Modem

One of the most popular areas of application of a personal computer is the work in the global Internet. To it, the computer is connected via a regular telephone or a special line using a device called a modem (Modulator + Demodulator). The digital data coming to the modem from the computer is converted into it by modulation into a special continuous signal, which is sent to the transmission line. The modem-receiver carries out the inverse transformation of the signal (demodulation) and forwards the restored digital data to its computer. The data transfer rate of modems is 33,600 or 56,000 bits/s and depends on the transmission protocol that the modem supports.

The first modem for personal computers was the Micromodem II device for the Apple II personal computer, released in 1979 by Hayes Microcomputer Products. It cost $380 and worked at a speed of 110/300 bps. In 1981, Hayes released the 300Mbps Smartmodem modem, the command system of which (Hayes-command) became the de facto standard in the industry.

The modem can be designed as a separate device, connected to the computer through one of its standard serial inputs or as an internal device located inside the computer's system unit. Developing digital data transmission technologies that require significantly higher transmission rates and communication quality require the use of digital modems. The digital modem is not really a modulator-demodulator of signals, since both the input and the output have a pulse signal. Such modems are available in different modifications for specific digital networks and the transmitter speed ranges from 300 Kbps to 2-5 Mbps. Cellular modems are also available for work in the cellular communication system and radio modems that receive and transmit within a line of sight a pair of modems and operate on ultrashort waves.

According to the principle of operation, the modems are divided into: hardware modems - all signal conversion operations, the support of physical exchange protocols is built into the modem by a calculator (for example, using a DSP or microcontroller). Also in the hardware there is a ROM in which the firmware controlling the modem is recorded; software modems (soft modems) - all operations for signal coding, error control and protocol management are implemented programmatically and are performed by the central processor of the computer. The modem contains only input / output analog circuits and converters (DAC and ADC), as well as an interface controller; semi-program - modems in which part of the modem functions is performed by the computer to which the modem is connected.

Modern telecommunication technology ADSL (Asymmetric digital subscriber lines), which allows data to be transmitted at high speed via ordinary telephone lines, has a difference in the speeds of data exchange in directions to the subscriber and vice versa. The asymmetric type modem is ideal for modern applications where the amount of data transmitted to a subscriber in the overwhelming majority of cases far exceeds the amount of data going from the subscriber to the side of the network. Using the ADSL modem it is possible to transmit data at a speed of up to 8 Mbit/s in the direction of the subscriber and up to 1 Mbit/s in the reverse direction. Using it, no additional cable is required, since an existing telephone line is used to access the network, and the phone is free during connection to the network.