In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.
The term wavelength-division multiplexing is commonly applied to an optical carrier, which is typically described by its wavelength, whereas frequency-division multiplexing typically applies to a radio carrier which is more often described by frequency. This is purely convention because wavelength and frequency communicate the same information.
A WDM system uses a multiplexer at the transmitter to join the several signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer.There are three different WDM types, where of one is called "WDM", the notation "xWDM" is normally used when discussing the technology as such.
DWDM System Advantages
Less fiber cores to transmite and receive high capacity data
A single core fiber cable could divided into multiple channels instead of using 12 fiber core.
Easy network expansion, especially for limited fiber resource, no need extra fiber but add wavelength, Low cost for expansion, because no need to replace many components such as optical amplifiers, Can move to STM-64 when economics improve
DWDM systems capable of longer span lengths, TDM approach using STM-64 is more costly and more susceptible to chromatic and polarization mode dispersion
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