Jason O. Hallstrom

Responsibilities and rewards: specifying design patterns

Design patterns provide guidance to system designers on how to structure individual classes or groups of classes, as well as constraints on the interactions among these classes, to enable them to implement flexible and reliable systems. Patterns are usually described informally. While such informal descriptions are useful and even essential, if we want to be sure that designers precisely and unambiguously understand the requirements that must be met when applying a given pattern, and be able to reliably predict the behaviors the resulting system exhibits, we also need formal characterizations of the patterns. In this paper, we develop an approach to formalizing design patterns. The requirements that a designer must meet with respect to the structures of the classes, as well as with respect to the behaviors exhibited by the relevant methods, are captured in the responsibilities component of the patterns specification; the benefits that results by applying the pattern, in terms of specific behaviors that the resulting system is guaranteed to exhibit, are captured in the rewards component. One important aspect of many design patterns is their flexibility; our approach is designed to ensure that this flexibility is retained in the formalization of the pattern. We illustrate the approach by applying it to a standard design pattern.

Teaching CS unplugged in the high school (with limited success)

CS Unplugged is a set of active learning activities designed to introduce fundamental computer science principles without the use of computers. The program has gained significant momentum in recent years, with proponents citing deep engagement and enjoyment benefits. With these benefits in mind, we initiated a one-year outreach program involving a local high school, using the CS Unplugged program as the foundation. To our disappointment, the results were at odds with our enthusiasm --- significantly. In this paper, we describe our approach to adapting the CS Unplugged materials for use at the high school level, present our experiences teaching it, and summarize the results of our evaluation.

An RPC design for wireless sensor networks

Wireless sensor networks (WSNs) will profoundly influence the ubiquitous computing landscape. Their utility derives not from the computational capabilities of any single sensor node, but from the emergent capabilities of many communicating sensor nodes. Consequently, the details of communication within and across single hop neighborhoods is a fundamental component of most WSN applications. But these details are often complex, and popular embedded languages for WSNs do not provide suitable communication abstractions. We propose that the absence of such abstractions contributes to the difficulty of developing large-scale WSN applications. To address this issue, we present the design and implementation of a remote procedure call (RPC) abstraction for nesC and TinyOS, the defacto standard for developing WSN applications. We present the key language extensions, operating system services, and automation tools that enable the proposed abstraction. We illustrate these contributions in the context of a small case study, and draw preliminary conclusions regarding the suitably of our approach to resource-constrained sensor nodes

The South Carolina Digital Watershed: End-to-End Support for Real-Time Management of Water Resources

Water resources are under unprecedented strain. The combined effects of population growth, climate change, and rural industrialization have led to greater demand for an increasingly scarce resource. Ensuring that communities have adequate access to water—an essential requirement for community health and prosperity—requires finegrained management policies based on real-time in situ data, both environmental and hydrological. To address this requirement at the state level, we have developed the South Carolina Digital Watershed, an end-to-end system for monitoring water resources. In this paper, we describe the design and implementation of the core system components: (i) in situ sensing hardware, (ii) collection and uplink facilities, (iii) data streaming middleware, and (iv) back-end repository and presentation services. We conclude by discussing key organizational and technical challenges encountered during the development process.

Implementation of strong mobility for multi-threaded agents in Java

Strong mobility, which allows multithreaded agents to be migrated transparently at any time, is a powerful mechanism for implementing a peer-to-peer computing environment, in which agents carrying a computational payload find available computing resources. Existing approaches to strong mobility either modify the Java virtual machine or do not correctly preserve the Java semantics when migrating multithreaded agents. We give an overview of our implementation strategy for strong mobility in which each agent thread maintains its own serializable execution state at all times, while thread states are captured just before a move. We explain how to solve the synchronization problems involved in migrating a multithreaded agent and how to cleanly terminate the Java threads in the originating virtual machine. We present experimental results that indicate that our implementation approach is feasible in practice

