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. Keywords: all-optical networks, virtual topology design, multilayer optimization, scheduling. Transparent optical networks have been proposed to decrease costs and increase capacity in the Future Internet, i.e., "converged" multi-granular network architectures [1], [2] . In transparent optical networks, traffic is carried onto lightpaths which occupy one transmission wavelength in each traversed link. The carried traffic is electronically processed at the ingress and egress nodes of the lightpath, but not at intermediate transit nodes, saving electronic switching costs, and providing the data plane with a sort of traffic format transparency. The Wavelength Switch Fabric (WSF) is the central optical part of a switching node in transparent optical networks. A WSF can have a fixed or reconfigurable structure. In non-reconfigurable switching fabrics, connections between the input and output ports of the WSF are manually hard-wired. However, if the WSF is a device implemented using reconfigurable optical add/drop multiplexers (R-OADM) or reconfigurable wavelength crossconnects (R-WXC), then connections from input to output ports can be dynamically reconfigured allowing lightpaths to change along the time. In this paper, we focus on the offline planning of transparent optical networks for a given periodic traffic demand, which changes along a sequence of time intervals. The objective of our planning problem is to find the most cost-effective (i) scheduled virtual topology design, and (ii) routing of the electronic flows on top of the virtual topology. Scheduled virtual topology design determines the number of lightpaths to be established between every input-output pair of nodes over time. This resolves the number of transceivers needed in the network, which is a common cost figure of interest. Naturally, in the lower layer, each lightpath in the virtual topology has to be routed over the physical topology and assigned a wavelength. This problem is called the Routing and Wavelength Assignment (RWA) problem [3] . In this paper, we assume that the network links support a sufficient number of wavelengths for any RWA scheme. Thus, the physical-layer constraints do not apply and we can remove the RWA subproblem from the global network planning optimization problem. This assumption realistically depicts several network scenarios, such as metro-area optical networks with an over-dimensioned fiber plant. This paper proposes two MILP formulations to solve two variants of the "Scheduled Virtual Topology Design" problem (SVTD). The input data of a SVTD problem are a series of traffic matrices and a physical topology. The solution to a SVTD problem consists of one or more virtual topology designs, together with associated flow routings. The first problem variant is denoted as SVTD-NR (SVTD-Non-Reconfigurable). In this case, the WSFs are assumed to be non-reconfigurable or hard-wired. Hence, the virtual topology is constrained to be constant along time. In the second problem alternative, denoted as SVDT-R (SVTDReconfigurable), reconfigurable switching nodes are assumed, i.e., the virtual topology design can change along time. Intuitively, fewer transceivers should be necessary to carry a given periodic traffic demand with reconfigurable equipment. We briefly summarize related contributions in this research field. Real traffic traces, such as the Abilene backbone network [4] suggest a periodic nature of traffic, making the expected traffic load in the network fairly
In REF a set of MILP formulations is proposed to solve several MH-VTD-NR and MH-VTD-R problem variants minimizing solely the number of transceivers.
MILP formulations for scheduling lightpaths under periodic traffic
2009 11th International Conference on Transparent Optical Networks
2009 11th International Conference on Transparent Optical Networks
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