@phdthesis{Fink2014, author = {Fink, Martin}, title = {Crossings, Curves, and Constraints in Graph Drawing}, publisher = {W{\"u}rzburg University Press}, isbn = {978-3-95826-002-3 (print)}, doi = {10.25972/WUP-978-3-95826-003-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-98235}, school = {W{\"u}rzburg University Press}, pages = {222}, year = {2014}, abstract = {In many cases, problems, data, or information can be modeled as graphs. Graphs can be used as a tool for modeling in any case where connections between distinguishable objects occur. Any graph consists of a set of objects, called vertices, and a set of connections, called edges, such that any edge connects a pair of vertices. For example, a social network can be modeled by a graph by transforming the users of the network into vertices and friendship relations between users into edges. Also physical networks like computer networks or transportation networks, for example, the metro network of a city, can be seen as graphs. For making graphs and, thereby, the data that is modeled, well-understandable for users, we need a visualization. Graph drawing deals with algorithms for visualizing graphs. In this thesis, especially the use of crossings and curves is investigated for graph drawing problems under additional constraints. The constraints that occur in the problems investigated in this thesis especially restrict the positions of (a part of) the vertices; this is done either as a hard constraint or as an optimization criterion.}, subject = {Graphenzeichnen}, language = {en} } @phdthesis{Jarschel2014, author = {Jarschel, Michael}, title = {An Assessment of Applications and Performance Analysis of Software Defined Networking}, issn = {1432-8801}, doi = {10.25972/OPUS-10079}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-100795}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {With the introduction of OpenFlow by the Stanford University in 2008, a process began in the area of network research, which questions the predominant approach of fully distributed network control. OpenFlow is a communication protocol that allows the externalization of the network control plane from the network devices, such as a router, and to realize it as a logically-centralized entity in software. For this concept, the term "Software Defined Networking" (SDN) was coined during scientific discourse. For the network operators, this concept has several advantages. The two most important can be summarized under the points cost savings and flexibility. Firstly, it is possible through the uniform interface for network hardware ("Southbound API"), as implemented by OpenFlow, to combine devices and software from different manufacturers, which increases the innovation and price pressure on them. Secondly, the realization of the network control plane as a freely programmable software with open interfaces ("Northbound API") provides the opportunity to adapt it to the individual circumstances of the operator's network and to exchange information with the applications it serves. This allows the network to be more flexible and to react more quickly to changing circumstances as well as transport the traffic more effectively and tailored to the user's "Quality of Experience" (QoE). The approach of a separate network control layer for packet-based networks is not new and has already been proposed several times in the past. Therefore, the SDN approach has raised many questions about its feasibility in terms of efficiency and applicability. These questions are caused to some extent by the fact that there is no generally accepted definition of the SDN concept to date. It is therefore a part of this thesis to derive such a definition. In addition, several of the open issues are investigated. This Investigations follow the three aspects: Performance Evaluation of Software Defined Networking, applications on the SDN control layer, and the usability of SDN Northbound-API for creation application-awareness in network operation. Performance Evaluation of Software Defined Networking: The question of the efficiency of an SDN-based system was from the beginning one of the most important. In this thesis, experimental measurements of the performance of OpenFlow-enabled switch hardware and control software were conducted for the purpose of answering this question. The results of these measurements were used as input parameters for establishing an analytical model of the reactive SDN approach. Through the model it could be determined that the performance of the software control layer, often called "Controller", is crucial for the overall performance of the system, but that the approach is generally viable. Based on this finding a software for analyzing the performance of SDN controllers was developed. This software allows the emulation of the forwarding layer of an SDN network towards the control software and can thus determine its performance in different situations and configurations. The measurements with this software showed that there are quite significant differences in the behavior of different control software implementations. Among other things it has been shown that some show different characteristics for various switches, in particular in terms of message processing speed. Under certain circumstances this can lead to network failures. Applications on the SDN control layer: The core piece of software defined networking are the intelligent network applications that operate on the control layer. However, their development is still in its infancy and little is known about the technical possibilities and their limitations. Therefore, the relationship between an SDN-based and classical implementation of a network function is investigated in this thesis. This function is the monitoring of network links and the traffic they carry. A typical approach for this task has been built based on Wiretapping and specialized measurement hardware and compared with an implementation based