Sebastian Orlowski

Dynamic routing at different layers in IP-over-WDM networks - Maximizing energy savings

We estimate potential energy savings in IP-over-WDM networks achieved by switching off router line cards in low-demand hours. We compare three approaches to react on dynamics in the IP traffic over time, Fufl, Dufl and Dudl. They provide different levels of freedom in adjusting the routing of lightpaths in the WDM layer and the routing of demands in the IP layer. Using MILP models based on three realistic network topologies as well as realistic demands, power, and cost values, we show that already a simple monitoring of the lightpath utilization in order to deactivate empty line cards (Fufl) may bring substantial benefits. The most significant savings, however, are achieved by rerouting traffic in the IP layer (Dufl). A sophisticated reoptimization of the virtual topology and the routing in the optical and electrical domains for every demand scenario (Dudl) yields nearly no additional profits in the considered networks. These results are independent of the ratio between the traffic demands and capacity granularity, the time scale, distribution of demands, and the network topology for Dufl and Dudl. The success of Fufl, however, depends on the spatial distribution of the traffic as well as on the ratio of traffic demands and lightpath capacity.

On cut-based inequalities for capacitated network design polyhedra

In this article, we study capacitated network design problems. We unify and extend polyhedral results for directed, bidirected, and undirected link capacity models. Valid inequalities based on a network cut are known to be strong in several special cases. We show that regardless of the link model, facets of the polyhedra associated with such a cut translate to facets of the original network design polyhedra if the two subgraphs defined by the network cut are (strongly) connected. Our investigation of the facial structure of the cutset polyhedra allows to complement existing polyhedral results for the three variants by presenting facet-defining flow-cutset inequalities in a unifying way. In addition, we present a new class of facet-defining inequalities, showing as well that flow-cutset inequalities alone do not suffice to give a complete description for single-commodity, single-module cutset polyhedra in the bidirected and undirected case – in contrast to a known result for the directed case. The practical importance of the theoretical investigations is highlighted in an extensive computational study on 27 instances from the Survivable Network Design Library (SNDlib).

Complexity of column generation in network design with path‐based survivability mechanisms

Abstract in Undetermined his survey deals with computational complexity of column generation problems arising in the design of survivable communication networks. Such problems are often modeled as linear programs based on noncompact multicommodity flow network formulations. These formulations involve an exponential number of path-flow variables, and therefore require column generation to be solved to optimality. We consider several path-based protection and restoration mechanisms and present results, both known and new, on the complexity of the corresponding column generation (also called pricing) problems. We discuss results for the case of single link or single node failures scenarios, and extend the considerations to multiple link failures. Further, we classify the design problems corresponding to different survivability mechanisms according to the structure of their pricing problem. Eventually, we show that almost all the encountered pricing problems are hard to solve for scenarios admitting multiple failures, while a great deal of them are NP-hard already for single failure scenarios. (Less)

Single-layer Cuts for Multi-layer Network Design Problems

We study a planning problem arising in SDH/WDM multi-layer telecommunication network design. The goal is to find a minimum cost installation of link and node hardware of both network layers such that traffic demands can be realized via grooming and a survivable routing. We present a mixed-integer programming formulation for a predefined set of admissible logical links that takes many practical side constraints into account, including node hardware, several bit-rates, and survivability against single physical node or link failures. This model is solved using a branch-and-cut approach with cutting planes based on either of the two layers. On several realistic two-layer planning scenarios, we show that these cutting planes are still useful in the multi-layer context, helping to increase the dual bound and to reduce the optimality gaps.

Comparing restoration concepts using optimal network configurations with integrated hardware and routing decisions

We investigate the impact of link and path restoration on the cost of telecommunication networks. The main observation is that the cost of an optimal network configuration is almost independent of the restoration concept if (i) the installation of network elements (ADMs, DXCs, or routers) and interface cards, (ii) link capacities, and (iii) working and restoration routings are simultaneously optimized.We present a mixed-integer programming model which integrates all these decisions. Using a branch-and-cut algorithm (with column generation to deal with all admissible routing paths), we solve structurally different real-world based problem instances and show that the cost of optimal solutions is almost independent of the used restoration concept.In addition, we optimize spare capacities based on predetermined shortest working paths with respect to different link weights. On our test instances, the additional cost of solutions obtained with this sequential approach, compared to simultaneous optimization of working and restoration routings, varies between 0 and 164%.

A computational study for demand-wise shared protection

In this paper, we compare the new resilience mechanism demand-wise shared protection (DSP) with dedicated and shared path protection. The computational study on five realistic network planning scenarios reveals that that the best solutions for DSP are on average 15% percent better than the corresponding 1+1 dedicated path protection solutions, and only 15% percent worse than shared path protection.

The Effect of Hop Limits on Optimal Cost in Survivable Network Design

We investigate the impact of hop-limited routing paths on the total cost of a telecommunication network. For different survivability settings (dedicated path protection, link and path restoration), the optimal network cost without restrictions on the admissible path set is compared to the results obtained with two strategies to impose hop limits on routing paths. Using optimal solutions for nine real-world based problem instances, we evaluate how much the restriction to short paths can increase network cost.

Overview of resilience mechanisms based on multipath structures

Multipath structures are the base for many recently developed rerouting and protection switching mechanisms. All of these methods show a similar path layout, rely on traffic distribution, and promise resilience with only little backup capacity. Therefore, it is hard to recognize their commonalities and differences at first sight. This paper provides an overview of these related mechanisms and a comparative analysis regarding their applicability in optical and packet switched technologies, their path layout, their reaction time, their dynamic adaptability, and many other aspects.

Branch-and-Cut Techniques for Solving Realistic Two-Layer Network Design Problems

We study a planning problem arising in SDH/WDM multilayer telecommunication network design. The goal is to find a minimum cost installation of link and node hardware of both network layers such that traffic demands can be realized via grooming and a survivable routing. We present a mixed-integer programming formulation for a predefined set of admissible logical links that takes many practical side constraints into account, including node hardware, several bit rates, and survivability against single physical node or link failures. This model is solved using a branch-and-cut approach with problem-specific preprocessing, MIPbased heuristics, and cutting planes based on either of the two layers. On several realistic two-layer planning scenarios, we show that these ingredients can be very useful to reduce the optimality gaps in the multilayer context.

Estimating trenching costs in FTTx network planning

In this paper we assess to which extent trenching costs of an FTTx network are unavoidable, even if technical side constraints are neglected. For that purpose we present an extended Steiner tree model. Using a variety of realistic problem instances we demonstrate that the total trenching cost can only be reduced by about 5 percent in realistic scenarios. This work has been funded by BMBF (German Federal Ministry of Education and Research) within the program “KMU-innovativ”.