Cost-optimized dimensioning of translucent WDM networks with Mixed-Line-Rate spectrum-flexible channels

In order for transport networks to cost-effectively provide higher capacity, it is expected that channel bit-rates beyond 100 Gb/s will be accomplished by resorting to a flexible WDM grid with variable channel spacing. Among the implications of this concept is the need for planning tools that fully exploit the additional degrees of freedom enabled by a flexible grid to further optimize network cost and spectral efficiency. This paper proposes an optimization framework to minimize the transponder and regenerator deployment cost in a translucent WDM network featuring channel bit-rates of 40, 100 and 400 Gb/s and multiple transmission formats per bit-rate, each characterized by its own spectral width, optical reach and cost properties. Firstly, we formulate the problem via a novel Integer Linear Programming (ILP) model, whose resolution finds the optimal (cheapest) feasible network configuration. Secondly, we propose an efficient heuristic called Narrowest First-Iterative Cost Reduction (NF-ICR) to handle network scenarios for which solving the ILP entails an unreasonable computational burden. The NF-ICR heuristic is shown to provide tight optimality bounds where the benchmark given by the ILP solution is attainable. For larger networks, we show that the use of a flexible grid and multiple format options for each bit-rate results in around 10% less cost in transponders and regenerators for metro networks, and a substantial increase in the total traffic load supported by the network. We also conclude that a distinction emerges between metro/regional scenarios and long-haul networks with long paths, wherein the shorter transparent reach of 400 Gb/s channels drives up the cost due to extra regeneration, favoring the use of parallelized solutions of lower bit-rate channels.