Bent Thomsen

A modal process logic

A novel logic is introduced for the introduction of nondeterministic and concurrent processes expressed in a process algebra. For a process algebra to be useful as a process language, it must possess compositionality, i.e. it should be possible to decompose the problem of correctness for a combined system with respect to a given specification of similar and simpler correctness problems for the components of the system. The logic presented allows such specifications to be expressed. It is an extension of process algebra in the sense that process constructs are included as connectives in the logic. Moreover, the formulas of the logic are given an operational interpretation based on which a refinement ordering between formulas is defined.<<ETX>>

Plain CHOCS A second generation calculus for higher order processes

In this paper we present a calculus of communicating systems which allows one to express sending and receiving processes. We call this calculus Plain CHOCS. The calculus is a refinement of our earlier work on the calculus of higher order communicating systems (CHOCS).Essential to the new calculus is the treatment of restriction as a static binding operator on port names. The new calculus is given an operational semantics using labelled transition systems which combines ideas from the applicative transition systems described by Abramsky and the transition systems used for CHOCS. The new calculus enjoys algebraic properties which are similar to those of CHOCS only needing obvious extra laws for the static nature of the restriction operator.Processes as first class objects enable description of networks with changing interconnection structure, and there is a close connection between the Plain CHOCS calculus and the π-Calculus described by Milner, Parrow and Walker: the two calculi can simulate one another.Recently object oriented programming has grown into a major discipline in computational practice as well as in computer science. From a theoretical point of view object oriented programming presents a challenge to any metalanguage since most object oriented languages have no formal semantics. We show how Plain CHOCS may be used to give a semantics to a prototype object oriented language calledO.

Seamless Indoor/Outdoor Positioning Handover for Location-Based Services in Streamspin

This paper presents the implementation of a novel seamless indoor/outdoor positioning service for mobile users.The service is being made available in the Streamspin system(www.streamspin.com), an open platform for the creation and delivery of location-based services. Streamspin seeks to enable the delivery of truly ubiquitous location-based services by integrating GPS and Wi-Fi location fingerprinting. The paper puts focus on key aspects of the seamless handover between outdoor to indoor positioning. Several different handover solutions are presented,and their applicability is evaluated with respect to positioning accuracy and battery consumption of the mobile device.

Model-based schedulability analysis of safety critical hard real-time Java programs

In this paper, we present a novel approach to schedulability analysis of Safety Critical Hard Real-Time Java programs. The approach is based on a translation of programs, written in the Safety Critical Java profile introduced in [21] for the Java Optimized Processor [18], to timed automata models verifiable by the Uppaal model checker [23]. Schedulability analysis is reduced to a simple reachability question, checking for deadlock freedom. Model-based schedulability analysis has been developed by Amnell et al. [2], but has so far only been applied to high level specifications, not actual implementations in a programming language. Experiments show that model-based schedulability analysis can result in a more accurate analysis than possible with traditional approaches, thus systems deemed non-schedulable by traditional approaches may in fact be schedulable, as detected by our analysis. Our approach has been implemented in a tool, named SARTS, successfully used to verify the schedulability of a real-time sorting machine consisting of two periodic and two sporadic tasks. SARTS has also been applied on a number of smaller examples to investigate properties of our approach.

A Facile Tutorial

The Facile system combines language, compiler and distributed system technology into a programming environment supporting the rapid construction of reliable and sophisticated end-user applications operating in distributed computing environments. In particular, Facile is well suited for construction of systems based on the emerging “mobile agents” principle.

A Profile for Safety Critical Java

We propose a new, minimal specification for real-time Java for safety critical applications. The intention is to provide a profile that supports programming of applications that can be validated against safety critical standards such as DO-178B (1992). The proposed profile is in line with the Java specification request JSR-302: Safety Critical Java Technology, which is still under discussion. In contrast to the current direction of the expert group for the JSR-302 we do not subset the rather complex Real-Time Specification for Java (RTSJ). Nevertheless, our profile can be implemented on top of an RTSJ compliant JVM

Some facile chemistry

In this paper we use the chemical abstract machine (CHAM) framework [BeB90, BeB92, Bou94] for discussing various semantics for the Facile programming language [GMP89, GMP90, FAR93] and for formalising (parts of) its implementations. We use these formal descriptions to argue (informally) about implementability and cost of implementation in terms of low level machinery needed to implement the given semantics.We take the Facile language as source for discussion, but the results also apply to several other new languages such as CML [Rep91, BMT92] and Poly/ML [Mat91]. Characteristic for all these languages is that they combine ideas from the λ-calculus and process algebra, such as CCS [Mil80, Mil89], to support high level constructs for programming concurrent, parallel and/or distributed systems.The full version of this extended summary can be found in [LeT94].

A predictable Java profile: rationale and implementations

A Java profile suitable for development of high integrity embedded systems is presented. It is based on event handlers which are grouped in missions and equipped with respectively private handler memory and shared mission memory. This is a result of our previous work on developing a Java profile, and is directly inspired by interactions with the Open Group on their on-going work on a safety critical Java profile (JSR-302). The main contribution is an arrangement of the class hierarchy such that the proposal is a generalization of Real-Time Specification for Java (RTSJ). A further contribution is to integrate the mission concept as a handler, such that mission memory becomes a handler private memory and such that mission initialization and finalization are scheduled activities. Two implementations are presented: one directly on an open source JVM using Xenomai and another, based on delegation, on an RTSJ platform.

WCET analysis of Java bytecode featuring common execution environments

We present a novel tool for statically determining the Worst Case Execution Time (WCET) of Java Bytecode-based programs called Tool for Execution Time Analysis of Java bytecode (TetaJ). This tool differentiates itself from existing tools by separating the individual constituents of the execution environment into independent components. The prime benefit is that it can be used for execution environments featuring common embedded processors and software implementations of the JVM. TetaJ employs a model checking approach for statically determining WCET where the Java program, the JVM, and the hardware are modelled as Networks of Timed Automata (NTA) and given as input to the state-of-the-art UPPAAL model checking tool. The tool is evaluated through a case study based on the classic text-book example of a hard real-time control system in a mine pump. The system is hosted on an execution environment featuring an interpretation-based JVM, called Hardware near Virtual Machine (HVM) that runs on an Atmel AVR ATmega2560 processor.

Algorithmic strategies for adapting to environmental changes in 802.11 location fingerprinting

Ubiquitous and accurate indoor positioning represents a key capability of an infrastructure that enables indoor location-based services. At the same time, such positioning has yet to be achieved. Much research uses commercial, off-the-shelf 802.11 (Wi-Fi) hardware for indoor positioning. In particular, the dominant fingerprinting technique uses a database (called a radio map) of manually collected Wi-Fi signal strengths and is able to achieve positioning accuracies that enable a wide range of location-based services. However, a major weakness of fingerprinting occurs when changes occur in the indoor environment that cause the signal propagation patterns and thus signal strength to change. Under such circumstances, a radio map collected at one time is unable to offer accurate positioning at all times. We propose a data-centric approach to achieving accurate positioning in changing environments. Unlike previous work, our approach does not require the deployment of special sensors that capture current signal strength phenomena, but rather lends itself towards ubiquitous indoor positioning. An empirical comparison of our proposals against conventional, static radio maps demonstrates very significant improvements in positioning accuracy in changing environments.