Modellierung und Ablaufplanung adaptiver Multimedia-Dokumente

Multimedia documents are of importance in several application areas, such as education, advertising, and entertainment. Multimedia documents present discrete media objects, such as graphic or text, and continuous media objects, such as video, audio, or animation, in an orchestrated manner to the user. In a distributed environment, multimedia documents are typically stored on servers from which they are retrieved for presentation. The actual presentation takes place at a presentation terminal, such as a PC, a Set-Top-Unit, or even a mobile device. When a user initiates the presentation of a document at a terminal, the terminal takes over the responsibility for orchestrating this presentation, i.e. it schedules the access to remote servers, the playout of data units etc. In order to present a multimedia document, a certain amount of resources is needed. For example, processing and buffer resources are needed at the terminal to present the document, while network resources are required to transfer the media objects associated with the document from the server to the terminal. Since multimedia documents comprise continuous media, the presentation of those documents may require a significant amount of resources. The amount of resources available strongly depends on the system configuration and the current system load. Clearly, a workstation connected to a high-speed network allows for a higher quality presentation than a Personal Digital Assistant (PDA) linked to a radio network. One approach to overcome this problem of heterogeneity is to have different versions of the same logical document, one for each potential configuration. Another approach is to have one document that is able to adapt to the capabilities of the underlying system. A comparison of the approaches shows that the second approach has advantages, since it avoids redundancy and does not have to predict numerous configurations. Even if we consider one type of terminal and one type of network, variations in the system load may cause different amounts of resources to be available. Without resource reservation, the resources available for a presentation may change while the presentation is in progress. If the underlying system provides for resource reservation, the resources needed to present a (monomedia) object (e.g., a video-clip) can be determined and reserved prior to its presentation. However, reservation in general cannot prevent resource shortages to occur during the presentation