Abstract: Context aware systems need to acquire and share various kinds of context knowledge, which not only include the concrete values of environmental parameters, but the vocabularies used to describe the environmental states, i. e.,concepts. Context knowledge is inherently dynamic. It is difficult to predict the context information involved in the system at the moment of design. Most of existing systems lack of effective mechanisms for dynamic context knowledge maintenance, thus unable to support run-time scalability of context aware applications. This paper introduced an ontology based hierarchical context model. On the basis of this model, a dynamic context knowledge acquisition and sharing infrastructure(DCASI) was proposed. The mechanism of distributed acquisition on demand ensures that each class of context knowledge comes from the entity with the most competence to define it, which makes the context knowledge in the system sufficient but not redundant. hhe mechanism of centralized sharing with doublcrepository maintains two knowledge repositories according to different functions, thereby efficiently supports the definition and discovery of new context knowledge. A prototype system was implemented and the proposed infrastructure was validated.

Key words: Context aware, Dynamic context knowledge, Context model, Context infrastructure