on OpenFlow switches and a special SDN control application. The results of the comparison show that the SDN version can compete in terms of measurement accuracy for bandwidth and delay estimation with the traditional measurement set-up. However, a compromise has to be found for measurements below the millisecond range. Another question regarding the SDN control applications is whether and how well they can solve existing problems in networks. Two programs have been developed based on SDN in this thesis to solve two typical network issues. Firstly, the tool "IPOM", which enables considerably more flexibility in the study of effects of network structures for a researcher, who is confined to a fixed physical test network topology. The second software provides an interface between the Cloud Orchestration Software "OpenNebula" and an OpenFlow controller. The purpose of this software was to investigate experimentally whether a pre-notification of the network of an impending relocation of a virtual service in a data center is sufficient to ensure the continuous operation of that service. This was demonstrated on the example of a video service. Usability of the SDN Northbound API for creating application-awareness in network operation: Currently, the fact that the network and the applications that run on it are developed and operated separately leads to problems in network operation. SDN offers with the Northbound-API an open interface that enables the exchange between information of both worlds during operation. One aim of this thesis was to investigate whether this interface can be exploited so that the QoE experienced by the user can be maintained on high level. For this purpose, the QoE influence factors were determined on a challenging application by means of a subjective survey study. The application is cloud gaming, in which the calculation of video game environments takes place in the cloud and is transported via video over the network to the user. It was shown that apart from the most important factor influencing QoS, i.e., packet loss on the downlink, also the type of game type and its speed play a role. This demonstrates that in addition to QoS the application state is important and should be communicated to the network. Since an implementation of such a state conscious SDN for the example of Cloud Gaming was not possible due to its proprietary implementation, in this thesis the application "YouTube video streaming" was chosen as an alternative. For this application, status information is retrievable via the "Yomo" tool and can be used for network control. It was shown that an SDN-based implementation of an application-aware network has distinct advantages over traditional network management methods and the user quality can be obtained in spite of disturbances.}, subject = {Leistungsbewertung}, language = {en} } @inproceedings{SchlosserJarschelDuellietal.2010, author = {Schlosser, Daniel and Jarschel, Michael and Duelli, Michael and Hoßfeld, Tobias and Hoffmann, Klaus and Hoffmann, Marco and Morper, Hans Jochen and Jurca, Dan and Khan, Ashiq}, title = {A Use Case Driven Approach to Network Virtualization}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55611}, year = {2010}, abstract = {In today's Internet, services are very different in their requirements on the underlying transport network. In the future, this diversity will increase and it will be more difficult to accommodate all services in a single network. A possible approach to cope with this diversity within future networks is the introduction of support for running isolated networks for different services on top of a single shared physical substrate. This would also enable easy network management and ensure an economically sound operation. End-customers will readily adopt this approach as it enables new and innovative services without being expensive. In order to arrive at a concept that enables this kind of network, it needs to be designed around and constantly checked against realistic use cases. In this contribution, we present three use cases for future networks. We describe functional blocks of a virtual network architecture, which are necessary to support these use cases within the network. Furthermore, we discuss the interfaces needed between the functional blocks and consider standardization issues that arise in order to achieve a global consistent control and management structure of virtual networks.}, subject = {Virtualisierung}, language = {en} } @article{AtienzadeCastroCortesetal.2012, author = {Atienza, Nieves and de Castro, Natalia and Cort{\´e}s, Carmen and Garrido, M. {\´A}ngeles and Grima, Clara I. and Hern{\´a}ndez, Gregorio and M{\´a}rquez, Alberto and Moreno-Gonz{\´a}lez, Auxiliadora and N{\"o}llenburg, Martin and Portillo, Jos{\´e} Ram{\´o}n and Reyes, Pedro and Valenzuela, Jes{\´u}s and Trinidad Villar, Maria and Wolff, Alexander}, title = {Cover contact graphs}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-78845}, year = {2012}, abstract = {We study problems that arise in the context of covering certain geometric objects called seeds (e.g., points or disks) by a set of other geometric objects called cover (e.g., a set of disks or homothetic triangles). We insist that the interiors of the seeds and the cover elements are pairwise disjoint, respectively, but they can touch. We call the contact graph of a cover a cover contact graph (CCG). We are interested in three types of tasks, both in the general case and in the special case of seeds on a line: (a) deciding whether a given seed set has a connected CCG, (b) deciding whether a given graph has a realization as a CCG on a given seed set, and (c) bounding the sizes of certain classes of CCG's. Concerning (a) we give efficient algorithms for the case that seeds are points and show that the problem becomes hard if seeds and covers are disks. Concerning (b) we show that this problem is hard even for point seeds and disk covers (given a fixed correspondence between graph vertices and seeds). Concerning (c) we obtain upper and lower bounds on the number of CCG's for point seeds.