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.
DWDM - Dense WaveLength Division Multiplex
DWDM (Dense Wavelength Division Multiplexing) is the ability to combine a group of optical wavelengths with a single optical fiber for transmission. This is a laser technology used to increase bandwidth on existing fiber optic backbone networks. More precisely, this technology multiplexes the tight spectral spacing of a single fiber carrier in a specified optical fiber in order to take advantage of the achievable transmission performance (for example, to minimize dispersion or attenuation). In this way, under a given information transmission capacity, the total number of optical fibers required can be reduced.
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