When two computers use telephone lines to transmit data signals, what are the necessary equipment?

Modem is what we often call modem, which is actually the abbreviation of modulator and demodulator. It is called modem in Chinese. The information in the computer is a digital signal composed of "0" and "1", but only analog electrical signals are transmitted on the telephone line (analog signals are continuous and digital signals are discontinuous).

Therefore, when two computers want to transmit data through telephone lines, a device is needed to convert the digital to analog. This digital-to-analog converter is the modem we are going to discuss here. When the computer sends data, the digital signal is first converted into the corresponding analog signal by the modem. This process is called "modulation" and also called D/A conversion.

Before the modulation signal is transmitted to another computer by telephone carrier, the modem of the receiver is also responsible for restoring the analog signal to a digital signal that can be recognized by the computer. This process is called "demodulation" and also called A/D conversion. It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that the remote communication between two computers is realized. ?

Extended data

fundamental principle

The average person's voice frequency range is 300-3400 Hz. In order to transmit voice signals in ordinary telephone system and allocate a certain bandwidth to it on the line, the international standard takes 4KHz as the bandwidth occupied by a standard telephone channel.

In this transmission process, a voice signal is input at a frequency of 300-3400 Hz, and the phone of the sender converts this voice signal into an analog signal, which is converted by a frequency division multiplexer.

In this way, multiple analog signals can be transmitted on the line at the same time, and after reaching the receiving end, they will be restored to the original analog signals in this frequency band through a frequency reduction process, and then the analog signals will be converted into sound signals by the receiving phone. ?

The information in the computer is a digital signal composed of "0" and "1", but only analog electrical signals can be transmitted on the telephone line. If no measures are taken to transmit digital signals through analog channels, huge errors (distortions) will inevitably occur. Therefore, in order to transmit data on the ordinary telephone network, the digital signal must be converted into the audio frequency spectrum required by the original design of the telephone network (that is, 300 Hz-3400 Hz).

Modulation is to control a parameter of carrier waveform with baseband pulse to form a signal suitable for line transmission.

Demodulation means that when the modulated signal reaches the receiving end, the analog signal transformed by the modulator is removed from the carrier and restored to the original baseband digital signal.

Modems can also be used to convert audio signals into high-frequency signals and vice versa. Therefore, another purpose of modulation is to promote line reuse to improve line utilization.

According to the three main parameters of carrier signal, modulation methods can be divided into three types: amplitude modulation, frequency modulation and phase modulation.

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