@misc{HuelsermannJaegerKrumkeetal., author = {H{\"u}lsermann, Ralf and J{\"a}ger, Monika and Krumke, Sven and Poensgen, Diana and Rambau, J{\"o}rg and Tuchscherer, Andreas}, title = {Dynamic Routing Algorithms in Transparent Optical Networks An Experimental Study Based on Real Data}, url = {http://nbn-resolving.de/urn:nbn:de:0297-zib-7025}, number = {02-35}, abstract = {Today's telecommunication networks are configured statically. Whenever a connection is established, the customer has permanent access to it. However, it is observed that usually the connection is not used continuously. At this point, dynamic provisioning could increase the utilization of network resources. WDM based Optical Transport Networks (OTNs) will shortly allow for fast dynamic network reconfiguration. This enables optical broadband leased line services on demand. Since service requests competing for network resources may lead to service blocking, it is vital to use appropriate strategies for routing and wavelength assignment in transparent optical networks. We simulate the service blocking probabilities of various dynamic algorithms for this problem using a well-founded traffic model for two realistic networks. One of the algorithms using shortest path routings performs best on all instances. Surprisingly, the tie-breaking rule between equally short paths in different wavelengths decides between success or failure.}, language = {en} } @misc{HeinzKrumkeMegowetal., author = {Heinz, Stefan and Krumke, Sven and Megow, Nicole and Rambau, J{\"o}rg and Tuchscherer, Andreas and Vredeveld, Tjark}, title = {The Online Target Date Assignment Problem}, url = {http://nbn-resolving.de/urn:nbn:de:0297-zib-8945}, number = {05-61}, abstract = {Many online problems encountered in real-life involve a two-stage decision process: upon arrival of a new request, an irrevocable first-stage decision (the assignment of a specific resource to the request) must be made immediately, while in a second stage process, certain ``subinstances'' (that is, the instances of all requests assigned to a particular resource) can be solved to optimality (offline) later. We introduce the novel concept of an \emph{Online Target Date Assignment Problem} (\textsc{OnlineTDAP}) as a general framework for online problems with this nature. Requests for the \textsc{OnlineTDAP} become known at certain dates. An online algorithm has to assign a target date to each request, specifying on which date the request should be processed (e.\,g., an appointment with a customer for a washing machine repair). The cost at a target date is given by the \emph{downstream cost}, the optimal cost of processing all requests at that date w.\,r.\,t.\ some fixed downstream offline optimization problem (e.\,g., the cost of an optimal dispatch for service technicians). We provide general competitive algorithms for the \textsc{OnlineTDAP} independently of the particular downstream problem, when the overall objective is to minimize either the sum or the maximum of all downstream costs. As the first basic examples, we analyze the competitive ratios of our algorithms for the par ticular academic downstream problems of bin-packing, nonpreemptive scheduling on identical parallel machines, and routing a traveling salesman.}, language = {en} } @misc{GroetschelHinrichsSchroeeretal., author = {Gr{\"o}tschel, Martin and Hinrichs, Hinrich and Schr{\"o}er, Klaus and Tuchscherer, Andreas}, title = {Ein gemischt-ganzzahliges lineares Optimierungsmodell f{\"u}r ein Laserschweißproblem im Karosseriebau}, url = {http://nbn-resolving.de/urn:nbn:de:0297-zib-9141}, number = {06-21}, abstract = {Wir besch{\"a}ftigen uns mit dem Problem der Betriebsplanung von Laserschweißrobotern im Karosseriebau. Gegeben ist eine Menge von Schweißn{\"a}hten, die innerhalb einer Fertigungszelle an einem Karosserieteil gefertigt werden m{\"u}ssen. Die Schweißn{\"a}hte werden durch mehrere parallel betriebene Roboter bearbeitet. Die Aufgabe besteht darin, f{\"u}r jeden Roboter eine Reihenfolge und eine zeitliche Koordinierung seiner Bewegungen zu finden, so dass alle Schweißn{\"a}hte innerhalb der Taktzeit der Fertigungszelle bearbeitet werden und so wenig Laserquellen wie m{\"o}glich eingesetzt werden. Dabei m{\"u}ssen einige Nebenbedingungen ber{\"u}cksichtigt werden. F{\"u}r dieses spezielle Schweißproblem haben wir eine Formulierung als gemischt-ganzzahliges lineares Programm entwickelt, welches sich f{\"u}r die untersuchten praktischen F{\"a}lle sehr schnell l{\"o}sen l{\"a}sst.}, language = {de} } @misc{HeinzKaibelPeinhardtetal., author = {Heinz, Stefan and Kaibel, Volker and Peinhardt, Matthias and Rambau, J{\"o}rg and Tuchscherer, Andreas}, title = {LP-Based Local Approximation for Markov Decision Problems}, url = {http://nbn-resolving.de/urn:nbn:de:0297-zib-9131}, number = {06-20}, abstract = {The standard computational methods for computing the optimal value functions of Markov Decision Problems (MDP) require the exploration of the entire state space. This is practically infeasible for applications with huge numbers of states as they arise, e.\,g., from modeling the decisions in online optimization problems by MDPs. Exploiting column generation techniques, we propose and apply an LP-based method to determine an \$\varepsilon\$-approximation of the optimal value function at a given state by inspecting only states in a small neighborhood. In the context of online optimization problems, we use these methods in order to evaluate the quality of concrete policies with respect to given initial states. Moreover, the tools can also be used to obtain evidence of the impact of single decisions. This way, they can be utilized in the design of policies.}, language = {en} }