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| Rigid fixed-grid wavelength division multiplexing (WDM) optical networks can no longer keep up with the emerging bandwidth-hungry and highly dynamic services in an efficient manner. As the available spectrum in optical fibers becomes occupied and is approaching fundamental limits, the research community has focused on seeking more advanced optical transmission and networking solutions that utilize the available bandwidth more effectively. To this end, the flexible/ elastic optical networking paradigm has emerged as a way to offer efficient use of the available optical resources. In this work, we provide a comprehensive view of the different pieces composing the ''flexible networking puzzle'' with special attention given to capturing the occurring interactions between different research fields. Only when these interrelations are clearly defined, an optimal network-wide solution can be offered. Physical layer technological aspects, network optimization for flexible networks, and control plane aspects are examined. Furthermore, future research directions and open issues are discussed.

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Elastic flexgrid optical networks (FG-ON) are considered a very promising solution for next-generation optical networks. In this article we focus on lightpath adaptation under variable traffic demands in FG-ON. Specifically, we explore the elastic spectrum allocation (SA) capability of FG-ON and, in this context, we study the effectiveness of three alternative SA schemes in terms of the network performance. To this end, we formulate a Multi-Hour Routing and Spectrum Allocation (MH-RSA) optimization problem and solve it by means of both Integer Linear Programming (ILP) and efficient heuristic algorithms. Since, as numerical results show, the effectiveness of SA schemes highly depends on the traffic demand profile, we formulate some indications on the applicability of elastic SA in FG-ON.

Abstract-In translucent optical networks the physical layer impairments degrading the optical signal are considered in the network design. In this paper we investigate the offline problem of Routing and Wavelength Assignment (RWA) and Regenerator Placement (RP) in translucent networks. Given a network topology, an estimation of the traffic demands, the objective is to minimize the cost of the regeneration equipment used, and to avoid the lightpath blocking. We formulate an optimal ILP model of the problem, to the best of the authors' knowledge, for the first time in the literature. Its simplicity allows us to test it for small and medium size networks. Despite of this merit, the problem is NP-hard. For larger problem instances we propose two heuristic methods: Lightpath Segmentation and 3-Step method. The latter guarantees that no lightpath blocking is produced by signal degradation. We also provide a lower bound for the regenerator equipment cost. The performance and the scalability of our proposals are then investigated by carrying out extensive tests, considering different network topologies, number of wavelengths per fiber, traffic load conditions and network link lengths. Results reveal that the solutions obtained by the heuristic algorithms are optimal or close-to-optimal and require low computation times. In addition, the results help to capture the trends in the regenerator equipment cost in different network instances.