DWDM systems maintain more stable wavelength than those needed for CWDM because of the closer spacing of the wavelengths, thus DWDM Mux/Demux provides greater maximum capacity and it is used at a higher level in the communications hierarchy than CWDM. GB-Link MUX join any numbers of any wavelength together at the transmitter, and the Demux at the receiver to split them apart, extra upgrade(express)/tap monitor port can be added to the Mux/Demux based on customer needs. WDM Mux/Demux are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber.