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MODEM [modulator/demodulator], a device for converting digital  information into a form suitable for transmission over a telephone line,  radio link, or other channel. The device is a combination of a modulator and a demodulator, hence the name modem. The modulator (or transmitter) portion converts binary digits (bits) into an outgoing signal that is compatible with the communication channel and sends the signal over the channel. The demodulator (or receiver) portion converts an incoming signal from the channel into bits. A modem links two or more  computers by translating data signals for transmission over telecommunication links.

It is a device that accepts a serial stream of bits as input and produces a carrier modulated by one (or more) of these methods (or vice versa) is called a modem (for modulater-demodulater). The modem is inserted between the (digital) computer and the (analog) telephone system.


Working of Modem

Modem allows a computer or terminal to transmit data over a standard telephone line. It converts digital pulses from the computer to audio tones that an analog telephone line is set up to handle and vice versa..

The term usually refers to 56 Kbps modems (V.92, V.90), the current top speed, or to older 28.8 Kbps modems (V.34). The term may also refer to higher-speed cable or DSL modems or to ISDN terminal adapters, which are all digital and technically not modems.

 Modem is used to transmit and receive digital data over a communications line normally used for analog signals. A modem attached to a converts digital data to an analog signal that it uses to modulate a carrier frequency. This frequency is transmitted over a line, frequently as an audio signal over a telephone line, to another modem that converts it back into a copy of the original data.

Synchronous data transmission uses timing signals in the data stream along with transmitted bits of uniform duration and interval. This permits the receiving modem to ignore spurious signals that do not conform to the anticipated signal. Asynchronous data transmission relies instead on various error-correcting protocols. The 8-bit parity protocol, for example, transmits a control bit after every 8 data bits that indicates whether the sum of the data bit values was odd or even. Although most modems are either of the synchronous or asynchronous variety, some employ both methods of communication.

Modem is usually already built into modern personal computers, provides "connectivity." Modems permit two computers to communicate by telephone lines in order to access databases, transmit files, upload and download fax transmissions, and send and receive e-mail. Early transmission speeds using this equipment were relatively slow—300 bps. Most modems sample 2400times/sec and focus on getting more bits per sample.

Some modems now operate at speeds of 56,000 bps and have error-checking and data-compression features. In many areas, connections to the Integrated Services Digital Network (ISDN) are available; with such connections, modems may operate at speeds as high as 128,000 bps.

 The most common use is to change analog signals into a form that can be manipulated by a digital computer, as in data communications; a modem, or data set, is a device that converts the digital signals produced by computers and terminals into analog signals that telephone circuits are designed to carry and then back to digital signals at the other end of the communication link.

For the home user, the equipment required includes a computer terminal, a telephone, and a modem, which enables the terminal and the database (usually some type of search-service system) to intercommunicate.

 A personal computer must be equipped with a modem to send and receive data over twisted-pair telephone lines. A modem converts the binary (two-level) data stream from the computer into a multilevel stream for transmission; the multilevel modulation makes it possible to transmit data at rates much greater than the 3-kHz bandwidth of a telephone channel. The multilevel signal is more vulnerable to errors from noise, however, which blurs the distinction between adjacent levels. Advanced modems reduce noise error in received multilevel signals by reducing distortion in pulses before extracting data from them.

Using a modem, cellular phones can also communicate data from laptop computers (for example, access the Internet for electronic mail and World Wide Web pages) and send and receive short text messages by facsimile (fax) transmission.

All advanced modems use a combination of modulation techniques to transmit multiple bits per baud.

 Baud- measure of the rate at which signals are transmitted over a telecommunications link. It is equivalent to the number of elements or pulses transmitted in one second, e.g., in computer input/output, 2400 baud equals 2400 bits per second (bps) if each pulse encodes one bit (either 0 or 1). Many modems permit the encoding of several bits per baud. A 9600-bps modem that operates at 2400 baud sends four bits per baud by using a range of 16 tones (representing the four-bit combinations 0000 to 1111) to transmit data, and data compression can boost the effective transmission rate even higher. Because of this, the data transfer rate of a modem is measured in bits per second. During each baud, one symbol is sent. Thus, an n-baud line transmits n-symbol/sec.


