Much attention has been focused on the problem of deadlock detection in distributed databases, resulting in the publication of numerous algorithms to accomplish this function. The algorithms published to date differ greatly in many respects: timing, location, information collection, and basic approach. The emphasis of this research has been on theory and proof of correctness, rather than on practical application. Relatively few attempts have been made to implement the algorithms. The impact of the characteristics of the underlying database management system, transaction model, and communications network upon the effectiveness and performance of the proposed deadlock detection algorithms has largely been ignored. It is the intent of this study to examine more closely the interaction between a deadlock detection algorithm and one aspect of the environment in which it is implemented: namely, the communications network. / Master of Science

<i>Survivability</i> is a means of introducing redundancy in a network architecture which is utilized to maintain communication capability under failure conditions. The need for survivability exists in most communication networks. Fiber optic networks concentrate large amounts of traffic within a few spans, using their high capacity to achieve a cost advantage. It is crucial to incorporate survivability at minimum cost in the design of such networks. This problem is conventionally tackled using techniques such as <i>Automatic Protection Switching</i> (APS), <i>Dual homing and Self-Healing Ring</i> (SHR). These techniques, although relatively simple to implement, are not efficient in utilizing network capacity. Interconnected ring topologies use the network capacity more efficiently at the expense of complex fault detection and routing methodologies. The <i>toroidal grid</i> is shown to be an optimized form of a generalized survivable interconnected ring topology. It is shown to have significantly lower capacity requirements than the simple self healing ring networks. The <i>modified toroidal</i> grid is an improvement on the toroidal grid because it has a planar graph and shorter average link length. A quantity called <i>link-traffic summation</i> is defined as a metric of total link capacity requirement in a network. Analytical values for link-traffic summation are obtained for simple ring, generalized interconnected ring, toroidal grid and modified toroidal grid networks. Simulations are performed for the modified toroidal grid and their results are compared with the analytical values obtained. / Master of Science

A need exists for the State of Virginia to have a data network for its televised distance education program. A combination of a terrestrial and a satellite data network can be utilized for the purpose. The network is analyzed and its strengths and weaknesses are presented. A data protocol has been written to control such a network. Delays and throughput of the network have been calculated. The leased telephone line network can be utilized for transferring data from distant class sites to Blacksburg. Six pages of text for 75 off-campus students can be transferred from distant class sites to Blacksburg using this network, in one hour. When the terrestrial network is used for voice and data communication at the same time, a delay of approximately 30 seconds is introduced between two voice connections. This delay is too high for a distance education network. The satellite data network should be utilized for transferring data from Blacksburg to all distant class sites. A very good 19.2 kbps carrier is available with bit error rate (BER) of 1E-6 or less. A very small aperture terminal (VSAT) network has also been proposed for the purpose. / Master of Science

A computer-mediated communication (CMC) system oriented towards changing health-related behavior was developed and evaluated. Stress management training was used to demonstrate the basic technology. Formative research and pilot-testing was conducted, to identify psychological and communication variables that are potentially critical to facilitating behavior change through the CMC medium. The resulting system was used to compare two forms of CMC training with face-to-face intervention: CMC Intensive Intervention (CII) and CMC Non-Intensive Intervention (CNI), with 9 matched subjects per treatment condition. CII subjects used their personal computers and modems to exchange messages with a therapist (the author), as well as to interact with automated system functions. Such functions included cognitive/behavioral assessment, l recording of self-monitored progress in applying specific and general coping strategies to managing stress, and instantaneous graphic and verbal feedback on such progress. The CNI form of intervention relied primarily on message exchanges with the therapist, and presentation of general information on stress management. The face-to-face (FFI) treatment was a "traditional" stress management workshop, comprising weekly one-hour sessions over a six-week period. FFI subjects’ assessment, information presentation, self-monitoring, and feedback were analogous to their CII counterparts. Major findings were that the CII treatment was as effective as the FFI treatment, while the CNI intervention was less effective than the other two treatments, through 3 months follow-up. This finding suggests that active involvement of subjects in practicing specific coping strategies may be critical to efficacious intervention through the CMC medium. Other findings were that: (1) The CMC message-exchange function can establish a "client-therapist relationship", which potentially overcomes a limitation of "computerized treatment" identified by previous researchers. (2) CMC-based intervention, at least for stress management-related problems, appears to be more cost-effective than face-to-face intervention, for individual treatment but not for group treatment. Implications for extensions to other types of behavior-change intervention and research are discussed. / Ph. D. / incomplete_metadata

The inherent properties of optical fibers such as small size and weight, EMI/RFI immunity, low attenuation, and large bandwidth provide many advantages over wire conductors that make fiber well-suited for communications. Fiber optic local area networks are particularly suited for use in electrically noisy and space sensitive industrial environments. The diversity of communication requirements that exist in a typical factory situation can be accommodated by the use of a hierarchical communications structure consisting of multiple tiers of fiber optic networks. The lowest tier of this structure would be inexpensive feeder networks used to connect devices such as sensors, actuators, PLCs, robots, and small computers on a factory floor. The emphasis at this level is low cost, but while providing interconnection to higher tiers. An approach which satisfies the lowest tier requirements is a non-shared medium scheme which is link based, consisting of an active star architecture using a roll-call polling access method. The centralized intelligence structure of a master/slave access method allows one to concentrate on the cost optimization of the optical data links. The use of low-cost optical components such as LEDs, PIN diodes, and plastic fiber as well as the potential for a significant amount of common hub equipment provides considerable economies. / Master of Science

Dynamic hierarchical networks represent an architectural strategy for employing adaptive behavior in applications sensitive to highly variable external demands or uncertain internal conditions. The characteristics of such architectures are described, and the significance of adaptive capability is discussed. The necessity for assessing cost/benefit tradeoffs leads to the use of queueing network models. The general model, a network of M/M/1 queues in a random environment, is introduced and then is simplified so that the links may be treated as isolated M/M/1 queues in a random environment. This treatment yields a formula for approximate mean network delay by combining matrix-geometric results (mean queue length and mean delay) for the individual links. Conditions under which the analytic model is considered valid are identified through comparison with a discrete event simulation model. Last, performance of the dynamic hierarchy is compared with that of the static hierarchy. This comparison establishes conditions for which the dynamic architecture enables performance equal or nearly equal to performance of the static architecture. / Ph. D. / incomplete_metadata

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