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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.

ata centers (DCs) are currently the largest closedloop systems in the information technology (IT) and networking worlds, continuously growing toward multi-million-node clouds [1]. DC operators manage and control converged IT and network infrastructures in order to offer a broad range of services and applications to their customers. Typical services and applications provided by current DCs range from traditional IT resource outsourcing (storage, remote desktop, disaster recovery, etc.) to a plethora of web applications (e.g., browsers, social networks, online gaming). Innovative applications and services are also gaining momentum to the point that they will become main representatives of future DC workloads. Among them, we can find high-performance computing (HPC) and big data applications [2]. HPC encompasses a broad set of computationally intensive scientific applications, aiming to solve highly complex problems in the areas of quantum mechanics, molecular modeling, oil and gas exploration, and so on. Big data applications target the analysis of massive amounts of data collected from people on the Internet to analyze and predict their behavior.All these applications and services require huge data exchanges between servers inside the DC, supported over the DC network (DCN): the intra-DC communication network. The DCN must provide ultra-large capacity to ensure high throughput between servers. Moreover, very low latencies are mandatory, particularly in HPC where parallel computing tasks running concurrently on multiple servers are tightly interrelated. Unfortunately, current multi-tier hierarchical tree-based DCN architectures relying on Ethernet or Infiniband electronic switches suffer from bandwidth bottlenecks, high latencies, manual operation, and poor scalability to meet the expected DC growth forecasts [3].These limitations have mandated a renewed investigation D Abstract Applications running inside data centers are enabled through the cooperation of thousands of servers arranged in racks and interconnected together through the data center network. Current DCN architectures based on electronic devices are neither scalable to face the massive growth of DCs, nor flexible enough to efficiently and cost-effectively support highly dynamic application traffic profiles. The FP7 European Project LIGHTNESS foresees extending the capabilities of today's electrical DCNs through the introduction of optical packet switching and optical circuit switching paradigms, realizing together an advanced and highly scalable DCN architecture for ultra-high-bandwidth and low-latency server-to-server interconnection. This article reviews the current DC and high-performance computing (HPC) outlooks, followed by an analysis of the main requirements for future DCs and HPC platforms. As the key contribution of the article, the LIGHTNESS DCN solution is presented, deeply elaborating on the envisioned DCN data plane technologies, as well as on the unified SDN-enabled control plane architectural solution that will empower OPS and OCS transm...

The ever-increasing Internet Protocol (IP) traffic volume has finally brought to light the high inefficiency of current wavelength-routed over rigid-grid optical networks in matching the client layer requirements. Such an issue results in the deployment of large-size, expensive, and powerconsuming IP/Multi-Protocol Label Switching (MPLS) layers to perform the required grooming/aggregation functionality. To deal with this problem, the emerging flexgrid technology, allowing for reduced-size frequency grids (usually referred to as frequency slots), has recently attracted much attention among network operators, component and equipment suppliers, and the research community. In this paper, we tackle the multilayer IP/MPLS-over-flexgrid optimization problem. To this end, an integer linear programing formulation and a greedy randomized adaptive search procedure (GRASP) metaheuristic are provided. Using GRASP, we analyze the cost implications that a set of frequency slot widths have on the capital expenditure investments required to deploy such a multilayer network. For the sake of a compelling analysis, exhaustive numerical experiments are carried out considering a set of realistic network topologies, network equipment costs, and traffic instances. Results show that investments in optical equipment capable of operating under slot widths of 12.5 GHz, or even 25 GHz, are more appropriate, given the expected traffic evolution.

Abstract. Power management strategies that allow network infrastructures to achieve advanced functionalities with limited energy budget are expected to induce significant cost savings and positive effects on the environment, reducing Green House Gases (GHG) emissions. Power consumption can be drastically reduced on individual network elements by temporarily switching off or downclocking unloaded interfaces and line cards. At the state-of-the-art, Adaptive Link Rate (ALR) and Low Power Idle (LPI) are the most effective local-level techniques for lowering power demands during low utilization periods. In this paper, by modeling and analyzing in detail the aforementioned local strategies, we point out that the energy consumption does not depend on the data being transmitted but only depends on the interface link rate, and hence is throughput-independent. In particular, faster interfaces require lower energy per bit than slower interfaces, although, with ALR, slower interfaces require less energy per throughput than faster interfaces. We also note that for current technologies the energy/bit is the same both at 1 Gbps and 10 Gbps, meaning that the increase in the link rate has not been compensated at the same pace by a decrease in the energy consumption.