The wavelength division multiplexing is similar to the frequency division multiplexing and uses fiber optical cable to transmit the signals. This technique is also used for combining a large number of optical carrier signals. It is based upon a laser that is used to emit single colors of light. Each signal that needs to transmit is attached to the laser that will emit a colored light beam and the color will be different for different signal. Theses light beams are then sent simultaneously. The receiver at the other end then splits the individual colors.
There are two types of wavelength division multiplexing.
Coarse Wavelength Division
This technique uses a small number of channels, usually four or eight. Channel spacing used in it is of 20 nm and the nominal wavelengths are taken in between 1310 to 1610. Wavelength tolerance provided in the transmitters is quite large, for example ~+mn~3 nm. Bit rate of the signal channel is in between 1 and 3.125 gigabit/second.
Dense wavelength division
Dense wavelength division multiplexing is for transferring very large data. This technique uses a large number of channels such as 40, 80, and 160. However, the channel spacing is very small such as 12.5, 25, and 50. Instead, the transmitters are required to meet the wavelength tolerance accurately. Bit-rate of the signal channel might be in between 1 and 10 GB/s.