Belen Garcia-Manrubia

Offline Impairment Aware RWA Algorithms for Cross-Layer Planning of Optical Networks

Transparent optical networks are the enabling infrastructure for converged multi-granular networks in the future Internet. The cross-layer planning of these networks considers physical impairments in the network layer design. This is complicated by the diversity of modulation formats, transmission rates, amplification and compensation equipments, or deployed fiber links. Thereby, the concept of quality of transmission (QoT) attempts to embrace the effects of the physical layer impairments, to introduce them in a multi-criterium optimization and planning process. This paper contributes in this field by the proposal and comparative evaluation of two novel offline impairment aware planning algorithms for transparent optical networks, which share a common QoT evaluation function. The first algorithm is based on an iterative global search driven by a set of binary integer linear programming formulations. Heuristic techniques are included to limit the binary programming complexity. The second algorithm performs different pre-orderings of the lightpath demand, followed by a sequential processing of the lightpath demands. The performance and the scalability of both approaches are investigated. Results reveal great scalability properties of the global search algorithm, and a performance similar to or better than the sequential schemes.

Virtual topology design and flow routing in optical networks under multihour traffic demand

This article addresses the problem of finding a static virtual topology design and flow routing in transparent optical wavelength division multiplexing networks under a time-varying (multihour) traffic demand. Four variants of the problem are considered, using fixed or dynamically adaptable (meaning variable) flow routing, which can be splittable or unsplittable. Our main objective is to minimize the number of transceivers needed which make up for the main network cost. We formulate the problem variants as exact integer linear programs (ILPs) and mixed ILPs. For larger problem instances, we also propose a family of heuristics based on the concept of domination between traffic matrices. This concept provides the theoretical foundations for a set of techniques proposed to reduce the problem complexity. We present a lower bound to the network cost for the case in which the virtual topology could be dynamically reconfigured along time. This allows us to assess the limit on the maximum possible benefit that could be achieved by using optical reconfigurable equipment. Extensive tests have been conducted, using both synthetically generated and real-traced traffic demands. In the cases studied, results show that combining variable routing with splittable flows obtains a significant, although moderate, cost reduction. The maximum cost reduction achievable with reconfigurable virtual topologies was shown to be negligible compared to the static case in medium and high loads.

(Non-)reconfigurable virtual topology design under multihour traffic in optical networks

This paper investigates offline virtual topology design in transparent optical networks under a multihour traffic demand. The main problem variant addressed here designs a reconfigurable virtual topology that evolves over time to more efficiently utilize network resources (the MH-VTD-R problem). The case of designing a static non-reconfigurable virtual topology that can accommodate the time-varying traffic (the MH-VTD-NR problem) is also considered. The objectives are to minimize: 1) the number of transceivers, which make up for the main network cost; and 2) the frequency of reconfiguration (for MH-VTD-R), which incurs additional overhead and potential service disruption. We formulate this multiobjective problem as an exact mixed integer linear program (MILP). Due to its high complexity, we propose a very efficient heuristic algorithm called Greedy Approach with Reconfiguration Flattening (GARF). GARF not only solves both (non-)reconfigurable problem variants, but it allows for tuning of the relative importance of the two objectives. Exhaustive experiments on real and synthetic traffic and comparison to previous proposals and bounds reveal the merits of GARF with respect to both solution quality and execution time. Furthermore, the obtained results indicate that the maximal transceiver cost savings achieved by the fully reconfigurable case may not be enough to justify the associated increase in reconfiguration cost. However, results show that an advantageous tradeoff between transceiver cost savings and reconfiguration cost can be achieved by a allowing a small number of virtual topology reconfigurations over time.

Scheduled Virtual Topology Design under periodic traffic in transparent optical networks

This paper investigates offline planning and scheduling in transparent optical networks for a given periodic traffic demand. The main objective is to minimize the number of transceivers needed which make up for the main network cost. We call this problem “Scheduled Virtual Topology Design” and consider two variants: non-reconfigurable and reconfigurable equipment. We formulate both problems as exact MILPs (Mixed Integer Linear Programs). Due to their high complexity, we propose a more scalable tabu search heuristic approach, in conjunction with smaller MILP formulations for the associated subproblems. The main motivation of our research efforts is to assess the benefits of using reconfigurable equipment, realized as a reduction in the number of required transceivers. Our results show that the achieved reductions are not very significant, except for cases with large network loads and high traffic variability.

