Bluetooth Low Energy (BLE) beacons raised high expectations when introduced as a cheap, easy to handle indoor positioning solution that is widely supported by smartphones. Experiments soon showed that RSSI values measured by devices vary significantly even when conducted under identical setups. As a result, BLE beacons can be used for what they are designed, i.e., proximity detection. However, setting up indoor positioning systems that cover complete floors and buildings with acceptable accuracy and out of the box is far from being straightforward. This paper evaluates setups for BLE indoor positioning at an inhouse fair to position dedicated smartphones among visitors and exhibition stands. It investigates how established positioning approaches, namely trilateration, fingerprinting, and centroid, respond to the impact of the crowd. Experiments show for the .95 percentile that the weighted centroid technique WCWCL achieves the highest position accuracy of 5.7 meters in the crowded space. Fingerprinting, on the other hand, is heavily impacted by the crowd. While most accurate in an empty/uncrowded space, its accuracy decreases to 11 meters in the crowded space. Further, the paper shows that for a setup with randomly distributed beacons a density of one beacon per 40 square meter is a sweet spot beyond which accuracy does not further increase.

Mobile devices capture, process, and exchange sensory data about their operating environment making them attractive service providers for pervasive computing. Composing services from different devices supports context-aware applications for smart spaces. However, carrier mobility and participation autonomy cause frequent changes to the network and service topology and impose a high failure probability on composites. Decentralised composition solutions assign service providers at runtime to provide for flexibility and to avoid a single point of failure. However, existing solutions rely on a pre-established service overlay network and employ a conservative allocation strategy which limits their applicability for highly dynamic environments. This paper presents a novel service composition protocol that allocates and invokes service providers opportunistically to minimise the impact of topology changes and to reduce failure. The protocol supports service sequences and parallel service flows. Automated model checking verifies that the protocol does not deadlock and that it terminates in a valid end state after having allocated the correct number of service providers for all required sub-services. The results of the simulation-based evaluation demonstrate that the opportunistic approach generally reduces composition failure. At the same time, the evaluation reveals the protocol's limits with regard to composite complexity, network density, and service demand.

Dynamic service composition has emerged as a promising approach to build complex runtime-adaptable applications as it allows for binding service providers only shortly before service execution. However, the dynamic and ad hoc nature of mobile computing environments poses a significant challenge for dynamic service composition. In particular, the lack of central control and the potential volatility of service providers increase the complexity and failure probability of the composition process. Although, current research has led to decentralised composition algorithms and failure recovery strategies, the key question of how to reduce the failure probability of a composition still remains. We address this question and propose opportunistic service composition, an optimised execution model for complex service requests. The model merges the execution phase into the dynamic binding phase and supports the immediate fulfilment of partially composed service requests. We evaluated our model in mobile ad hoc network simulations. The results show an improvement over a baseline approach regarding composition success, response time, and communication effort.

Disclosing personal and contextual information enables a widespread use of services in the internet in a comfortable working environment adapted to individual user needs and preferences. On the other hand, information disclosure raises serious privacy issues. In this paper, we propose a novel integrated approach for achieving a good balance between convenience and privacy. We integrate privacy- enhancing identity management (PIM) with extended role- based access control mechanisms (RBAC) in an extended concept and architecture. In combination, both mechanism enable a fine grained control of what private and contextual information is disclosed to whom in what situation. We have implemented a privacy-enhanced adaptive communicator application (PEAC) to validate our approach.

Following Mark Weiser's vision of ubiquitous computing and calm technology, computer systems should run in the background, preferably without the user noticing it at all. The gathering and disclosure of contextual information on the one hand enables the improvement of system behaviour towards a more autonomous and adaptive behaviour but on the other hand raises privacy issues by disclosing personal data. Thus, a major challenge in ubiquitous computing environments is achieving a good balance between convenience and control over personal data. In this paper we describe an access control mechanism for context data that enables the user to control his personal data in a convenient and non-intrusive way. The approach is based on existing role-based access control mechanisms but extends them as follows. Firstly, our approach is owner-centric, i.e. it is under control of each user, to whom his context is propagated throughout the system. Secondly, our approach does not only control the access to context data but also utilizes context information to simplify the management of these control mechanisms to make the handling of access control more convenient to the user. And thirdly, it introduces individual roles for each user and thus replaces the centrally defined role model of common role-based access control by distinct models for each user. We have validated our approach based on an extended instant messaging system called Adaptive Multimedia Messenger, providing varying buddy information dependent on the access permission of the requesting user