Section: New Results

Optical networks

Participants : Nizar Bouabdallah, Gerardo Rubino.

Typical optical-based backbones are in general underutilized. This is not due to the lack of transmission needs, but to other factors, among which we underline two: the bottleneck effects at access networks, and the somehow rigid and inefficient way of using the optical infrastructure as nowadays available in the current technology. In the team, we work for providing new techniques in order to utilize efficiently the tremendous transmission capacity offered by the optical equipments. To achieve this, we propose intelligent traffic aggregation mechanisms and efficient routing algorithms [9] .

Optical technology has increased significantly the transmission capacity of today's transport networks, and it is playing important roles in supporting the rapidly increasing data traffic. Meanwhile, congestion issues are definitively relieved in such core networks. Nonetheless, the rigid and large routing granularity (i.e. wavelength) entailed by such an approach could lead to bandwidth waste [67] . In this regard, increasing research interest is now focusing on the development of new concept of traffic aggregation in optical networks. The main objective of our work is therefore eliminating both the bandwidth underutilization and the scalability concerns that are typical of all-optical wavelength-routed networks.

Specifically, we propose using multiple-access lightpaths (i.e., optical circuits) instead of the traditional point-to-point lightpaths. By doing so, we aim at increasing the lightpath utilization since its capacity is shared by multiple connections instead of a single end-to-end connection. To achieve this and a first main contribution of our work, we conceived new medium access and sharing mechanisms adapted to such very high speed networks. Such mechanisms lead to significant cost savings.

We also provide new sharing policy techniques to ensure QoS-aware protection schemes. The main objective is to give multiple grades of service for different clients, with various availability requirements, sharing the same protection paths. Indeed the resulting network reliability depends mainly on the deployed redundancy capacity and how such capacity is shared among the different classes of service. In view of this, we aim at proposing new priority-enabled shared-protection mechanisms as well as new models to evaluate their effectiveness. To do so, we elaborate new analytical models for the proposed priority-enabled schemes [9] .

In the design of an optical backbone, a commonly applied requirement is to ensure the existence of at least two node-disjoint-paths between pairs of distinguished nodes. The problem of finding a topology verifying this restriction is known as the Steiner Two-Node-Survivable Network Problem (denoted by STNSNP), an NP-hard problem. In [24] we introduce a Greedy Randomised Adaptive Search Procedure (GRASP) for designing low-cost topologies for the STNSNP model. The heuristic was tested over a large problem set containing heterogeneous topologies with different characteristics, including instances with hundreds of nodes. The numerical results were highly satisfactory, accomplishing in all cases good quality local-optimal solutions.