Impact of channel spacing on the design of a mixed-line-rate optical network

Due to the increasing heterogeneity and the growing volume of traffic, telecom backbone networks are going through new innovations and paradigm shift. The wavelength-division multiplexed (WDM) optical networks may cost-effectively support the growing heterogeneity of traffic demands by having mixed line rates (MLR) over different wavelength channels. The coexistence of wavelength channels with different line rates in the same fiber brings up the important issue of the choice of the channel spacing that one can have in these MLR networks. The channel spacing affects the signal quality in terms of bit-error rate (BER), and hence affects the maximum reach of the lightpaths, which is a function of line rates. Various approaches to set an opportunistic width of the channel spacing can be considered: i) on a practical side, one may choose uniform fixed channel spacing specified by the ITU-T grid (typically 50 GHz); ii) alternatively, to optimize the usage of the fiber spectrum, one can explore different channel spacing for different line rates, also referred to as the "one-size-does-not-fit-all" approach; iii) else an optimal value of channel spacing that leads to minimum cost can be sought. In this work, we investigate the third case by evaluating the cost of a MLR network for different channel spacings. Our results show that, even under the assumption of uniform channel spacing for a MLR network, it is possible to identify optimal values of channel spacing for a minimum-cost MLR network design.