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Mobile computing poses different requirements on middleware than more traditional desktop systems interconnected by fixed networks. Not only the characteristics of mobile network technologies as for example lower bandwidth and unreliability demand for customized support. Moreover, the devices employed in mobile settings usually are less powerful than their desktop counterparts. Slow processors, a fairly limited amount of memory, and smaller displays are typical properties of mobile equipment, again requiring special treatment. Furthermore, user mobility results in additional requirements on appropriate middleware support. As opposed to the quite static environments dominating the world of desktop computing, dynamic aspects gain more importance. Suitable strategies and techniques for exploring the environment e.g. in order to discover services available locally are only one example. Managing resources in a fault-tolerant manner, reducing the impact ill-behaved clients have on system stability define yet another exemplary prerequisite. Most state of the art middleware has been designed for use in the realm of static, resource rich environments and hence is not immediately applicable in mobile settings as set forth above. The work described throughout this thesis aims at investigating the suitability of different middleware technologies with regard to application design, development, and deployment in the context of mobile networks. Mostly based upon prototypes, shortcomings of those technologies are identified and possible solutions are proposed and evaluated where appropriate. Besides tailoring middleware to specific communication and device characteristics, the cellular structure of current mobile networks may and shall be exploited in favor of more scalable and robust systems. Hence, an additional topic considered within this thesis is to point out and investigate suitable approaches permitting to benefit from such cellular infrastructures. In particular, a system architecture for the development of applications in the context of mobile networks will be proposed. An evaluation of this architecture employing mobile agents as flexible, network-side representatives for mobile terminals is performed, again based upon a prototype application. In summary, this thesis aims at providing several complementary approaches regarding middleware support tailored for mobile, cellular networks, a field considered to be of rising importance in a world where mobile communication and particularly data services emerge rapidly, augmenting the globally interconnecting, wired Internet.
Hardware bugs can be extremely expensive, financially. Because microprocessors and integrated circuits have become omnipresent in our daily live and also because of their continously growing complexity, research is driven towards methods and tools that are supposed to provide higher reliability of hardware designs and their implementations. Over the last decade Ordered Binary Decision Diagrams (OBDDs) have been well proven to serve as a data structure for the representation of combinatorial or sequential circuits. Their conciseness and their efficient algorithmic properties are responsible for their huge success in formal verification. But, due to Shannon's counting argument, OBDDs can not always guarantee the concise representation of a given design. In this thesis, Parity Ordered Binary Decision Diagrams are presented, which are a true extension of OBDDs. In addition to the regular branching nodes of an OBDD, functional nodes representing a parity operation are integrated into the data structure, thus resulting in Parity-OBDDs. Parity-OBDDs are more powerful than OBDDs are, but, they are no longer a canonical representation. Besides theoretical aspects of Parity-OBDDs, algorithms for their efficient manipulation are the main focus of this thesis. Furthermore, an analysis on the factors that influence the Parity-OBDD representation size gives way for the development of heuristic algorithms for their minimization. The results of these analyses as well as the efficiency of the data structure are also supported by experiments. Finally, the algorithmic concept of Parity-OBDDs is extended to Mod-p-Decision Diagrams (Mod-p-DDs) for the representation of functions that are defined over an arbitrary finite domain.
