modem

Modem is designed for long-distance transmission of digital signals for data communication over analog channels with limited bandwidth. It is generally composed of several parts such as baseband processing, modulation and demodulation, signal amplification and filtering, and equalization. Modulation is the combination of a digital signal and an audio carrier to produce an audio signal (analog signal) suitable for transmission over a telephone line; demodulation recovers the digital signal from the audio signal.

Modems are generally divided into three types: external, built-in, and PC card. Through the telephone line or the special network cable, the external modem and the computer serial interface; The built-in type modem inserts directly in the computer expansion slot; The PC card type uses the notebook computer, inserts directly in the standard PCMCIA slot.

The performance and speed of the modem are directly related to the speed of transmitting information after networking. Modem rates are 14.4K, 19.2K, 28.8K, 33.6K, and 56K, etc. Currently, the use of 56K is more common. The CCITT recommends that the V.34 standard for modems has the biggest feature of "adaptive rate transmission", that is, in the transmission process, the product has the function of automatically adjusting the transmission rate according to the quality of the local subscriber line, which can make the area Poorly connected network users can also enjoy high-speed connectivity. The V.37 standard has 9600 ~ 128000bps signal rate, four-wire full-duplex communication, synchronization, single-sideband modulation and 60 ~ 108kHz base group circuit and other functions; v.42 standard with 56000bps signal rate, full-duplex communication Ways, synchronization, and data compression and error control technologies.