Marc Ruiz

Modeling the routing and spectrum allocation problem for flexgrid optical networks

Flexgrid optical networks are attracting huge interest due to their higher spectrum efficiency and flexibility in comparison with traditional wavelength switched optical networks based on the wavelength division multiplexing technology. To properly analyze, design, plan, and operate flexible and elastic networks, efficient methods are required for the routing and spectrum allocation (RSA) problem. Specifically, the allocated spectral resources must be, in absence of spectrum converters, the same along the links in the route (the continuity constraint) and contiguous in the spectrum (the contiguity constraint). In light of the fact that the contiguity constraint adds huge complexity to the RSA problem, we introduce the concept of channels for the representation of contiguous spectral resources. In this paper, we show that the use of a pre-computed set of channels allows considerably reducing the problem complexity. In our study, we address an off-line RSA problem in which enough spectrum needs to be allocated for each demand of a given traffic matrix. To this end, we present novel integer lineal programming (ILP) formulations of RSA that are based on the assignment of channels. The evaluation results reveal that the proposed approach allows solving the RSA problem much more efficiently than previously proposed ILP-based methods and it can be applied even for realistic problem instances, contrary to previous ILP formulations.

Dynamic routing and spectrum (re)allocation in future flexgrid optical networks

Future flexible-grid elastic optical networks are very promising due to their higher spectrum efficiency and flexibility comparing to the rigid spectrum grid optical networks realized with the traditional wavelength division multiplexing (WDM) technology. The maturity of key system components enabling flexgrid optical networks, such as advanced modulation techniques and multi-granular switching, is already high enough and thus their deployment is expected in the near future. The main feature of such networks is the removal of fix grid-space assignment (in general 50GHz) to the optical connections independently of the required bandwidth. In fact, the available optical spectrum in flexgrid network is divided into frequency slots of a fixed spectrum width and an optical connection can be allocated into the number of slots that better matches the actual bandwidth of the connection demand. Nonetheless, such allocation must satisfy two constraints, i.e. the slots must be (i) contiguous in the spectrum domain and (ii) continuous along the links on the routing path. These constraints result in a need for dedicated Routing and Spectrum Allocation (RSA) algorithms able to operate under dynamic traffic conditions. From the network design perspective, an important issue is the selection of the frequency slot width which may have an impact on the network performance. Last but not least, network dynamicity entails spectrum fragmentation, which significantly reduces the network performance. In this paper we address these topics and, in particular: (1) we present an RSA algorithm to be used in dynamic network scenarios, (2) we study the optimal slot width as a function of the foreseen traffic to be served, and (3) we propose an algorithm to reallocate already established optical connections so that to make room in the spectrum for the new ones. Exhaustive simulation results reveal that the proposed approach improves the blocking probability performance in flexgrid optical networks.

Elastic Spectrum Allocation for Time-Varying Traffic in FlexGrid Optical Networks

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.

Solving Routing and Spectrum Allocation Related Optimization Problems: From Off-Line to In-Operation Flexgrid Network Planning

Compared to wavelength switched optical networks (WSON), flexgrid optical networks provide higher spectrum efficiency and flexibility. To properly analyze, design, plan, and operate flexgrid networks, the routing and spectrum allocation (RSA) problem must be solved. The RSA problem involves two different constraints: the continuity constraint to ensure that the allocated spectral resources are the same along the links in the route and the contiguity constraint to guarantee that those resources are contiguous in the spectrum. As a consequence of its complexity, it is crucial that efficient methods are available to allow solving realistic problem instances in practical times. In this paper, we review different RSA-related optimization problems that arise within the life-cycle of flexgrid networks. Different methods to solve those optimization problems are reviewed along with the different requirements related to where those problems appear. Starting from its formulation, we analyze network life-cycle and indicate different solving methods for the kind of problems that arise at each network phase: from off-line to in-operation network planning. We tackle two representative use cases: i) a use case for off-line planning where a flexgrid network is designed and periodically upgraded, and ii) multilayer restoration as a use case for in-operation planning. Three solving methods are proposed for the off-line planning problem: mathematical programming, column generation and metaheuristics, whereas, as a result of its stringent required solving times, two heuristic methods are presented for the on-line problem.

Survivable IP/MPLS-Over-WSON Multilayer Network Optimization

Network operators are facing the problem of dimensioning their networks for the expected huge IP traffic volumes while keeping constant or even reducing the connectivity prices. Therefore, new architectural solutions able to cope with the expected traffic increase in a more cost-effective way are needed. In this work, we study the survivable IP/multi-protocol label switching (MPLS) over wavelength switched optical network (WSON) multilayer network problem as a capital expenditure (CAPEX) minimization problem. Two network approaches providing survivability against optical links, IP/MPLS nodes, and opto-electronic port failures are compared: the classical overlay approach where two redundant IP/MPLS networks are deployed, and the new joint multilayer approach which provides the requested survivability through an orchestrated interlayer recovery scheme which minimizes the over-dimensioning of IP/MPLS nodes. Mathematical programming models are developed for both approaches. Solving these models, however, becomes impractical for realistic networks. In view of this, evolutionary heuristics based on the biased random-key genetic algorithm framework are also proposed. Exhaustive experiments on several reference network scenarios illustrate the effectiveness of the proposed approach in minimizing network CAPEX.

CAPEX study for a multilayer IP/MPLS-over-flexgrid optical network

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 power-consuming 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.

Planning fixed to flexgrid gradual migration: drivers and open issues

Flexgrid technology has recently been presented as the most promising option for upgrading the currently operating fixed grid optical networks and extending their capacity to be able to deal with the massive traffic volumes forecast for the next decade. Although the current traffic is successfully supported on fixed grid networks, flexgrid technology brings features that are not offered by the fixed grid networks, such as transporting optical connections with a capacity beyond 100 Gb/s and elasticity against time-varying traffic. In light of this, a gradual fixed grid to flexgrid migration is generally accepted in order to add these useful features to the network. In this article, we study the migration process where flexgrid is deployed in the network progressively, and review the main drivers and open issues induced by its deployment.

Column generation algorithm for RSA problems in flexgrid optical networks

Finding optimal routes and spectrum allocation in flexgrid optical networks, known as the RSA problem, is an important design problem in transport communication networks. The problem is

Statistical Approach for Fast Impairment-Aware Provisioning in Dynamic All-Optical Networks

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