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.
The basic principle of
The average person's voice is 300-3400 hz frequency range, for the voice signal transmission in the ordinary telephone system, online the bandwidth of the assigned to it must be on the road, international standard take 4 KHZ for a (road occupied bandwidth. 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.
Modem chip as the processor of the brand as the product of different manufacturer, one of the biggest number was ROCKWELL chip, it accounts for some 70% of the global market share, Intel is the same as the status and processor on the market, the domestic most external Modem product USES ROCKWELL chip. The second is TI chip, the famous USR "big black cat" with TI chip. In addition, there are Curiss Logic products, but the use of this chip is relatively small. All in all, the ROCKWELL chip's Modem has better performance and stability than the Modem with other chips, but the USR's "big black cat" is a special case. TI chips in international markets are trading lower than ROCKWELL chips.
Notice the choose and buy
The Modem of foreign country is not necessarily better than domestic, many people like to buy foreign brand Modem. In fact, the majority of the Modem is Taiwan province OEM production, only in the material selection is more exquisite. Domestic Modem products are designed to take into account the actual situation in the country, the price is cheaper, and in performance is not inferior to foreign products. Don't focus on appearance, but focus on inner qualities. The Modem shell of the market is getting more and more flowers and colors are getting more and more colorful, but you must not be deceived by your appearance when buying. Many of the so-called series products introduced by manufacturers, vary widely in price, but actually use the same circuit board, just different shell. Buying a Modem doesn't have to cost hundreds of dollars to cover the shell.
The external Modem of USB interface, the external Modem of USB interface is mostly soft "cat", it is compact, does not need external power source, and can realize hot plug, the advantage is more. But many USB Modem connection rate is too low, not even 50K. And some USB Modem can have problems while dialing, so it's better to buy a serial interface with a Modem. Low price Modem performance is not necessarily poor. The price of the cheapest 56K Modem is only about 350 yuan, and superman himself is using only 300 yuan of tp-link 56K "cat". For a year or so, it's definitely not as frequent as the legend. The only problem is that the connection speed is only 48K