Modem (modem) is a device that converts "digital signals" and "analog signals" into each other; it converts digital signals into analog signals through modulation at the sending end, and then demodulates the analog signals at the receiving end. The signal is converted into a digital signal. A modem translates a computer's digital signals into analog signals that can be sent along an ordinary telephone line. These analog signals can then be received by another modem at the other end of the line and translated into a language that the computer can understand.
The operating environment of this tutorial: Windows 7 system, Dell G3 computer.
Modem refers to "modem", commonly known as "cat", which is a kind of computer hardware.
The main function of Modem is to convert between "digital signal" and "analog signal". Today's computers (computers) process digital signals, while telephone lines can only transmit analog signals. This is why you need to use a modem when dialing up the Internet through telephone lines (ADSL).
Modem can translate the computer's digital signals into analog signals that can be transmitted along ordinary telephone lines, and these analog signals can be received by another modem at the other end of the line. And translated into a language understandable by computers.
Basic introduction
Modem is actually the abbreviation of Modulator and Demodulator. It is a method of modulating the digital signal to be transmitted onto the carrier wave or separating the digital signal from the carrier wave.
The so-called modulation is to convert digital signals into analog signals transmitted on telephone lines; demodulation is to convert analog signals into digital signals, collectively called a modem. It converts digital signals into analog signals through modulation at the transmitting end, and then converts analog signals into digital signals through demodulation at the receiving end.
When the computer sends data, the Modem first converts the digital signal into the corresponding analog signal. This process is called "modulation". Before the modulated signal is transmitted to another computer through the telephone carrier, the receiver's Modem is responsible for restoring the analog signal to a digital signal that the computer can recognize. This process is called "demodulation." It is through such a digital-to-analog conversion process of "modulation" and "demodulation" that remote communication between two computers is achieved.
It is mainly used for data communication in telephone networks and plays an important role in network interconnection and remote access.
The current modem is very mature in terms of adaptive, data compression and network coding modulation technology. The data transmission of each voice channel can be as high as 56kbps, which is basically close to the theoretical maximum rate of telephone lines.
Basic Principle
The average voice frequency range is 300-3400Hz. In order to transmit the voice signal in the ordinary telephone system, a certain bandwidth is allocated to it on the line. , the international standard takes 4KHz as the frequency bandwidth occupied by a standard voice channel. During this transmission process: the voice signal is input at a frequency of 300-3400Hz. The sender's telephone converts the voice signal into an analog signal. This analog signal is changed through a frequency division multiplexer so that it can be transmitted simultaneously on the line. When the multi-channel analog signal reaches the receiving end, it goes through a frequency decoding process to restore it to the original analog signal in the frequency range, and then the receiving telephone converts the analog signal into a sound signal.
The information in the computer is a digital signal composed of "0" and "1", but what is transmitted on the telephone line can only be an analog electrical signal. Using analog channels to transmit digital signals without taking any measures will inevitably lead to large errors (distortion). Therefore, when transmitting data on ordinary telephone networks, the digital signals must be converted into the audio frequency spectrum required by the original design of the telephone network (i.e. 300Hz). -3400Hz).
Modulation is to use baseband pulses to control a certain parameter of the carrier waveform to form a signal suitable for line transmission.
Demodulation is to remove the carrier from the analog signal transformed by the modulator and restore it to the original baseband digital signal when the modulated signal reaches the receiving end.
Using a modem, you can also convert audio signals into higher frequency signals and convert higher frequency signals into audio signals. Therefore, another purpose of modulation is to facilitate line multiplexing in order to improve line utilization.
Based on the three main parameters of the carrier signal, the modulation methods can be divided into three types: amplitude modulation, frequency modulation and phase modulation.
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