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Many real-life phenomena, such as computer systems, communication networks, manufacturing systems, supermarket checkout lines as well as structural military systems can be represented by means of queueing models. Looking at queueing models, a controller may considerably improve the system's performance by reducing queue lengths, or increasing the throughput, or diminishing the overhead, whereas in the absence of a controller the system behavior may get quite erratic, exhibiting periods of high load and long queues followed by periods, during which the servers remain idle. The theoretical foundations of controlled queueing systems are led in the theory of Markov, semi-Markov and semi-regenerative decision processes. In this thesis, the essential work consists in designing controlled queueing models and investigation of their optimal control properties for the application in the area of the modern telecommunication systems, which should satisfy the growing demands for quality of service (QoS). For two types of optimization criterion (the model without penalties and with set-up costs), a class of controlled queueing systems is defined. The general case of the queue that forms this class is characterized by a Markov Additive Arrival Process and heterogeneous Phase-Type service time distributions. We show that for these queueing systems the structural properties of optimal control policies, e.g. monotonicity properties and threshold structure, are preserved. Moreover, we show that these systems possess specific properties, e.g. the dependence of optimal policies on the arrival and service statistics. In order to practically use controlled stochastic models, it is necessary to obtain a quick and an effective method to find optimal policies. We present the iteration algorithm which can be successfully used to find an optimal solution in case of a large state space.
Spatial Queues
(2000)
In the present thesis, a theoretical framework for the analysis of spatial queues is developed. Spatial queues are a generalization of the classical concept of queues as they provide the possibility of assigning properties to the users. These properties may influence the queueing process, but may also be of interest for themselves. As a field of application, mobile communication networks are modeled by spatial queues in order to demonstrate the advantage of including user properties into the queueing model. In this application, the property of main interest is the user's position in the network. After a short introduction, the second chapter contains an examination of the class of Markov-additive jump processes, including expressions for the transition probabilities and the expectation as well as laws of large numbers. Chapter 3 contains the definition and analysis of the central concept of spatial Markovian arrival processes (shortly: SMAPs) as a special case of Markov-additive jump processes, but also as a natural generalization from the well-known concept of BMAPs. In chapters 4 and 5, SMAPs serve as arrival streams for the analyzed periodic SMAP/M/c/c and SMAP/G/infinity queues, respectively. These types of queues find application as models or planning tools for mobile communication networks. The analysis of these queues involves new methods such that even for the special cases of BMAP inputs (i.e. non-spatial queues) new results are obtained. In chapter 6, a procedure for statistical parameter estimation is proposed along with its numerical results. The thesis is concluded by an appendix which collects necessary results from the theories of Markov jump processes and stochastic point fields. For special classes of Markov jump processes, new results have been obtained, too.
In der Forschungsliteratur ist Medienwandel bislang überwiegend als Wandel von Kommunikationstechnologienbehandelt worden, Theorien zum Medienwandel sind ein Forschungsdesiderat. Ziel der Arbeit ist es, anhand exemplarischer Untersuchungen zu klären, ob es bestimmteGelenkstellen gibt, an denen einerseits Probleme der Medientransformation sichtbar werdenund sich andererseits gleichzeitig Anforderungen an eine Theorie des Medienwandels ableitenlassen. In der Arbeit werden dabei explizit nicht nur die Kommunikatorseite, sondern auch die Angebote selbst und ihre Nutzung mit einbezogen. Für die Arbeit wurden empirische Untersuchungen in zwei Online-Redaktionen unterschiedlicher Muttermedien vorgenommen (teilnehmende Beobachtung, Leitfadeninterview). Die Onlineangebote wurden durch eine Studie zur Rezeption dieser Angebote in die Untersuchung mit einbezogen. Betrachtet werden die Redaktionsorganisation, Präsentationsmuster des Angebots und die Rezeptionsmuster, die sich bei der Nutzung der untersuchten Angebote zeigen. Die von den untersuchten Anbietern eingesetzte Strategie der Adaption erweist sich angesichts eines Mißverhältnisses zwischen Produktionskompetenz und -möglichkeiten der Macher und der Abstimmung der Angebote auf die Nutzungskompetenz ihrer Rezipienten als problematisch. Eine Theorie des Medienwandels muss mehrdimensional und integrativ sein, um den verschiedenen wechselseitigen Abhängigkeiten der Medienkommunikation gerecht zu werden.