Virtuelle Umgebungen erfreuen sich in den letzten Jahren einer großen Beliebtheit und können kontinuierlich wachsende Nutzerzahlen vorweisen. Deshalb wird erwartet, dass die ohnehin große Nutzergemeinde weiterhin wächst. Dieses Wachstum stellt jedoch die Entwickler und die Betreiber von virtuellen Umgebungen vor eine Herausforderung. Die meisten heutzutage erfolgreichen virtuellen Umgebungen basieren auf einer Client-Server-Infrastruktur, in der der Server für die Verwaltung der gesamten Umgebung zuständig ist. Bei einer gleichzeitigen Inanspruchnahme durch viele Nutzer werden die serverbasierten Umgebungen jedoch massiven Skalierbarkeits- und Lastverteilungsproblemen ausgesetzt. Diese Probleme führen beispielsweise dazu, dass Nutzer Wartezeiten beim Einloggen in Kauf nehmen müssen bzw. selbst im Falle eines erfolgreichen Logins die "überfüllten" virtuellen Regionen nicht betreten dürfen. Deshalb wird in dieser Arbeit untersucht, ob eine Verbesserung der Skalierbarkeit und der Lastverteilung in virtuellen Umgebungen durch die Verwendung alternativer Infrastrukturansätze erreicht werden kann. Außerdem werden Maßnahmen zur weiteren Entlastung der Basis-Infrastruktur entwickelt. Diese Verbesserung soll zum einen dadurch erzielt werden, dass anstelle eines Servers ein Server-Verbund (Peer-to-Peer-System) als eine Art Management-Schicht agieren soll, in der jeder Knoten nur einen bestimmten Teil der virtuellen Umgebung verwaltet. Dabei wird die gewünschte Lastverteilung durch die Aufteilung der Verantwortungsbereiche auf die Knoten der Management-Schicht erreicht. Gleichzeitig entsteht aber ein Mehraufwand für die Konstruktion bzw. Erhaltung einer solchen Management-Schicht. Deshalb werden in dieser Arbeit die Vor- und Nachteile eines Infrastrukturwechsels hin zu dezentralisierten Infrastruktur-Ansätzen untersucht. Zum anderen wird eine spürbare Entlastung der Management-Schicht und somit eine Verbesserung der Skalierbarkeit und der Lastverteilung durch die Übertragung einiger Verwaltungsaufgaben an die Clients angestrebt. Zu diesen Verwaltungsaufgaben zählen die Gruppenkommunikation und das Konsistenz-Handling. Dazu wird in dieser Arbeit erforscht, inwiefern die Clients, die sich innerhalb derselben virtuellen Region befinden, selbst die Gruppenkommunikation über eine Multicast-Infrastruktur regeln können bzw. in die Wahrung der Konsistenz eines gemeinsamen Zustandes und der gemeinsam genutzten Objekte involviert werden können. Bei dem clientbasierten Konsistenz-Handling sollte vor allem auf die besonderen Interaktivitäts- und Konsistenzanforderungen, die Nutzer an die virtuellen Umgebungen stellen, geachtet werden. Dazu wird ein neues Konsistenzmodell definiert, welches die Einhaltung der strengen Interaktivitätsanforderungen erzwingt und die Konsistenz mit einer bestimmten Wahrscheinlichkeit garantieren kann.
In dieser Arbeit präsentieren wir einen systematischen Ansatz zur Multithread Implementierung von Divide und Conquer sowie inkrementellen Algorithmen. Wir stellen für beide Kategorien von Algorithmen ein Framework vor. Beide Frameworks sollen die Behandlung von Threads erleichtern, die Implementierung von parallelen Algorithmen beschleunigen und die Rechenzeit verringern. Mit Hilfe der Frameworks parallelisieren wir beispielhaft zahlreiche Algorithmen insbesondere aus dem Bereich Computational Geometry, unter anderem: Sortieralgorithmen, konvexe Hülle Algorithmen und Triangulierungen. Der Programmcode zu diese Arbeit ist in C++ unter Verwendung templatisierter Klassen implementiert und basiert auf der LEDA Bibliothek.
