Christian Prehofer

A Distributed End-to-End Reservation Protocol for IEEE 802.11-Based Wireless Mesh Networks

This paper presents an end-to-end reservation protocol for quality-of-service (QoS) support in the medium access control layer of wireless multihop mesh networks. It reserves periodically repeating time slots for QoS-demanding applications, while retaining the distributed coordination function (DCF) for best effort applications. The key features of the new protocol, called distributed end-to-end allocation of time slots for real-time traffic (DARE), are distributed setup, interference protection, and scheduling of real-time data packets, as well as the repair of broken reservations and the release of unused reservations. A simulation-based performance study compares the delay and throughput of DARE with those of DCF and the priority-based enhanced distributed channel access (EDCA) used in IEEE 802.11e. In contrast to DCF and EDCA, DARE has a low, nonvarying delay and a constant throughput for each reserved flow

Ambient networks: a framework for future wireless internetworking

An increasingly wireless world faces new challenges due to the dynamicity of interactions, range of applications, multitude of available radio access technologies and network functionality. The ambient networks project recognizes these trends and enables the creation of innovative network solutions for mobile and wireless systems beyond 3G. These networks enables scalable and affordable wireless networking while providing pervasive, rich and easy-to-use communication. A specific focus lies on enabling advanced capabilities in environments with increased competition as well as cooperation, environments that are populated by a multitude of user devices, wireless technologies, network operators and business actors. The project adopts a modular architecture that enables plug-and-play control extensibility that supports a wide range of different applications and network technologies. Based on a small subset of common functionality, this approach supports the dynamic deployment of advanced internetworking capabilities, such as media and context-awareness or multi-radio access.

An adaptive trust control model for a trustworthy component software platform

Trust has been recognized as a vital factor for a component software platform. Inside the platform, trust of a platform entity can be controlled according to its assessment result. Special control modes can be applied in order to ensure a trustworthy system. In this paper, we present an adaptive trust control model in order to support autonomic trust management for the component software platform. This model is based on a Fuzzy Cognitive Map. It includes the quality attributes of the platform entity and a number of control modes supported by the platform in order to ensure the entitys trustworthiness. The parameters of this model can be adaptively adjusted in order to reflect real system context. The simulation results show that this model is effective for automatically predicting and selecting feasible control modes for a trustworthy platform. It also helps studying cross-influence of applied control modes on a number of quality attributes.

Distributed maintenance of resource reservation paths in multihop 802.11 networks

Supporting applications with high quality-of-service requirements in wireless multihop networks is challenging, mainly due to uncertainties about the packet forwarding times introduced by the data link layer at every node on the path to the destination. For instance, networks based on IEEE 802.11 exhibit such variable forwarding times due to their CSMA (carrier sense multiple access) mechanism. Previously, we designed a fully distributed reservation protocol that introduces time slots into this CSMA mechanism. We now analyze the performance of our protocol in scenarios where nodes randomly switch on and off. We show, by simulation, that our protocol achieves a lower average and less variable end-to-end delay.

Practical Web-Based Smart Spaces

Researchers from Nokia propose a Web-based framework that applies a resource-based HTTP style called Representational State Transfer (REST) to enable smart spaces to support pervasive applications in various devices. Mobile devices are evolving into hubs of content and context information. Therefore, our work focuses on pervasive applications in smart spaces that use locally available connectivity and device discovery. This approach allows, for example, sharing content and offering services locally with direct connections between devices. Although many research projects have shown the potential of such applications, the results of this research havent yet become widely deployed.

Approach for Dynamically Composing Decentralised Service Architectures with Cross-Cutting Constraints

The emergence of open, composable Internet services and mashups means that services cannot be composed in a centralised manner. Despite this, cross-cutting constraints might exist between services, stemming from, e.g., security. Especially when used with mobile devices, these service compositions need to be constructed at runtime. This paper proposes a knowledge-based approach for dynamically finding and validating decentralised service compositions while taking into account cross-cutting constraints. The approach is exemplified with a case of a shopping mall portal.

Semantic reasoning about feature composition via multiple aspect-weavings

In this paper, we consider semantic refinement for feature-oriented programming where components are built from features and weavings, which we use to adapt one feature to the context of another one. We address the question of semantic reasoning about multiple weavings. If we know the effect of feature A on X and of feature B on X, what can we conclude about adding both A and B to X? For this, we define conservative weavings which do not modify the state of another feature. We show that composition of several such weavings is however not compositional as it does not preserve semantics. In particular, weavings must consider that other weavings have already been applied. This explains why it is considerably more difficult to reason about multiple aspect weavings. We show criteria on the dependencies between weavings which allow modular, semantics-preserving application of weavings. This is formalized in a calculus for feature composition and also extended to conditional refinements.

From the Internet of Things to Trusted Apps for Things

We present a new approach for Internet of things (IoT) applications (apps) for domains where a high level of reliability is needed. The two main current approaches, using a central application server and sand boxing for applications on IoT devices, are discussed. Then we present our approach, which enables trusted local computation without the need for a sandbox. Instead, we use a model-based approach, where code is generated from state machine models. By using formal methods, we ensure the basic safety properties of the applications. We show an example of an application for a fridge and verify basic safety properties. The typical verification conditions as well as the new role of a trusted application provider are discussed.

Towards a Common Sensor Network API: Practical Experiences

Wireless sensor networks are becoming more and more popular and there are many vendors offering solutions with similar functionality, but utilizing different approaches to controlling the network and gathering the data. We have evaluated two very different types of networks, Arch Rock and Z-Wave, in three main areas: network configuration, access control, and fault tolerance. Even though some functionality is shared by both types of networks, there are conceptual differences that need to be taken into consideration. We propose an API for sensor networks, which considers the above practical issues of existing sensor networks.

Models at REST or modelling RESTful interfaces for the Internet of Things

We present a first approach to model complex IoT systems with sensors and actuators in UML models and then to generate RESTful interfaces from these. As shown for software development in the embedded domain, high-level models can significantly improve productivity. In particular, we model the composition of resources to structure IoT devices in an hierarchical way and model the behavior of actuators as state diagrams. From these models, we show how to generate rich RESTful interfaces for the discovery, reading and manipulation of resources for the Internet of things.