MILP formulations for scheduling lightpaths under periodic traffic

This paper addresses offline virtual topology design in transparent optical networks under given periodic traffic. We call this planning problem “Scheduled Virtual Topology Design”. Two problem variants are considered: for a network based on non-reconfigurable equipment and for a network based on reconfigurable equipment. Two MILP (Mixed Integer Linear Program) formulations are proposed, one for each alternative. The number of transceivers in the network is the selected cost figure to minimize. Tests are performed to evaluate the benefits of using reconfigurable equipment under different traffic conditions and network sizes. The reduction in the number of transceivers obtained by allowing temporal variations in the virtual topology seems low in all cases, indicating that using reconfigurable equipment may not be cost-effective for periodic traffic.

Multi-hour network planning based on domination between sets of traffic matrices

In multi-hour network design, periodic traffic variations along time are considered in the dimensioning process. Then, the non coincidence of traffic peaks along the day or the week can be exploited. This paper investigates the application of the traffic domination relation between sets of traffic matrices to multi-hour network planning. Two problem variants are considered: a network with a static, and with a dynamic traffic routing. We derive a set of techniques for, given a multi-hour traffic demand potentially composed of hundreds of matrices, obtaining a traffic series with a smaller number of matrices. The traffic domination relation guarantees that the network designed for the simplified series is suitable for the original one. Also, we apply the domination relation to derive lower bounds to the network cost, and upper bounds to the suboptimality incurred by simplifying the traffic demand. The algorithms proposed are tested in a case of study with the Abilene network. In our tests, a long traffic series could be reduced to a small number of traffic matrices, and be effective for network planning.

Heuristic Approaches for Periodic Reconfiguration of Lightpath-based Networks under Multi-hour Traffic

Periodic lightpath reconfiguration of virtual topologies in transparent optical networks has been recently investigated as a mechanism to more efficiently adapt the network to predictable periodic traffic variations along a day or week. Scheduling periodic reconfigurations involves tuning a trade-off between a lower network cost obtained through better resource allocation, and undesired traffic disruptions that these reconfigurations may cause. This paper presents and compares two algorithms for planning a reconfigurable virtual topology suitable for exploring this trade-off. The first is based on a Lagrangean Relaxation of the planning problem, and the second is based on a Tabu Search meta-heuristic. The merits of both algorithms are assessed for moderate network sizes through comparison with analytical lower bounds and exact solutions obtained by a MILP formulation.

Algorithms for virtual topology reconfiguration under multi-hour traffic using Lagrangian relaxation and Tabu search approaches

Periodic reconfiguration of the virtual topology in transparent optical networks has been recently investigated as a mechanism to more efficiently adapt the network to predictable periodic traffic variations along the day or week. The scheduling of periodic reconfigurations should consider the trade-off between a lower network cost obtained through better resource allocation, and the undesired traffic disruptions that these reconfigurations may cause. This paper presents and compares two algorithms for planning virtual topology reconfiguration suitable for exploring this trade-off. The first is based on a Lagrangian relaxation of the planning problem, and the second is based on a Tabu Search meta-heuristic. The merits of both algorithms are compared.

Balancing multifibre and wavelength converter cost in wavelength routing networks

This paper evaluates the balance between the cost of multifibres and the cost of wavelength converters in multilayer networks. A novel ILP formulation of the static planning problem is applied.

Introducing optical switching in high-capacity commercial routers

High-performance packet switches are costly, bulky and power-hungry. Their scalability is constrained by the limits of electronic technology at high binary rates. In this paper we study the benefits of introducing optical technology in the switch fabric architecture, to cope with this bottleneck. In particular, we study the replacement of the switch fabric boards in the Cisco 12816 architecture, by two optical architectures based on arrayed waveguide gratings (AWGs) and tunable wavelength converters (TWCs). The results illustrate the potential benefits in terms of power consumption and system scalability that these optical architectures could bring to commercial routers.