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Front of a V.90 modem

The, or the modem (from Mo dulator and The odulator) is a communication device to digital signals over long transmission paths exchange between two digital devices. The sending modem modulates a digital signal onto a carrier frequency in the high frequency range , and the receiving modem uses demodulation to recover the original information . Modems of the newer DSL standard work with higher carrier frequencies and greater bandwidths on the telephone line than the models based on the older "narrowband" standard. The term modem was more present in the 1970s to 1990s than later, because back then, using a modem was synonymous with going online , i.e. networking your computer with others.

Schematic diagram for data transmission using a modem

History of the modem in Germany

At the Deutsche Bundespost , modulation methods were used early on in broadcast technology and later in carrier frequency technology (wireless modulation). Use in wireline communication (wireline modulation) began in Germany in 1966 with the introduction of the first modems by the Deutsche Bundespost.

Postmodem D 200 S (above) with MODAP modem adapter from DEC

These devices had a sheet metal housing with the external dimensions of about 60 cm × 30 cm × 20 cm and were intended for wall mounting. They were called the D 1200 S transition modem . The number 1200 stood for the maximum transmission speed in bit / s, the letter S for "serial transmission" (to differentiate between modems with parallel transmission ). As a direct successor there were modems as a desktop device called Unity Post modem D 1200 S and Unity Post Modem D 200 S . These corresponded to the ITU-T recommendations V.23 (1200 bit / s, half duplex operation, options for an auxiliary channel and synchronous transmission using a clock module) or V.21 (asynchronous duplex transmission with 200 or 300 bit / s).

Until the mid-1980s, in Germany, as in many other countries, it was not allowed to connect modems other than Post's own to the telephone line. The modem counted as a network termination , which, like the line, also belonged to the telephone network and thus to the sovereignty of the state Deutsche Bundespost. However, when private modems were increasingly used because they were considerably cheaper, faster and more user-friendly than the Post Office modems, the modem monopoly could no longer be maintained and was lifted in 1986.

The Deutsche Bundespost also demanded that the devices connected to their modems were approved by the Central Telecommunications Office . Since this was difficult to achieve, especially with larger computer systems, some manufacturers switched to building so-called modem adapters . Apart from a few light-emitting diodes for displaying the modem control signals, these devices were practically inoperable, but were approved without any problems (see picture on the right with the MODAP from DEC under the D 200 S 03 ). The number "03" in the name indicates that this modem has been technically modified. Often existing devices were retrofitted by the Deutsche Bundespost in the Telecommunications Central Office in Elmshorn .

Modulation techniques

  • Telephone: ITU-T standards (until 1992 CCITT )
  • Cable: half duplex (one pair of wires), duplex (two pairs of wires)

Modem variants

Dial-up modem

At the end of the 1980s there were the first telephone modems as a further development of the acoustic coupler . Walking speeds of 300 baud were common here  . With the first modems, the step speed or symbol rate , expressed in the unit of baud, was still identical to the data transmission rate , often expressed in this context as the bit rate in bit / s or bps. This equality at the time resulted in the often incorrect equation of the symbol rate and the bit rate: While the symbol rate reached an upper limit of approx. 3000 baud due to the physically limited bandwidth of the telephone connection line, the achievable bit rate could be achieved through more efficient digital modulation methods and improved channel coding in the following years can be increased up to the theoretical limit of the channel capacity of the telephone connection line. The first modem with a bit rate of 2400 bit / s was presented at CeBIT in 1990 , followed a year later by the first post-approved modem with 9600 bit / s according to the ITU-T standard V.32, which uses data compression with uncompressed source data up to 38400 bit / s.

However, the Deutsche Bundespost only allowed the use of their approved modems on the German telephone networks, which could either be rented or bought monthly. Since these prices clearly exceeded the prices of other modem manufacturers, modems from other manufacturers were sometimes operated illegally on the German telephone network.

Telephone modem

Stack of different dial-up modems
Elsa-Microlink-33.6TQV, dialing in to T-Online in 2010

Telephone modems for analog landline connections are adapted to the particularities of the telephone network. Only the frequency band from 300 Hz to 3400 Hz is available for transmission. Connections to a digital exchange extend this band to 0 to 4000 Hz. The first modems were standardized internationally by the ITU-T in 1964; it was a question of devices with a transmission speed of 300 bit / s (V.21, duplex) and 1200 bit / s (V.23, half duplex).

The first telephone modems for the computer mass market were not yet electrically connected to the telephone network, but instead established the connection using suitable sound transducers - the so-called acoustic couplers - via the receiver of an existing telephone. This was mechanically attached to the modem. Background noise quickly led to transmission errors, which is why the shells of the acoustic couplers were often wrapped in cushions. Transmission was carried out at 110 baud, later 300 baud with frequency shift keying (FSK). Since the data to be transmitted were typically coded in accordance with the standards for serial data transmission on lines ( V.24 or RS-232 ) and the start, stop and parity bits were also transmitted, the effective user data rate was lower. These early modem types had to be subjected to a type approval test by the German Federal Post Office , which led to high sales prices.

