Diss. (sammanfattning) Uppsala : Univ., 2002. / Härtill 8 uppsatser.

Zugl.: München, Techn. Univ., Diss., 2007

Zugl.: München, Techn. Univ., Diss., 2007

Zugl.: Braunschweig, Techn. Univ., Diss., 2008

Time-triggered communication is widely used throughout several industry do- mains, primarily for reliable and real-time capable data transfers. However, existing time-triggered technologies are designed for terrestrial usage and not directly applicable to space applications due to the harsh environment. In- stead, specific hardware must be developed to deal with thermal, mechanical, and especially radiation effects. SpaceWire, as an event-triggered communication technology, has been used for years in a large number of space missions. Its moderate complexity, her- itage, and transmission rates up to 400 MBits/s are one of the main ad- vantages and often without alternatives for on-board computing systems of spacecraft. At present, real-time data transfers are either achieved by prior- itization inside SpaceWire routers or by applying a simplified time-triggered approach. These solutions either imply problems if they are used inside dis- tributed on-board computing systems or in case of networks with more than a single router are required. This work provides a solution for the real-time problem by developing a novel clock synchronization approach. This approach is focused on being compatible with distributed system structures and allows time-triggered data transfers. A significant difference to existing technologies is the remote clock estimation by the use of pulses. They are transferred over the network and remove the need for latency accumulation, which allows the incorporation of standardized SpaceWire equipment. Additionally, local clocks are controlled decentralized and provide different correction capabilities in order to handle oscillator induced uncertainties. All these functionalities are provided by a developed Network Controller (NC), able to isolate the attached network and to control accesses. / Zeitgesteuerte Datenübertragung ist in vielen Industriezweigen weit verbreitet, primär für zuverlässige und echtzeitfähige Kommunikation. Bestehende Technologien sind jedoch für den terrestrischen Gebrauch konzipiert und aufgrund der rauen Umgebung nicht direkt auf Weltraumanwendungen anwendbar. Stattdessen wird spezielle Hardware entwickelt, um Strahlungseffekten zu widerstehen sowie thermischen und mechanischen Belastungen standzuhalten. SpaceWire wurde als ereignisgesteuerte Kommunikationstechnologie entwickelt und wird seit Jahren in einer Vielzahl von Weltraummissionen verwendet. Dessen erfolgreiche Verwendung, überschaubare Komplexität, und Übertragungsraten bis zu 400 MBit/s sind einige seiner Hauptvorteile. Derzeit werden Datenübertragungen in Echtzeit entweder durch Priorisierung innerhalb von SpaceWire Router erreicht, oder durch Anwendung von vereinfachten zeitgesteuerten Ansätzen. Diese Lösungen implizieren entweder Probleme in verteilten Systemarchitekturen oder in SpaceWire Netzwerken mit mehreren Routern. Diese Arbeit beschreibt eine Uhrensynchronisation, die bestimmte Eigenschaften von SpaceWire ausnutzt, um das Echtzeitproblem zu lösen. Der Ansatz ist dabei kompatibel mit verteilten Systemstrukturen und ermöglicht eine zeitgesteuerte Datenübertragung.

The use of materialized views in a data warehouse installation is a common tool to speed up mostly aggregation queries. The problems coming along with materialized aggregate views have triggered a huge variety of proposals, such as picking the optimal set of aggregation combinations, transparently rewriting user queries to take advantage of the summary data, or synchronizing pre-computed summary data as soon as the base data changes. The paper focuses on the problem of view selection in the context of distributed data warehouse architectures. While much research was done with regard to the view selection problem in the central case, we are not aware to any other work discussing the problem of view selection in distributed data warehouse systems. The paper proposes an extension of the concept of an aggregation lattice to capture the distributed semantics. Moreover, we extend a greedy-based selection algorithm based on an adequate cost model for the distributed case. Within a performance study, we finally compare our findings with the approach of applying a selection algorithm locally to each node in a distributed warehouse environment.