OpenFlow-based load balancing for wireless mesh infrastructure

Wireless mesh-based backhaul infrastructure is intended to provide reliable data transmission, with high throughput across large-scale networks. Load balancing is essential over a long period of operation to provide high throughput and uninterrupted service to end users. Load across mesh nodes is highly variable as the traffic depends on the number of clients connected to the nodes, as well as the services they use. Existing load balancing solutions are based on theoretical analysis and simulations. Most require distributed routing algorithms executed over compute-intensive routing nodes. It is also challenging to provide practical support for real-time traffic redirection using traditional mesh nodes. In this paper, we develop a prototype mesh infrastructure where flows from a source node can take multiple paths through the network. OpenFlow, an emerging technology that makes network switches programmable via a standard interface, allows flexible control of data flow paths. The Better Approach To Mobile Ad-hoc Networking (B.A.T.M.A.N) mesh protocol is used to provide mesh topology and link quality information. The OpenFlow controller decides the best data path based on this information to ensure high throughput data transfer. To demonstrate the usefulness of our approach, we have implemented three test cases to enable data path setup and redirection with low complexity and overhead. Our test case measurements confirm that OpenFlow is a promising complementary technology to traditional mesh routing protocols for wireless networks.

Characterizing data deliverability of greedy routing in wireless sensor networks

As a popular routing protocol in wireless sensor networks (WSNs), greedy routing has received great attention. The previous works characterize its data deliverability in WSNs by the probability of all nodes successfully sending their data to the base station. Their analysis, however, neither provides the information of the quantitative relation between successful data delivery ratio and transmission power of sensor nodes nor considers the impact of the network congestion or link collision on the data deliverability. To address these problems, in this paper, we characterize the data deliverability of greedy routing by the ratio of successful data transmissions from sensors to the base station. We introduce η-guaranteed delivery which means that the ratio of successful data deliveries is not less than η, and study the relationship between the transmission power of sensors and the probability of achieving η-guaranteed delivery. Furthermore, with considering the effect of network congestion and link collision, we provide a more precise and full characterization for the deliverability of greedy routing. Extensive simulation and real-world experimental results show the correctness and tightness of the upper bound of the smallest transmission power for achieving η-guaranteed delivery.

An Interactive, Source-Centric, Open Testbed for Developing and Profiling Wireless Sensor Systems

The difficulty of developing wireless sensor systems is widely recognized. Problems associated with testing, debugging, and profiling are key contributing factors. While network simulators have proven useful, they are unable to capture the subtleties of underlying hardware, nor the dynamics of wireless signal propagation and interference; and physical experimentation remains a necessity. To this end, developers increasingly rely on shared deployments exposed for physical experimentation. Sensor network testbeds are under development across the world. We present a complementary testbed architecture that derives its novelty from three characteristics. First, the system is interactive; users can profile source and network level components across a network in real time, as well as inject transient state faults and external network traffic. Second, the system is source-centric; it enables automated source code analysis, instrumentation, and compilation. Finally, the design is open; developers can extend the set of exposed inter faces as appropriate to particular projects without modifying the underlying middleware. We present the testbed design and implementation, a graphical user interface, a shell-based macro programming interface, example scenarios that illustrate their use, and discuss the testbeds application in the research and teaching activities at client institutions.

Injecting rapid feedback and collaborative reasoning in teaching specifications

We describe an approach to teaching formal interface specifications using aspects of the Collaborative Reasoning Paradigm. The module requires students to construct test cases independently and cooperatively based on their understanding of a given set of method specifications. Students are supported by software-based reasoning assistants that guide them through their exercises and provide realtime feedback as they work --- both for the students and the instructor. We describe the design of the course module, the supporting reasoning assistant, and representative reasoning exercises. We conclude with a discussion of evaluation results from a recent pilot study conducted at Clemson University.

Towards Ontology-based Data Quality Inference in Large-Scale Sensor Networks

This paper presents an ontology-based approach for data quality inference on streaming observation data originating from large-scale sensor networks. We evaluate this approach in the context of an existing river basin monitoring program called the Intelligent River®. Our current methods for data quality evaluation are compared with the ontology-based inference methods described in this paper. We present an architecture that incorporates semantic inference into a publish/subscribe messaging middleware, allowing data quality inference to occur on real-time data streams. Our preliminary benchmark results indicate delays of 100ms for basic data quality checks based on an existing semantic web software framework. We demonstrate how these results can be maintained under increasing sensor data traffic rates by allowing inference software agents to work in parallel. These results indicate that data quality inference using the semantic sensor network paradigm is viable solution for data intensive, large-scale sensor networks.