Locating and Detecting Arrays for Interaction Faults

The identification of interaction faults in component-based systems has focused on indicating the presence of faults, rather than their location and magnitude. While this is a valuable step in screening a system for interaction faults prior to its release, it provides little information to assist in the correction of such faults. Consequently tests to reveal the location of interaction faults are of interest. The problem of nonadaptive location of interaction faults is formalized under the hypothesis that the system contains (at most) some number d of faults, each involving (at most) some number t of interacting factors. Restrictions on the number and size of the putative faults lead to numerous variants of the basic problem. The relationships between this class of problems and interaction testing using covering arrays to indicate the presence of faults, designed experiments to measure and model faults, and combinatorial group testing to locate faults in a more general testing scenario, are all examined. While each has some definite similarities with the fault location problems for component-based systems, each has some striking differences as well. In this paper, we formulate the combinatorial problems for locating and detecting arrays to undertake interaction fault location. Necessary conditions for existence are established, and using a close connection to covering arrays, asymptotic bounds on the size of minimal locating and detecting arrays are established. A final version of this paper appears in J Comb Optim (2008) 15: 17-48

Modelling and monitoring ad hoc networks

A mobile ad hoc network (MANET) is a collection of mobile wireless nodes that self-organize without fixed infrastructure or centralized control. MANETs present a system of many factors, ranging from variables in the protocol stack to the movement of nodes. Choosing the appropriate values for internal parameters in response to external changes in conditions, i.e., adapting the parameters to optimize performance while conditions change, is a fundamental problem. Optimizations for fixed conditions are uninformative in scenarios where conditions change rapidly. Through screening experiments, this dissertation demonstrates that factors in a MANET that contribute to measured responses vary as a function of node speed. At certain speeds one set of factors dictates performance, while at other speeds the set of factors is different. These experiments suggest that a linear model is inappropriate. To effectively optimize systems of this nature, one must optimize reactively—i.e., in an online manner, while the network runs. Two approaches are explored here. The first is rooted in statistical modelling and process monitoring. The second applies machine learning to the problem. Exploring a statistical approach develops profile-driven regression, a new modelling methodology. Profile-driven regression alleviates difficulties in modelling large sets of factors that display nonlinearity. Additionally, a cumulative score based model-monitoring scheme is developed. These methods are used to develop, monitor, and improve a model of throughput, as well as optimize throughput under changing node speeds. The examination of machine learning focusses on artificial neural networks as a means of "learning" throughput conditions. Evaluation of several neural network topologies yields varying results. The best performing topology suggests throughput is affected nonlinearly by some factors, which agrees with the results obtained statistically. Both examinations yield significant performance improvements over default parameters, though the studies conducted reveal limitations in both techniques. Because models are not explicitly represented in their topology, neural networks, while effective in system optimization, prove a poor choice for model monitoring and optimization. Conversely, while the statistical techniques explored provide useful and robust models of throughput under varying conditions, the potentially prohibitive number of experiments is currently more suitable for simulation than real experimentation