By implementing modems using digital signal processing , the achievable bit rates could be increased further. During this time, the telecommunications company Hayes implemented today's de facto standard for modems, the so-called AT command set , which was later adopted and expanded by numerous manufacturers. More powerful modulation methods such as phase shift keying (PSK) and quadrature amplitude modulation (QAM) laid the foundation, followed by methods such as trellis-coded modulation (TCM), which is a first form of coded modulation and was also used in other data transmission methods in the following years. By continuously measuring and negotiating the maximum possible bit rate for the line and the remote station, these transmission methods enable a data transmission speed that is optimally adapted to the respective situation.

Due to the increasing efficiency of the digital signal processors used in modems and in particular the transmission methods, for example through compression, there are further performance features that go beyond the basic framework of the modem concept. The fax function ( fax modem ) and the answering machine function (voice modem) should be mentioned in particular . Many models contain integrated SIP adapters for IP telephony (VoIP). There are also modems with integrated fax ( T.38 ) and e-mail protocols ( POP3 / SMTP ) as well as Short Message Service functionality (SMS) for autonomous sending and receiving of messages.

In the analog telephone network, in which the transmission bandwidth is limited to 3.1 kHz, according to the Shannon theorem, the maximum data transmission rate is limited to 30 to 40 kbit / s with normal line quality. A download data rate of 56 kbit / s ( V.90 , V.92 ) is only possible in the telephone network with an analog connection that is coupled to a digital exchange . The modem is synchronized with the converter clock of the exchange. The sending remote station, e.g. B. a dial-in node , but must be fully digital. However, the upload speed remains limited in the same way.

Modem connections via the telephone network are often referred to as dial-up connections , since a dialing process is necessary before the data connection can be established. Examples of dial-up modem connections are e.g. B. BTX , Datex-P or analog dial-up to the Internet via an Internet provider . The first dial-in at the end of the 80s with 300 baud modems, e.g. B. at the WDR, but took place by entering the telephone number. Dialing in via ISDN differs from this in that everything is digital, i.e. it is no longer modulated and no modem is involved. However, the misnomer ISDN modem is often used for an ISDN connection device or an ISDN card. The first ISDN connection options were presented in 1992 at CeBit.

While teletyping tasks were mainly delegated to modems in the early phase, the mailbox scene developed later , which worked with network protocols such as Kermit or Z-Modem . In addition, X.25 was used as a network layer in the commercial area . Today, IP for data exchange via telephone modem dominates the network layer.

Fax modem

Internal fax modem (FerrariFax)
V.34 data / fax modem as PCMCIA card for notebooks

In addition to the usual function of data transmission, fax modems also have a protocol for the transmission of faxes . Most fax modems can transmit faxes at 14400 bps. The transmission takes place via ordinary telephone lines.

With the help of a fax modem, a computer can be used as a fax machine - often only for sending, but receiving is also possible.

Callback modem

There were also call-back modems for remote maintenance of mainframes and servers. A phone number was programmed in when the modem was configured. If it is selected from the outside, a password is requested. After entering the password, the modem "hangs up" and dials back the number stored in the configuration. If a hacker dials this modem, he may be able to enter the password correctly, but is then "disconnected" because the callback goes to the legitimate IT employee who owns the configured phone number.

Softmodem / Winmodem

Soft modems are special modems in which some parts of the hardware functionality have been outsourced to the device driver for cost reasons . Often the sound card or the corresponding assembly on the motherboard takes on the actual modem functions; physically only the contacts to the telephone network are available. Winmodems are soft modems that were specially developed for Microsoft Windows . They mostly only work with the Windows versions that were current at the time they were manufactured. On alternative operating systems such as B. Linux they could not be used at first, or only with difficulty, since there were initially no drivers.

Leased line modem

Leased line modem

In contrast to telephone modems, leased line modems offer a point-to-point connection. So you are always permanently connected to the same remote station; there is no need to dial before the connection is established.

Leased lines are mostly used by banks and large companies. Comparable, more cost-effective solutions for the home area are usually only quasi-leased lines, as the provider often provides for a forced disconnection of the connection in the daily cycle.

While the bandwidth in the analog telephone network is limited due to technical and economic considerations, the normal - but partly additionally shielded - two-wire telephone lines allow higher bandwidths in this case.

DSL modem

DSL modems enable much higher transmission rates. In order to be able to transmit the conventional telephone signal and the additional high-speed signal simultaneously on the same line, the high-speed signal is up-modulated into a higher frequency range before both signals are placed on the same line. On the receiver side, the signals are separated again by a so-called splitter (a crossover network) and the high-speed signal is fed to the demodulation in the local modem.

Common data rates with ADSL reach up to 25 Mbit / s in the download. The upload is limited to a lower value with ADSL. The data rate is subject to limitations for technical reasons if the distance to the exchange is great. In the exchange it is usually connected to a purely digital network backbone .

Fiber optic modem

Fiber optic modems (ONT Optical Network Termination) allow significantly higher transmission rates than DSL modems. A conventional telephone signal cannot be transmitted via fiber optics at all (voice communication only takes place via VoIP), the fiber optic modem only provides a data connection. The AON or PON technologies (technology used in Germany) are used for this purpose .