Uses of Modems

          Modems are widely used in homes, retail stores, and business offices. External modems attach to the device being used; internal modems are imbedded in the device. Facsimile (fax) machines use modems to transmit and receive scanned images of documents. Modems connect personal computers  with Internet service providers to access the Internet and the World Wide Web. Point-of-sale terminals in stores and at gasoline pumps use modems to dial up central computers to verify the validity of credit cards. Modems have even been installed in vending machines to report sales and inventory levels to a company’s home office. Modem technology is used in pagers to receive numeric and text messages transmitted by radio, and in digital cordless cellular telephones to transmit and receive digitized speech. Very high speed modems have been developed for home, portable, and automobile receivers to accept digital radio and TV broadcasts with compact-disk quality sound and pictures. (In traditional broadcast systems, the receivers contain the demodulators and the broadcast centers contain the modulators. Digital satellite receivers also contain a low-speed modulator for communicating with the service provider by telephone line to report the monthly use of pay-per-view television shows.)

Modems that simultaneously send and receive, such as those in cellular phones and computers, are known as full duplex modems. Half-duplex modems, such as those in fax machines, transmit or receive, but not at the same time.




            The simplest modems for telephone lines transmit or receive only 1200 bits per second (bps). These modems employ a technique called frequency-shift keying (FSK), which converts digital information into tones at different frequencies for transmission. For example, the binary digit 0 can be transmitted as a tone of 1000 cycles per second (hertz, Hz), and the binary digit 1 as a 2000-Hz tone. At the other end of the channel, an FSK modem detects the frequency of the received tone and converts it to the appropriate binary digit. In an alternative FSK scheme, groups of bits can be transmitted in this way. For example, the binary bits 00 can  be sent as a 1000-Hz tone, 01 as 1300 Hz, 10 as 1600 Hz, and 11 as 1900 Hz. Each group of bits is called a symbol.

Transmission rates are also expressed in terms of baud; named after the French telegrapher Mile Baudot (1845–1903), a baud is a unit equal to the number of signal events per second. In binary signals, 1 baud equals 1 bps.

Modem transmission rates are limited by the tendency of transmission media to blur the distinction between bits. A transmission channel can weaken the signal, making frequency differences difficult to detect by a distant modem. Some channels may also introduce frequency shifts (distortion) that cause errors. Engineers continue to develop coding, compression, and modulation methods that boost transmission rates.

Higher-speed modems (1200 to 33,600 bps) use such techniques as quadrature amplitude modulation (QAM), which encodes symbols into changes in amplitude and phase of a tone. (A phase change is a change between sine and cosine waveforms, which differ by 90° in phase.) For example, a 28,800-bps QAM modem encodes a symbol of 8 bits into one of 256 possible amplitude and phase combinations and transmits the symbols at a rate of 3600 times per second.

These multilevel modems correct errors that may result from signal impairment in the transmission process. They add extra bits (redundant data) to the signal so that the receiving modem can recover bits that have been incorrectly received because of attenuation, noise, or distortion. The modems also compress data to increase the transmission rate; for example, data segments composed of identical bits or of repetitive patterns can often be transmitted as a single symbol rather than many symbols. The receiving modem is programmed to recognize the compressed data and expand it into the proper sequence of bits. The higher the rate, the faster large quantities of data can be transmitted and the more realistic the appearance of sound and video.

Modems provide much higher transmission rates in the Integrated  Services Digital Network (ISDN), optical fiber links, coaxial cable systems, and digital radio and television broadcasting, where the transmission media have much greater capacity than telephone lines. In hybrid optical fiber–coaxial cable systems, for instance, modems transmit and receive at 30 million bps (or megabits per second).

To increase the effective data rate further, many modems compress the data before transmitting it, to get an effective data rate higher than 33,600 bps. On the other hand, nearly all modems test the line before starting to transmit the user data, and if they find the quality lacking, cut back to a speed lower than the rated maximum. Thus, the effective modem speed observed by the user can be lower, equal to, or higher than the official rating.

All  modern modems allow traffic in both directions at the same time (by using different frequencies for different directions). A connection that allows traffic in both directions simultaneously is called full duplex. A connection that allows traffic either way, but only one way at a time is called a half duplex. A connection that allows traffic only one way is called simplex.


New Developments


Modems introduced in 1997 were designed to eliminate another major source of error and serious speed limitation—quantization noise, which occurs when an analog signal is sampled for conversion into a digital signal. These modems are intended expressly for fast downloading of files into personal computers from the Internet and the Web. They receive at 56,000 bps in the downstream direction (Internet service provider to personal computer) because the telephone company accepts pulse code modulation (PCM) signals directly from the Internet service provider, without subjecting them to the usual digital-to-analog conversion. These modems are limited to about 30,000 bps in the upstream direction (personal computer to Internet service provider) because an analog-to-digital conversion is unavoidable in this direction. The asymmetrical rates are not a serious limitation for many users, since the bulk of Internet traffic is in the downstream direction.




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