In WDM an optical carrier is modulated with a baseband signal of typically hundred of Mbit/s.
The operation of multiplexing and demultiplexing the single subcarriers is carried out electronically.
This gives an advantage over a pure WDM access, due to the lower cost of the electrical components if compared with an optical multiplexer.
[1] SCM has the disadvantage of being limited in maximum subcarrier frequencies and data rates by the available bandwidth of the electrical and optical components.
Therefore, SCM must be used in conjunction with WDM in order to take advantage of most of the available fiber bandwidth, but it can be used effectively for lower-speed, lower-cost multiuser systems.