Allan I. McInnes

Utility of algebraic connectivity metric in topology design of survivable networks

In studies of survivable networks, it is important to be able to differentiate network topologies by means of a robust numerical measure that indicates the levels of immunity of these topologies to failures of their nodes and links. Ideally, such a measure should be sensitive to the existence of nodes or links which are more important than others, for example, if their failures cause the networks disintegration. In this paper, we suggest using an algebraic connectivity metric, adopted from spectral graph theory, namely the 2nd smallest eigenvalue of the Laplacian matrix of the network topology, instead of the average nodal degree that is usually used to characterize network connectivity in studies of the spare capacity allocation problem. Extensive simulation studies confirm that this metric is a more informative and more accurate parameter than the average nodal degree for characterizing network topologies in survivability studies.

Self-Tracking Mobile Station Controls Its Fast Handover in Mobile WiMAX

This paper proposes a mobile station (MS)-controlled fast MAC-layer handover (HO) scheme to reduce the HO latency in Mobile WiMAX. Based on the received signal strength (RSS) from any Base Station (BS), the MS can estimate its present distance from that BS. This capability allows the MS, through utilization of the broadcast messages by its Serving BS (SBS) and through few scannings of the Neighbouring BSs (NBS), to self-track its own direction of motion relative to the SBS and the NBSs and thus look ahead to perform an intelligent choice of the Target BS (TBS) for the impending HO. Simulation studies have shown a considerable reduction in the HO latency in the proposed method.

Using CSP to Model and Analyze TinyOS Applications

The TinyOS concurrency model, although easier to reason about than shared-state threads, may still produce undesirable behavior as a result of unexpected interleaving of concurrent activities. This is problematic, since TinyOS applications are typically intended to run unattended for long periods of time, and must be reliable. In this paper, we describe a technique for modeling the interactions between TinyOS application components, and between an application and the TinyOS scheduling and preemption mechanisms, using the process algebra CSP. Analysis of the resulting process models can help TinyOS application developers to discover and diagnose concurrency-related errors in their designs that might otherwise go undetected until deployment of the application.

Model-checking the Flooding Time Synchronization Protocol

Large-scale wireless sensor networks must be reliable, since they are intended to be operated without human intervention. Using well-understood building-blocks is one method of increasing confidence in the reliability of a sensor network design. In this paper, we use model-checking to analyze and characterize the Flooding Time Synchronization Protocol, a synchronization protocol that is distributed along with the TinyOS sensor network operating system. We apply a number of abstraction techniques to keep the model state-space small, and as a result are able to verify several properties of FTSP networks that have not previously been checked. Our results provide greater confidence in FTSP, and also establish some limitations on the size of FTSP networks. Our FTSP model provides a basis for further model-checking of FTSP.

A simple metric for turn-taking in emergent communication

To facilitate further research in emergent turn-taking, we propose a metric for evaluating the extent to which agents take turns using a shared resource. Our measure reports a turn-taking value for a particular time and a particular timescale, or “resolution,” in a way that matches intuition. We describe how to evaluate the results of simulations where turn-taking may or may not be present and analyze the apparent turn-taking that could be observed between random independent agents. We illustrate the use of our turn-taking metric by reinterpreting previous work on turn-taking in emergent communication and by analyzing a recorded human conversation.

Model Checking a TTCAN Implementation

This paper describes how the SPIN model checker has been applied to find and correct problems in the software design of a distributed vessel control system currently under development at a control systems specialist in New Zealand. The system under development is a mission critical control system used on large marine vessels. Hence, the requirement to study the architecture and verify the implementation of the system. The model checking work reported here focused on analysing the implementation of the Time-Triggered Controller-Area-Network (TTCAN) protocol, as this is used as the backbone for communications between devices and thus is a crucial part of the control system. The starting point was to develop a set of general techniques for model checking TTCAN-like protocols. The techniques developed include modelling the progression of time efficiently in SPIN, TTCAN message transmission, TTCAN error handling, and CAN bus arbitration. These techniques form the basis of a set of models developed to check the implementation of TTCAN in the control system as well as the fault tolerance schemes added to the system. Descriptions of the models and properties developed to check the correctness of the implementation are given, and verification results are presented and discussed. This application of model checking to an industrial design problem has uncovered and corrected a number of potentially costly issues in the original design.

