Line ports, sometimes called common ports, are one of the must-have ports on CWDM and DWDM Mux/Demux. External fibers connect to the Mux/Demux unit through this port and are usually labeled Tx and Rx. All WDM channels are multiplexed and demultiplexed through this. 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. The light from each fiber is first collimated. The collimated beams are then combined using a dichroic filter, with typically the longer wavelength transmitted from port T, the shorter. As we all know, there are several different ports on WDM multiplexers and multiplexers.