C. M. Overstreet

Interactive distance learning over intranets

Many distance learning systems claim to be interactive, but few can offer two-way video, on-the-fly interaction and application sharing. To address these limitations, our research group built the Interactive Remote Instruction (IRI) system, which links sites over a high-speed intranet, allowing students at geographically dispersed satellite campuses and community colleges to take a class together. Access from home PCs through a Windows NT port is planned but not yet available. IRI improves on the Old Dominion Universitys Teletechnet system in five areas: video resolution, asymmetrical video presence, interaction, teacher support and computer simulations.

Model composability as a research investment: responses to the featured paper

Responses to the featured paper (see ibid. Composable simulations, Kasputis, S.; Ng, H.C., p. 1577-84) are provided by four authors who represent different elements of the simulation research community: industry, private research laboratory, and university. As is evident from the reactions given, these perspective provide both shared and distinct observations on model composability as an opportunity for research investment.

The simulation project life-cycle: models and realities

Various issues regarding simulation life-cycle models are discussed in the panel session. Simulation life-cycles models have received little attention, and interest generation in this topic is sought and it is hoped that new ideas for development, teaching, and use of these models are stimulated

IRI-h, a Java-based distance education system: architecture and performance

We used our Original Interactive Remote Instruction (IRI) system to teach scores of university classes over the past years at sites up to 300 km apart. While this system is a prototype, its use in real classes allows us to deal with crucial issues in distributed education instruction systems. We describe our motivation and vision for a reimplementation of IRI that supports synchronous and asynchronous distance education. This new version, called IRI-h (h for hetergeneous), is coded in Java and executes on several different platforms. IRI-h extends IRI both to multiple platforms and heterogeneous network experiences with the developing prototype, including preliminary performance evaluation, and also unresolved issues still to be addressed.

Characterizations and relationships of world views

We describe a characterization the three classical world views of event scheduling, activity scanning, and process interaction and discuss transformations among them. We believe that one advantage of each is to allow more concise model descriptions by allowing a model specifier to take advantage of contextual information. Automated transformation among world views is difficult due to a modelers use of contextual information. We illustrate this by transforming and then simplifying a model representation creating a version, similar to what a programmer or modeler might generate.

Model diagnosis using the condition specification: from conceptualization to implementation

Creating efficient implementations for discrete event models with complex behaviors is often difficult. The task of correctly specifying intended behaviors even without dealing with implementation and run-time details is challenging. We describe ongoing work with Condition Specifications intended to support a modeler in the creation of a model specification. A Condition Specification includes no low-level implementation details and is amenable to static analysis which can 1) identify some types of specification errors, 2) provide feedback to the model specifier which might allow the modeler to detect specification errors, or 3) assist creation of an efficient implementation. We review previous work on Condition Specifications, present some important results related to what is achievable in analysis of specifications, describe how analysis can be performed and discuss some current activities in this area.

Dynamic bandwidth allocation in a network

Recently protocols have been introduced which enable us to integrate periodic traffic (voice or video) and aperiodic traffic (data) and to extend the size of local area networks without any loss in speed and capacity. One of these, the DRAMA protocol, is based on broadband technology and allows for dynamic allocation of bandwidth to clusters of nodes in the total network. In this paper we propose a distributed algorithm to allocate bandwidth in a fair manner, where we have defined fair to mean that every node in the network, regardless of its location, should have the same expected access delay to the net. We show by means of simulation that the algorithm is stable and within a few percentage points of the optimal solution. Stability is measured in terms of the time it takes the system to return to near optimal distribution of the bandwidth after strong disturbances. The algorithm is shown to handle total reallocation of the resources within O(100ms).

Model testing: is it only a special case of software testing?

Effective testing of software is an important concern in the software engineering community. While many techniques regularly used for testing software apply equally well to testing the implementations of simulation models, we believe that testing simulations often raises issues that occur infrequently in other types of software. We believe that many code characteristics that commonly occur in simulation code are precisely those that the software testing community has identified as making testing challenging. We discuss many of the techniques that the software engineering community has developed to deal with those features and evaluate their applicability to simulation development.

Use of Web technology for interactive remote instruction

Advancements in web technology are changing learning paradigms. In this paper we relate various learning paradigms to existing technology and describe two approaches in IRI to use web technology for synchronous sessions: web steering and control and automatic content synthesis using the web. IRI (for Interactive Remote Instruction) is a computer-based system to support distance education developed at Old Dominion University; it is being used to teach courses between sites up to 200 miles apart.

A carrier sensed multiple access protocol high data rate ring networks

This paper presents a significant extension of the CSMA network access protocol. The protocol is based on the facts that, at high data rates, networks can contain multiple messages simultaneously over their span, and that in a ring, nodes needs only to detect the presence of a message arriving from the immediate up stream neighbor. When an incoming signal is detected, the node truncates the message it is presently sending instead of aborting it. The system has been named Carrier Sensed Multiple Access Ring Network, CSMA/RN.Analytical studies using three performance factors, wait or access time, service time and response or end-to-end travel time are presented. The service time is basically a function of the network rate; it changes by a factor of 4 between no load and full load. Wait time, which is zero for no load, remains small for load factors up to 70% of full load. Response time, which adds travel time while on the network to wait and service time, is mainly a function of network length, especially for longer distance networks.Simulation results are shown for CSMA/RN where messages are removed at the destination. Destination removal on an average increases network load capacity by a factor of 2, i.e., a 1 Gbps network can handle a 2 Gbps load. A wide range of local and metropolitan area network parameters including variations in message size, network length and node count are studied. In all cases performance is excellent, and message fracture usually remains less than a factor of 4. Throughput, even at overload conditions, remains high for the protocol. The nominal network rate is 1 Gbps; however, performance remains good for data rates as low as 200 Mbps. Finally, a scaling factor based upon the ratio of message to network length demonstrates that the results of this paper, and hence, the CSMA/RN protocol, are applicable to wide area networks.