}, subject = {Informatik}, language = {de} } @phdthesis{Hock2014, author = {Hock, David Rog{\´e}r}, title = {Analysis and Optimization of Resilient Routing in Core Communication Networks}, issn = {1432-8801}, doi = {10.25972/OPUS-10168}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-101681}, school = {Universit{\"a}t W{\"u}rzburg}, pages = {175}, year = {2014}, abstract = {Routing is one of the most important issues in any communication network. It defines on which path packets are transmitted from the source of a connection to the destination. It allows to control the distribution of flows between different locations in the network and thereby is a means to influence the load distribution or to reach certain constraints imposed by particular applications. As failures in communication networks appear regularly and cannot be completely avoided, routing is required to be resilient against such outages, i.e., routing still has to be able to forward packets on backup paths even if primary paths are not working any more. Throughout the years, various routing technologies have been introduced that are very different in their control structure, in their way of working, and in their ability to handle certain failure cases. Each of the different routing approaches opens up their own specific questions regarding configuration, optimization, and inclusion of resilience issues. This monograph investigates, with the example of three particular routing technologies, some concrete issues regarding the analysis and optimization of resilience. It thereby contributes to a better general, technology-independent understanding of these approaches and of their diverse potential for the use in future network architectures. The first considered routing type, is decentralized intra-domain routing based on administrative IP link costs and the shortest path principle. Typical examples are common today's intra-domain routing protocols OSPF and IS-IS. This type of routing includes automatic restoration abilities in case of failures what makes it in general very robust even in the case of severe network outages including several failed components. Furthermore, special IP-Fast Reroute mechanisms allow for a faster reaction on outages. For routing based on link costs, traffic engineering, e.g. the optimization of the maximum relative link load in the network, can be done indirectly by changing the administrative link costs to adequate values. The second considered routing type, MPLS-based routing, is based on the a priori configuration of primary and backup paths, so-called Label Switched Paths. The routing layout of MPLS paths offers more freedom compared to IP-based routing as it is not restricted by any shortest path constraints but any paths can be setup. However, this in general involves a higher configuration effort. Finally, in the third considered routing type, typically centralized routing using a Software Defined Networking (SDN) architecture, simple switches only forward packets according to routing decisions made by centralized controller units. SDN-based routing layouts offer the same freedom as for explicit paths configured using MPLS. In case of a failure, new rules can be setup by the controllers to continue the routing in the reduced topology. However, new resilience issues arise caused by the centralized architecture. If controllers are not reachable anymore, the forwarding rules in the single nodes cannot be adapted anymore. This might render a rerouting in case of connection problems in severe failure scenarios infeasible.}, subject = {Leistungsbewertung}, language = {en} } @article{WolffRutter2012, author = {Wolff, Alexander and Rutter, Iganz}, title = {Augmenting the Connectivity of Planar and Geometric Graphs}, series = {Journal of Graph Algorithms and Applications}, journal = {Journal of Graph Algorithms and Applications}, doi = {10.7155/jgaa.00275}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-97587}, year = {2012}, abstract = {In this paper we study connectivity augmentation problems. Given a connected graph G with some desirable property, we want to make G 2-vertex connected (or 2-edge connected) by adding edges such that the resulting graph keeps the property. The aim is to add as few edges as possible. The property that we consider is planarity, both in an abstract graph-theoretic and in a geometric setting, where vertices correspond to points in the plane and edges to straight-line segments. We show that it is NP-hard to � nd a minimum-cardinality augmentation that makes a planar graph 2-edge connected. For making a planar graph 2-vertex connected this was known. We further show that both problems are hard in the geometric setting, even when restricted to trees. The problems remain hard for higher degrees of connectivity. On the other hand we give polynomial-time algorithms for the special case of convex geometric graphs. We also study the following related problem. Given a planar (plane geometric) graph G, two vertices s and t of G, and an integer c, how many edges have to be added to G such that G is still planar (plane geometric) and contains c edge- (or vertex-) disjoint s{t paths? For the planar case we give a linear-time algorithm for c = 2. For the plane geometric case we give optimal worst-case bounds for c = 2; for c = 3 we characterize the cases that have a solution.