Fiber optic modems are typically installed separately on the WAN side in front of a router, as there are not yet many routers with integrated fiber optic modems. The connection is then not established by the fiber optic modem itself, but by the router, which communicates with the exchange, while the fiber optic modem only converts the WAN-side optical signals into the LAN-side electrical signals and vice versa.

Cable modem

Cable modem

Cable modems for data transmission over broadband cable networks can be seen as a type of dedicated line modem . In the past, a telephone modem was also used in some cases to enable a return channel. This technology is no longer necessary today, as providers such as Vodafone or Unitymedia are now expanding their cable network with a return channel. Today, cable modems offer higher data rates than data transmission using DSL modems. The cable network operators usually also implement telephony via the cable modems.

Power line modem

The modulation of data signals on power lines is also possible. A special but simple application is the so-called ripple control technology of the energy supply companies , with which z. B. the switching of the electricity meter between day and night electricity is accomplished.

In the recent past, attempts have been made to market high-speed modems (mostly up to around 1 Mbit / s) under the collective term PLC (PowerLine Communication), which never got beyond the test phase and ultimately did not affect the price-performance ratio or the transmission security of DSL technology could reach. The RF interference generated by this technology is also not without problems. The technology typically uses numerous individual carrier frequencies in the range between 500 kHz and 10 MHz for modulation and demodulation of the user data.

Radio modem

Radio modem

Radio modems are often integrated in other devices - for example in cell phones - and the respective channel is used multiple times (e.g. in ringer systems for voice and data).

In the radio sector there are numerous applications with which telecontrol or remote control tasks can be solved by modulation. One example is radio remote controls in model making .

The GSM , UMTS and LTE networks use modulation methods with digital modulation for data transmission by radio , while the previous system, the C network, still used analog modulation . The useful signal is digital, which is converted into a radio signal by the modulator . This makes the transmission significantly less sensitive to interference, since the digital useful signal, in contrast to the continuous analog signal, can only assume known, discrete values ​​that the receiver can still deduce in the event of (not too strong) interference in the transmission.

Satellite modem

ipstar satellite

Satmodems or satellite modems are used for data transmission via a communications satellite. To do this, satellite modems convert an input bit stream into a radio signal and vice versa. There are a wide variety of satellite modems available, from inexpensive devices for internet access at home to expensive multifunctional devices for business use. Devices that only have a demodulator for downloading data via satellite are called satellite modems. In this case, the uploaded data is transmitted through a conventional telephone modem, DSL modem or ISDN.

Modem designs

UMTS modem with USB connection from Huawei
RS-232 modem Smarty 28.8 TI from Dr. Neuhaus
USB modem from ELSA MicroLink 56k (1999)
Different designs of the same modem

The typical PC modem is an external device in flat design with a serial interface. It is usually connected to a computer via RS-232 or increasingly via USB . The power supply is normally provided by a plug-in power supply unit or later via the USB interface. To display the status, there is often a line of light-emitting diodes on the front, which indicate the status of the interface lines.

In the professional sector there is a design that allows installation in 19-inch housing. In the industrial sector, a housing for DIN rail mounting in the control cabinet has become established for modems .

An alternative design for modems is the plug-in card form for a standardized slot (e.g. PCI for stationary PCs , Mini-PCI or PCMCIA for notebooks ) or a proprietary socket. There is usually an additional encapsulation or shielding to avoid mutual interference between PC and telephone line. The status of the modem is usually displayed by a PC software.

Modem hardware manufacturer

Web links

Commons : Modem  - collection of pictures, videos and audio files
Wiktionary: Modem  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Duden | Modem | Spelling, meaning, definition, origin. Retrieved June 21, 2019 .
  2. ^ Slides for teaching, as of April 1, 1974, Central Telecommunications Office, Section B 34, slide archive. Series 64.01, data transmission, DK 621.391, 11.1966 Description on the slides: DBP, FTZ / FSA Dmst, Urh: FTZ VIIIB / 1966
  3. ^ "Yes, but" on the new modem concept of the Post - Liberalization: Board of Directors against Prejudice. Computer Week, July 25, 1986
  4. Albrecht Darimont: Btx and DFU on the PC (see literature), page 67
  5. Activation of an existing callback option
  6. ^ A. Wittneben: Specialist internship digital modulation. ETH Zurich ( Memento from January 11, 2006 in the Internet Archive )
  7. Modems from Keymile ( Memento from May 27, 2014 in the Internet Archive )
  8. Snapdragon Modem Comparison | Qualcomm . In: Qualcomm . ( [accessed January 24, 2018]).


  • Karl-Heinz Schmidt (Hrsg.): Terminals on the analog telecommunications network. Rev Deckers's Verlag, Heidelberg 1992, ISBN 3-7685-4889-9 , pages 295-334.
  • Albrecht Darimont: Btx and DFU on the PC Friedr. Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 1992, ISBN 978-3-528-05175-4 , pages 65-92