. Prior to the arrival of the standard, there were two competing industry standards for 56 kbit/s downstream signalling, X2 and K56flex. K56flex itself is a merged standard of K56 and 56flex.

V.92 A digital modem and analog modem pair for use on POTS at data signalling rates of up to 56 kbit/s downstream and up to 48 kbit/s upstream, using PCM encoding both ways, supporting Modem-on-Hold technology. This is a development of V.90 Mode 2.