In the analog carrier communication system, frequency division multiplexing is usually used to improve the transmission capacity of the system and make full use of the bandwidth resources of the cable, that is, the signals of several channels are transmitted simultaneously in the same cable, and the signals of each channel can be filtered out by using band-pass filter according to the different carrier frequencies at the receiving end. Similarly, frequency division multiplexing can be used to improve the transmission capacity of optical fiber communication system. Demultiplexer (equivalent to optical bandpass filter) is used at the receiving end to separate the optical carriers of each signal. Because of the big difference of signal frequency in the frequency domain of light, the wavelength is generally used to define the difference of frequency, and this multiplexing method is called WDM. WDM technology is to make full use of the single mode fiber low loss brought by the huge bandwidth resources, according to each channel frequency (or wavelength) of light waves of different fiber low loss Windows can be divided into several channel, light as a signal of the carrier, in the sender USES the wavelength division multiplexer (filter) to different wavelength of the signal light transmitted carrier combined into a single fiber. A multiplexing method at the receiving end in which these optical carriers carrying different signals at different wavelengths are separated by a wavelength division multiplexer (WDM). Because the optical carrier signals of different wavelengths can be regarded as independent of each other (not considering the fiber nonlinearity). The multiplexing transmission of optical signals can be realized in a single fiber. Two-way transmission can be realized by arranging the signals in two directions to transmit at different wavelengths. Depending on WDM, the number of wavelengths that can be multiplexed is also different, ranging from 2 to dozens, and generally commercialized as 8 wavelength and 16 wavelength systems, depending on the spacing of the allowed optical carrier wavelength.
WDM is essentially a frequency division multiplexing (FDM) technology in optical frequency. According to the transmission technology applied in China for decades, it follows the route of FDM-TDM-TDM FDM. At the beginning, FDM simulation technology was used for both open wire and coaxial cable, namely frequency division multiplexing technology on the electric domain. The bandwidth of each voice channel is 4 KHz, and each voice channel occupies a section of bandwidth of transmission medium (such as coaxial cable). PDH and SDH systems are TDM baseband digital signals transmitted on optical fiber, with each voice rate of 64KB/s. WDM technology is the optical fiber upper frequency division multiplexing technology, 16 (8) ×2.5Gb/s WDM system is the combination of FDM analog technology on optical frequency and TDM digital technology on electrical frequency.
WDM is essentially a frequency division multiplexing FDM technology in optical frequency, and each wavelength path is realized by frequency domain segmentation. Each wavelength path occupies the bandwidth of a segment of optical fiber, which is different from the former coaxial cable FDM technology : (1) different transmission media, WDM system is frequency segmentation on optical signals, while coaxial system is frequency segmentation and utilization on electrical signals. (2) In each channel, the coaxial cable system transmits analog signal 4 KHz voice signal, while the WDM system currently has digital signal SDH2.5GB/s or higher digital system on each wavelength channel.
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