Acoustic coupler modems used a telephone handset as the audio medium, with the user dialing the desired number and then pressing the handset into the modem to complete the connection. These systems generally operated at a speed of 300 bits per second.
Categorization for signal modulation based on data and carrier types
Collection of modems once used in Australia, including dial-up, DSL, and cable modems.
A low-frequency message signal (top) may be carried by an AM or FM radio wave.
TeleGuide terminal
Waterfall plot of a 146.52 MHz radio carrier, with amplitude modulation by a 1,000 Hz sinusoid. Two strong sidebands at + and - 1 kHz from the carrier frequency are shown.
The original 300-baud Hayes Smartmodem
A carrier, frequency modulated by a 1,000 Hz sinusoid. The modulation index has been adjusted to around 2.4, so the carrier frequency has small amplitude. Several strong sidebands are apparent; in principle an infinite number are produced in FM but the higher-order sidebands are of negligible magnitude.
USRobotics Sportster 14,400 Fax modem (1994)
Schematic of 4 baud, 8 bit/s data link containing arbitrarily chosen values
V.34 modem implemented as an internal ISA card
V.34 data/fax modem as PC card for notebooks
Dial-up modem bank at an ISP
The Novation CAT acoustically coupled modem
A PCI Winmodem soft modem (on the left) next to a conventional ISA modem (on the right)
DSL modem
Cable modem
A bluetooth radio module with built-in antenna (left)
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An ONT providing data, telephone and television service
Null modem adapter

A modem transmits data by modulating one or more carrier wave signals to encode digital information, while the receiver demodulates the signal to recreate the original digital information.

- Modem

A modem (from modulator–demodulator), used in bidirectional communication, can perform both operations.

- Modulation
Acoustic coupler modems used a telephone handset as the audio medium, with the user dialing the desired number and then pressing the handset into the modem to complete the connection. These systems generally operated at a speed of 300 bits per second.

8 related topics

Alpha

Categorization for signal modulation based on data and carrier types

Symbol rate

Categorization for signal modulation based on data and carrier types

In a digitally modulated signal or a line code, symbol rate or modulation rate is the number of symbol changes, waveform changes, or signaling events across the transmission medium per unit of time.

The term baud rate has sometimes incorrectly been used to mean bit rate, since these rates are the same in old modems as well as in the simplest digital communication links using only one bit per symbol, such that binary "0" is represented by one symbol, and binary "1" by another symbol.

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Orthogonal frequency-division multiplexing

Type of digital transmission and a method of encoding digital data on multiple carrier frequencies.

Type of digital transmission and a method of encoding digital data on multiple carrier frequencies.

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Subcarriers system of OFDM signals after FFT

Based on this feedback information, adaptive modulation, channel coding and power allocation may be applied across all subcarriers, or individually to each subcarrier.

The fact that OFDM can cope with this frequency selective attenuation and with narrow-band interference are the main reasons it is frequently used in applications such as ADSL modems.

Constellation diagram example for BPSK

Phase-shift keying

Constellation diagram example for BPSK
Constellation diagram for QPSK with Gray coding. Each adjacent symbol only differs by one bit.
Conceptual transmitter structure for QPSK. The binary data stream is split into the in-phase and quadrature-phase components. These are then separately modulated onto two orthogonal basis functions. In this implementation, two sinusoids are used. Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal. Note the use of polar non-return-to-zero encoding. These encoders can be placed before for binary data source, but have been placed after to illustrate the conceptual difference between digital and analog signals involved with digital modulation.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.
Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown at the top, and the total combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.
Signal doesn't pass through the origin, because only one bit of the symbol is changed at a time.
Difference of the phase between QPSK and OQPSK
Timing diagram for offset-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the half-period offset between the two signal components.
Dual constellation diagram for π/4-QPSK. This shows the two separate constellations with identical Gray coding but rotated by 45° with respect to each other.
Timing diagram for π/4-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note that successive symbols are taken alternately from the two constellations, starting with the "blue" one.
Constellation diagram for 8-PSK with Gray coding
BER comparison between DBPSK, DQPSK and their non-differential forms using Gray coding and operating in white noise
Differential encoding/decoding system diagram
Mutual information of PSK over the AWGN channel

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).

Historically, voice-band synchronous modems such as the Bell 201, 208, and 209 and the CCITT V.26, V.27, V.29, V.32, and V.34 used PSK.

Modern surface-mount electronic components on a printed circuit board, with a large integrated circuit at the top.

Baud

Common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.

Common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.

Modern surface-mount electronic components on a printed circuit board, with a large integrated circuit at the top.

It is the unit for symbol rate or modulation rate in symbols per second or pulses per second.

Example: Communication at the baud rate 1000 Bd means communication by means of sending 1000 symbols per second. In the case of a modem, this corresponds to 1000 tones per second; similarly, in the case of a line code, this corresponds to 1000 pulses per second. The symbol duration time is 1⁄1000 second (that is, 1 millisecond).

Digital clock. The time shown by the digits on the face at any instant is digital data. The actual precise time is analog data.

Digital data

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 digits.

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 digits.

Digital clock. The time shown by the digits on the face at any instant is digital data. The actual precise time is analog data.

A beacon is perhaps the simplest non-electronic digital signal, with just two states (on and off). In particular, smoke signals are one of the oldest examples of a digital signal, where an analog "carrier" (smoke) is modulated with a blanket to generate a digital signal (puffs) that conveys information.

More recently invented, a modem modulates an analog "carrier" signal (such as sound) to encode binary electrical digital information, as a series of binary digital sound pulses. A slightly earlier, surprisingly reliable version of the same concept was to bundle a sequence of audio digital "signal" and "no signal" information (i.e. "sound" and "silence") on magnetic cassette tape for use with early home computers.

An example of binary FSK

Frequency-shift keying

Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal.

Frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal.

An example of binary FSK

Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone.

Most early telephone-line modems used audio frequency-shift keying (AFSK) to send and receive data at rates up to about 1200 bits per second.

Example of QPSK carrier recovery phase error causing a fixed rotational offset of the received symbol constellation, X, relative to the intended constellation, O.

Demodulation

Extracting the original information-bearing signal from a carrier wave.

Extracting the original information-bearing signal from a carrier wave.

Example of QPSK carrier recovery phase error causing a fixed rotational offset of the received symbol constellation, X, relative to the intended constellation, O.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.

There are many types of modulation so there are many types of demodulators.

For example, in a modem, which is a contraction of the terms modulator/demodulator, a demodulator is used to extract a serial digital data stream from a carrier signal which is used to carry it through a telephone line, coaxial cable, or optical fiber.

Computer simulation, one of the main cross-computing methodologies.

Bit rate

Number of bits that are conveyed or processed per unit of time.

Number of bits that are conveyed or processed per unit of time.

Computer simulation, one of the main cross-computing methodologies.

where n is the number of parallel channels, Mi is the number of symbols or levels of the modulation in the i-th channel, and Ti is the symbol duration time, expressed in seconds, for the i-th channel.

The "connection speed" of a V.92 voiceband modem typically refers to the gross bit rate, since there is no additional error-correction code.