In telecommunication networks based on the current Ethernet technology, routing of traffic demands is based on multiple spanning trees: the network operator configures different routing spanning trees and assigns each demand to be routed in one of the selected spanning trees. A major optimization issue in this solution is the combined determination of (i) a set of appropriate spanning trees, and (ii) assignment of demands to the trees, in order to achieve an optimal load balancing on the links of the network. In this paper we consider models and solving techniques for lexicographical optimization of two load balancing objective functions. The first objective is the min-max optimization of the n worst link loads (with n up to the total number of network links), and the second objective is the minimization of the average link load (when n is smaller than the total number of network links). Besides exact methods, a heuristic technique that can compute both feasible solutions and lower bounds for the addressed optimization problem is proposed. Finally, we discuss effectiveness of different solution using results of a numerical study of realistic case studies.
In this paper we address link dimensioning and routing problems related to the area of resilient network design. We present two network design problems that assume different flow restoration schemes used to cope with network failures. In both cases we allow bifurcation of traffic flows in the normal (failure-less) network state. In the case of a failure, we assume that affected primary flows (i.e., flows applied in the normal state) are restored using assigned protection paths and that the primary flows are restored (also in a bifurcated manner) in a separate pool of (protection) capacity, distinct from the basic capacity used in the normal network operation state. The two presented models differ in the way protection paths are used to protect primary flows against different failure states. The first model, called state-independent flow restoration, assumes that once the backup path is assigned it must be used in every state in which the protected primary path fails. The second model allows different protection paths to be used in different network failure states and is called state-dependent flow restoration. The considered problems are formulated as linear programming (LP) problems using link-path (L-P) notation of multi-commodity flow network optimization. As the L-P notation is useful only when an effective column generation scheme is known, we discuss the applicability of this method on the basis of the theory of duality of LP. The paper presents and compares three different approaches and evaluates their usefulness for solving problem instances of practical size.
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