Modem is a kind of computer hardware, it can translate computer digital signal into the analog signal transmitted over regular telephone lines, and the analog signal can be line another modem at the other end of the receiving, and translated into computer language can understand.This simple process completes the communication between two computers.
A person's voice is 300-3400 hz frequency range, for transmission of voice signal in ordinary telephone system, the online road assign it a certain bandwidth, international standard take 4 KHZ for a road (the band width of the occupied.In the transmission process: speech signal to a 300-3400 hz frequency input, the sender's telephone transform the voice signal into analog signals, the analog signal through a frequency division multiplexer to change, can transmit multi-channel analog signal at the same time on line, later when they arrive in the receiving end after a XiePin process to restore it to its original frequency range of the analog signal, again by the receiving party telephone converts analog signals into sound signal.
The information in the computer is composed of "0" and "1" to form a digital signal, while the transmission on the telephone line can only be analog electrical signal.Do not take any measures to use analog channel to transmit digital signals inevitably there will be a big mistake (distortion), therefore, in the ordinary telephone network to transmit data, it must to digital signal conversion to telephone network within the required when the original design of the audio spectrum (that is, 300 hz - 3400 hz).
Modulation is to control the carrier wave waveform with the baseband pulse, forming a signal suitable for the transmission of the line.
The demodulation is that when the modulated signal arrives at the receiving end, the analog signal that has been transformed by the modulator will remove the carrier and return to the original baseband digital signal.
A modem can also convert an audio signal into a higher frequency signal and convert a higher frequency signal into an audio signal.The other purpose of modulation is to facilitate line reuse so as to improve line utilization.
Based on the three main parameters of carrier signal, modulation can be divided into three kinds: amplitude modulation, frequency modulation and phase modulation.
The external Modem is placed outside the chassis and connected to the host by serial communication port.This kind of Modem is convenient and easy to install, and the flashing lights are easy to monitor the working condition of Modem.But external Modem needs to use additional power and cable.
The built-in Modem needs to disassemble the chassis at installation, and the installation of the terminal and COM ports is cumbersome.The Modem takes up the expansion slot on the motherboard, but requires no extra power and cable, and a cheaper price than the external Modem.
Plug-in Modem is mainly used for notebook computers, and the volume is slim.With mobile phone, mobile office can be easily implemented.
A mobile Modem is equivalent to focusing a set of Modem on a box or casing and is powered by a unified power supply.Machine - frame Modem is mainly used in Internet/Intranet, telecommunication bureau, campus network, financial institution and other network center machine room.
Although the use of the ROCKWELL chip, but some "cat" love to drop the line, some "cat" rate cannot achieve nominal performance.The difference in performance of Modem is due to a variety of reasons, first of all the chip of the Modem used, followed by the selection and circuit design.Generally, the Modem circuit adopts the design of the common plate recommended by the chip manufacturer, and the difference in the selection is large.The Modem used by some small factories is used in second-hand components or inferior components of quality, leading to unstable phenomena due to heat and other causes when the Modem works long hours.In order to maintain their reputation, well-known manufacturers generally adopt high reliability components, which exceed small factories in terms of performance and stability.Because of this, the same is also using ROCKWELL's external Modem, the most expensive is 800 yuan, and the cheapest is only a few dozen yuan.