Multiplex-demultiplex (mux/demux) “groups” multiplex and demultiplex a predetermined maximum number of optical wavelengths. A method for assigning wavelengths of fiber links and mux/demux groups to given traffic (or traffic segments) in an optical communications network, minimizes over all terminals a total number of mux/demux groups required. The method (FIG. 5) involves (510) sorting the terminals in order of size of load. Further, for each traffic segment between pairs of first and second terminals, the method involves (516) assigning a smallest wavelength that is available between the first and second terminals and that is available on the segment path between the terminals; and in the terminals, (518) deploying mux/demux groups supporting the assigned wavelengths. A modified method (FIGS. 6A, 6B), as well as a method for dynamically assigning and deploying newly arriving traffic segments (FIG. 7), are also provided.