Coupling wavelength assignment in bidirectional lightpath: Is it worth the extra cost?

The introduction of end-to-end (multi hop) optical circuits (lightpaths) is often seen as the natural extension of a point-to-point (single hop) wavelength division multiplexing (WDM) transmission system, spanning across optical cross-connect nodes. Conventionally, a bidirectional lightpath (whether single or multi hop) comprises two lambda channels in fibers with opposite directions of signal propagation, which are assigned the same wavelength. This constraint is referred to as identical wavelength in bidirectional lightpath (IWBL). While IWBL is a natural and historical choice in point-to-point system, the authors are going to demonstrate that IWBL may unnecessarily increase the number of wavelength converters (WCs), which are required to establish lightpaths in the WDM network when the wavelength continuity constraint cannot be met. In the study, the IWBL constraint is relaxed, thus allowing the assignment of two distinct wavelengths to the same lightpath, one for each direction. A wavelength assignment (WA) algorithm is designed to both take advantage of the IWBL constraint relaxation and minimize the required number of WCs to establish a given set of lightpaths. The algorithm is then applied to a number of network topologies. The outcome is quite surprising, in that the amount of WC reduction obtained by relaxation of the IWBL constraint may be significant under certain conditions.