We are living in a connected world, surrounded by interwoven technical systems. Since they pervade more and more aspects of our everyday lives, a thorough understanding of the structure and dynamics of these systems is becoming increasingly important. However - rather than being blueprinted and constructed at the drawing board - many technical infrastructures like for example the Internet's global router network, the World Wide Web, large scale Peer-to-Peer systems or the power grid - evolve in a distributed fashion, beyond the control of a central instance and influenced by various surrounding conditions and interdependencies. Hence, due to this increase in complexity, making statements about the structure and behavior of tomorrow's networked systems is becoming increasingly complicated. A number of failures has shown that complex structures can emerge unintentionally that resemble those which can be observed in biological, physical and social systems. In this dissertation, we investigate how such complex phenomena can be controlled and actively used. For this, we review methodologies stemming from the field of random and complex networks, which are being used for the study of natural, social and technical systems, thus delivering insights into their structure and dynamics. A particularly interesting finding is the fact that the efficiency, dependability and adaptivity of natural systems can be related to rather simple local interactions between a large number of elements. We review a number of interesting findings about the formation of complex structures and collective dynamics and investigate how these are applicable in the design and operation of large scale networked computing systems. A particular focus of this dissertation are applications of principles and methods stemming from the study of complex networks in distributed computing systems that are based on overlay networks. Here we argue how the fact that the (virtual) connectivity in such systems is alterable and widely independent from physical limitations facilitates a design that is based on analogies between complex network structures and phenomena studied in statistical physics. Based on results about the properties of scale-free networks, we present a simple membership protocol by which scale-free overlay networks with adjustable degree distribution exponent can be created in a distributed fashion. With this protocol we further exemplify how phase transition phenomena - as occurring frequently in the domain of statistical physics - can actively be used to quickly adapt macroscopic statistical network parameters which are known to massively influence the stability and performance of networked systems. In the case considered in this dissertation, the adaptation of the degree distribution exponent of a random, scale-free overlay allows - within critical regions - a change of relevant structural and dynamical properties. As such, the proposed scheme allows to make sound statements about the relation between the local behavior of individual nodes and large scale properties of the resulting complex network structures. For systems in which the degree distribution exponent cannot easily be derived for example from local protocol parameters, we further present a distributed, probabilistic mechanism which can be used to monitor a network's degree distribution exponent and thus to reason about important structural qualities. Finally, the dissertation shifts its focus towards the study of complex, non-linear dynamics in networked systems. We consider a message-based protocol which - based on the Kuramoto model for coupled oscillators - achieves a stable, global synchronization of periodic heartbeat events. The protocol's performance and stability is evaluated in different network topologies. We further argue that - based on existing findings about the interrelation between spectral network properties and the dynamics of coupled oscillators - the proposed protocol allows to monitor structural properties of networked computing systems. An important aspect of this dissertation is its interdisciplinary approach towards a sensible and constructive handling of complex structures and collective dynamics in networked systems. The associated investigation of distributed systems from the perspective of non-linear dynamics and statistical physics highlights interesting parallels both to biological and physical systems. This foreshadows systems whose structures and dynamics can be analyzed and understood in the conceptual frameworks of statistical physics and complex systems.
Today, usage of complex circuit designs in computers, in multimedia applications and communication devices is widespread and still increasing. At the same time, due to Moore's Law we do not expect to see an end in the growth of the complexity of digital circuits. The decreasing ability of common validation techniques -- like simulation -- to assure correctness of a circuit design enlarges the need for formal verification techniques. Formal verification delivers a mathematical proof that a given implementation of a design fulfills its specification. One of the basic and during the last years widely used data structure in formal verification are the so called Ordered Binary Decision Diagrams (OBDDs) introduced by R. Bryant in 1986. The topic of this thesis is integration of structural high-level information in the OBDD-based formal verification of sequential systems. This work consist of three major parts, covering different layers of formal verification applications: At the application layer, an assertion checking methodology, integrated in the verification flow of the high-level design and verification tool Protocol Compiler is presented. At the algorithmic layer, new approaches for partitioning of transition relations of complex finite state machines, that significantly improve the performance of OBDD-based sequential verification are introduced. Finally, at the data structure level, dynamic variable reordering techniques that drastically reduce the time required for reordering without a trade-off in OBDD-size are described. Overall, this work demonstrates how a tighter integration of applications by using structural information can significantly improve the efficiency of formal verification applications in an industrial setting.