What is WDM and DWDM ?

WDM & DWDM

Wavelength-division multiplexing (WDM) 

is a method of network transmission along the fiber optic line to carry different wavelengths by a laser light. WDM can transmit data as 2.5 - 10 Gbps at 32 - 64 channels in the same time. Basically, WDM system uses "Multiplexer (mux)" to join the signals together before transmit out and a "De-multiplexer (demux)" at the receiver to split signals apart.

See the following picture for explanations.

Figure 1: mux and demux flow

Dense Wavelength-division multiplexing (DWDM) 

is higher performance transmission method than WDM. DWDM can transmit data as 2.5 - 10 Gbps at 160 channels in the same time. Moreover, DWDM is developed channel spacing up to 25-50 GHz then it can support transmission more than 1 Tbps.