Analog Modem


Definition of analog modem in Network Encyclopedia.

What is an Analog Modem?

Analog Modem is a modem used for asynchronous transmission of data over Plain Old Telephone Service (POTS) lines.

 Analog Modem (external version, serial interface)
Analog Modem

How it works

The word «modem» stands for “modulator/demodulator,” which refers to the fact that modems convert digital transmission signals to analog signals and vice versa. For example, in transmission, an analog modem converts the digital signals it receives from the local computer into audible analog signals that can be carried as electrical impulses over POTS to a destination computer or network.

To transmit data over a telephone channel, the modem modulates the incoming digital signal to a frequency within the carrying range of analog phone lines (between 300 Hz and 3.3 kHz). To accomplish this, multiplexing of the digital signal from the computer with a carrier signal is performed. The resulting modulated signal is transmitted into the local loop and transmitted to the remote station where a similar modem demodulates it into a digital signal suitable for the remote computer.

However, this basic process can transmit data only at speeds of about 1200 bps. To achieve the much higher speeds of today’s modems, advanced technologies must be applied, including echo canceling, training, data compression, and special modulation algorithms such as quadrature amplitude modulation (QAM). Using these technologies, modem speeds of 56 Kbps are now common.

Bell Labs in the 1960s and 1970s originally formulated modem standards, but after the breakup of Bell Telephone, the task of developing modem standards was taken over by the International Telegraph and Telephone Consultative Committee (CCITT), which is now called the International Telecommunication Union (ITU). According to ITU specifications, modem standards are classified by a series of specifications known as the V series.

See also: Modem

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