}, language = {en} } @phdthesis{Driewer2008, author = {Driewer, Frauke}, title = {Teleoperation Interfaces in Human-Robot Teams}, isbn = {978-3-923959-57-0}, doi = {10.25972/OPUS-2955}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-36351}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Diese Arbeit besch{\"a}ftigt sich mit der Verbesserung von Mensch-Roboter Interaktion in Mensch-Roboter Teams f{\"u}r Teleoperation Szenarien, wie z.B. robotergest{\"u}tzte Feuerwehreins{\"a}tze. Hierbei wird ein Konzept und eine Architektur f{\"u}r ein System zur Unterst{\"u}tzung von Teleoperation von Mensch-Roboter Teams vorgestellt. Die Anforderungen an Informationsaustausch und -verarbeitung, insbesondere f{\"u}r die Anwendung Rettungseinsatz, werden ausgearbeitet. Weiterhin wird das Design der Benutzerschnittstellen f{\"u}r Mensch-Roboter Teams dargestellt und Prinzipien f{\"u}r Teleoperation-Systeme und Benutzerschnittstellen erarbeitet. Alle Studien und Ans{\"a}tze werden in einem Prototypen-System implementiert und in verschiedenen Benutzertests abgesichert. Erweiterungsm{\"o}glichkeiten zum Einbinden von 3D Sensordaten und die Darstellung auf Stereovisualisierungssystemen werden gezeigt.}, subject = {Robotik}, language = {en} } @phdthesis{Hess2009, author = {Hess, Martin}, title = {Motion coordination and control in systems of nonholonomic autonomous vehicles}, isbn = {978-3-923959-55-6}, doi = {10.25972/OPUS-3794}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-46442}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {This work focuses on coordination methods and the control of motion in groups of nonholonomic wheeled mobile robots, in particular of the car-like type. These kind of vehicles are particularly restricted in their mobility. In the main part of this work the two problems of formation motion coordination and of rendezvous in distributed multi-vehicle systems are considered. We introduce several enhancements to an existing motion planning approach for formations of nonholonomic mobile robots. Compared to the original method, the extended approach is able to handle time-varying reference speeds as well as adjustments of the formation's shape during reference trajectory segments with continuously differentiable curvature. Additionally, undesired discontinuities in the speed and steering profiles of the vehicles are avoided. Further, the scenario of snow shoveling on an airfield by utilizing multiple formations of autonomous snowplows is discussed. We propose solutions to the subproblems of motion planning for the formations and tracking control for the individual vehicles. While all situations that might occur have been tested in a simulation environment, we also verified the developed tracking controller in real robot hardware experiments. The task of the rendezvous problem in groups of car-like robots is to drive all vehicles to a common position by means of decentralized control laws. Typically there exists no direct interaction link between all of the vehicles. In this work we present decentralized rendezvous control laws for vehicles with free and with bounded steering. The convergence properties of the approaches are analyzed by utilizing Lyapunov based techniques. Furthermore, they are evaluated within various simulation experiments, while the bounded steering case is also verified within laboratory hardware experiments. Finally we introduce a modification to the bounded steering system that increases the convergence speed at the expense of a higher traveled distance of the vehicles.}, subject = {Robotik}, language = {en} } @misc{Kaempgen2009, type = {Master Thesis}, author = {Kaempgen, Benedikt}, title = {Deskriptives Data-Mining f{\"u}r Entscheidungstr{\"a}ger: Eine Mehrfachfallstudie}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-46343}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Das Potenzial der Wissensentdeckung in Daten wird h{\"a}ufig nicht ausgenutzt, was haupts{\"a}chlich auf Barrieren zwischen dem Entwicklerteam und dem Endnutzer des Data-Mining zur{\"u}ckzuf{\"u}hren ist. In dieser Arbeit wird ein transparenter Ansatz zum Beschreiben und Erkl{\"a}ren von Daten f{\"u}r Entscheidungstr{\"a}ger vorgestellt. In Entscheidungstr{\"a}ger-zentrierten Aufgaben werden die Projektanforderungen definiert und die Ergebnisse zu einer Geschichte zusammengestellt. Eine Anforderung besteht dabei aus einem tabellarischen Bericht und ggf. Mustern in seinem Inhalt, jeweils verst{\"a}ndlich f{\"u}r einen Entscheidungstr{\"a}ger. Die technischen Aufgaben bestehen aus einer Datenpr{\"u}fung, der Integration der Daten in einem Data-Warehouse sowie dem Generieren von Berichten und dem Entdecken von Mustern wie in den Anforderungen beschrieben. Mehrere Data-Mining-Projekte k{\"o}nnen durch Wissensmanagement sowie eine geeignete Infrastruktur voneinander profitieren. Der Ansatz wurde in zwei Projekten unter Verwendung von ausschließlich Open-Source-Software angewendet.}, subject = {Data Mining}, language = {de} } @article{SchloerRingHotho2020, author = {Schl{\"o}r, Daniel and Ring, Markus and Hotho, Andreas}, title = {iNALU: Improved Neural Arithmetic Logic Unit}, series = {Frontiers in Artificial Intelligence}, volume = {3}, journal = {Frontiers in Artificial Intelligence}, issn = {2624-8212}, doi = {10.3389/frai.2020.00071}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-212301}, year = {2020}, abstract = {Neural networks have to capture mathematical relationships in order to learn various tasks. They approximate these relations implicitly and therefore often do not generalize well. The recently proposed Neural Arithmetic Logic Unit (NALU) is a novel neural architecture which is able to explicitly represent the mathematical relationships by the units of the network to learn operations such as summation, subtraction or multiplication. Although NALUs have been shown to perform well on various downstream tasks, an in-depth analysis reveals practical shortcomings by design, such as the inability to multiply or divide negative input values or training stability issues for deeper networks. We address these issues and propose an improved model architecture. We evaluate our model empirically in various settings from learning basic arithmetic operations to more complex functions. Our experiments indicate that our model solves stability issues and outperforms the original NALU model in means of arithmetic precision and convergence.}, language = {en} }