Rewards for pairs of Q-learning agents conducive to turn-taking in medium-access games

We describe a class of stateful games, which we call ‘medium-access games’, as a model for human and machine communication and demonstrate how to use the Nash equilibria of those games as played by pairs of agents with stationary policies to predict turn-taking behaviour in Q-learning agents based on the agents’ reward function. We identify which fixed policies exhibit turn-taking behaviour in medium-access games and show how to compute the Nash equilibria of such games by using Markov chain methods to calculate the agents’ expected rewards for different stationary policies. We present simulation results for an extensive range of reward functions for pairs of Q-learners playing medium-access games and we use our analysis for stationary agents to develop predictors for the emergence of turn-taking. We explain how to use our predictors to design reward functions for pairs of Q-learning agents that are conducive (or prohibitive) to the emergence of turn-taking in medium-access games. We focus on designing multi-agent reinforcement learning systems that deliberately produce coordinated turn-taking but we also intend our results to be useful for analysing emergent turn-taking behaviour. Based on our turn-taking related results, we suggest ways to use our methodology to designs rewards for quantifiable behaviours besides turn-taking.

A hierarchical control scheme for coordinated motion of mobile manipulators

Mobile manipulators are becoming more widespread, with growing commercial and scientific interest in their use. The addition of a mobile base to a manipulator greatly extends the workspace of the manipulator, but introduces complex control problems involving coordination of base and manipulator motion while simultaneously ensuring platform stability and good manipulability during task execution. This paper describes a hierarchical control scheme for a mobile manipulator designed to maintain manipulability and stability performance metrics within specified thresholds during the execution of manipulation tasks. We demonstrate the effectiveness of our control scheme by applying it to control of a simulated nonholonomic base with a 6 degree-of-freedom manipulator. Our simulation results show that the controller successfully completes a manipulation task involving multiple end-effector targets, and maintains stability and manipulability to the desired thresholds throughout.

A Modular Hierarchical Control Scheme for Mobile Manipulation

The addition of a mobile base to a robotic manipulator greatly extends the workspace of the manipulator, but introduces complex control problems involving coordination of base and manipulator motion. We describe a modular, hierarchical control scheme for a mobile manipulator, designed to coordinate motion of the manipulator and base to maintain various performance metrics. We demonstrate the effectiveness of our control scheme by developing a controller that maintains the stability and manipulability of a nonholonomic base with a 6 degree-of-freedom manipulator while executing manipulation tasks. We demonstrate the modularity of our control scheme by showing how the controller can be extended to avoid obstacles without requiring redesign of the rest of the controller. Simulation results show the controller completing a task involving multiple end-effector targets, avoiding simple obstacles, and maintaining stability and manipulability within desired limits.

Modeling and Analysis of TinyOS Sensor Node Firmware: A CSP Approach

Wireless sensor networks are an increasingly popular application area for embedded systems. Individual sensor nodes within a network are typically resource-constrained, event-driven, and require a high degree of concurrency. This combination of requirements motivated the development of the widely used TinyOS sensor node operating system. The TinyOS concurrency model is a lightweight nonpreemptive system designed to suit the needs of typical sensor network applications. Although the TinyOS concurrency model is easier to reason about than preemptive threads, it can still give rise to undesirable behavior due to unexpected interleavings of related tasks, or unanticipated preemption by interrupt handlers. To aid TinyOS developers in understanding the behavior of their programs we have developed a technique for using the process algebra Communicating Sequential Processes (CSP) to model the interactions between TinyOS components, and between an application and the TinyOS scheduling and preemption mechanisms. Analysis of the resulting models can help TinyOS developers to discover and diagnose concurrency-related errors in their designs that might otherwise go undetected until after the application has been widely deployed. Such analysis is particularly valuable for the TinyOS components that are used as building blocks for a large number of other applications, since a subtle or sporadic error in a widely deployed building block component could be extremely costly to repair.