A study of topologies and protocols for fiber optic local area network

The emergence of new applications requiring high data traffic necessitates the development of high speed local area networks. Optical fiber is selected as the transmission medium due to its inherent advantages over other possible media and the dual optical bus architecture is shown to be the most suitable topology. Asynchronous access protocols, including token, random, hybrid random/token, and virtual token schemes, are developed and analyzed. Exact expressions for insertion delay and utilization at light and heavy load are derived, and intermediate load behavior is investigated by simulation. A new tokenless adaptive scheme whose control depends only on the detection of activity on the channel is shown to outperform round-robin schemes under uneven loads and multipacket traffic and to perform optimally at light load. An approximate solution to the queueing delay for an oscillating polling scheme under chaining is obtained and results are compared with simulation. Solutions to the problem of building systems with a large number of stations are presented, including maximization of the number of optical couplers, and the use of passive star/bus topologies, bridges and gateways

Aerospace Medicine and Biology: A continuing bibliography with indexes, supplement 140

This bibliography lists 306 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1975

Aeronautical engineering: A continuing bibliography, supplement 122

This bibliography lists 303 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1980

A Comprehensive Analysis on the Healthcare Systems in Latin America

This comprehensive report summarizes and analyzes the current state of healthcare in Latin America, specifically in the countries of Mexico, Brazil, the Northern Triangle (El Salvador, Guatemala, and Honduras), the Andean Region (Ecuador, Peru, and Bolivia) and the Rioplatense Region (Uruguay, and Argentina). Each country's healthcare system is examined in depth, in terms of health infrastructure, the healthcare providing industry, and the effects of healthcare on the countries' respective populations.Mosser, Michael W.International Relations and Global Studie

Vegetation Canopy Cover Effects on Sediment and Salinity Loading in the Upper Colorado River Basin Mancos Shale Formation, Price, Utah

With future climate change and increased water demand and scarcity in the Colorado River Basin, the Bureau of Reclamation estimates that the costs of salinity damage will increase for Colorado River users and will exacerbate the current salinity challenges. This study focuses on saline and sodic soils associated with the Mancos Shale formation in order to investigate the mechanisms driving sediment and salinity loads in the Price-San Rafael River Basin of the upper Colorado River. A Walnut Gulch rainfall simulator was operated with a variety of slope angles and rainfall intensities at two field sites (Price, Dry-X) near Price, Utah in order to evaluate how the amount and spatial distribution of vegetation affects salinity in runoff. For each simulated rainfall event, the time-varying concentrations of major cations, anions, and sediment in runoff were measured. Principal component analysis revealed that the two field sites are generally different in runoff water chemistry and soil chemistry, likely due to the difference in parent material and soil indicative of their location on different geologic members. The Dry-X site also has substantially greater total dissolved solids (TDS) and sediment in runoff, soil sodium absorption ratio (SAR), and soil cation exchange capacity (CEC) than the Price site. Despite these differences, a consistent positive linear relationship between the plot-averaged sediment and TDS concentration was found across both sites. The Rangeland Hydrology Erosion Model (RHEM) was calibrated to provide unbiased estimates of sediment in runoff from 23 runs of the rainfall simulator. RHEM simulated the plot-plot variability best at Dry-X compared to Price. Sensitivity analysis of the RHEM input parameters showed that the splash and sheet erodibility coefficient (Kss) and the effective saturated conductivity coefficient (Ke) had the largest influence on the model’s sediment and discharge outputs, respectively. The regression that predicted TDS concentration from sediment was applied to RHEM outputs to show that the model could be used to provide salinity estimates for different storm intensities on this part of the Mancos Shale. The potential influence of vegetation canopy cover on sediment production from these two sites was inferred by running RHEM with canopy cover values ranging from 0% to 100%. This changed sediment output by 111% to -91% relative to the present vegetation cover. Measures of the geometry of soil and vegetation patches at Dry-X, such as fractal dimension index and proximity index, showed a relationship to error residuals from RHEM. As the vegetation becomes less isolated, more uniform, and the tortuosity of the bare soil area increases, observed sediment decreases relative to RHEM predictions. The results of this study will help land management agencies assess the feasibility of mitigation strategies for reducing sediment and salinity loads from the saline and sodic soils of the Mancos Shale formation and indicate a possible benefit to incorporating the parameters that describe the spatial pattern of vegetation in RHEM

Surface oscillation in peatlands: How variable and important is it?

Hydrology, particularly the water table position below the surface, is an important control on biogeochemical and ecological processes in peatlands. The position of the water table is a function of total storage changes, drainable porosity and peatland surface oscillation (PSO). Because the absolute level of the peat surface (ASL) oscillates in a peatland, we can assign two different water table positions: the water table depth below the surface (relative water level, RWL) and the water table position above an absolute elevation datum eg. sea level (absolute water level, AWL). A review of 37 studies that report peatland surface oscillation indicate a wide range (0.4-55 cm), which is to the same order as (or one order smaller than) water storage changes and RWL fluctuations. PSO can vary substantially across a single peatland and through time. A set of mechanisms (flotation, compression/shrinkage, gas volume changes and freezing) is hypothesised to cause ASL changes. The potential of PSO to reduce RWL fluctuations trended (mean in %) floating peatlands (63) > bogs (21), fens (18) > disturbed peatlands (10) with respect to peatland types. To investigate the spatiotemporal variability of peatland surface oscillation, AWL and ASL were monitored continuously over a one-year period (one site) and monthly (23 sites) in a warm-temperate peatland that is dominated by Empodisma minus (Restionaceae). A new measurement method was developed by pairing two water level transducers, one attached to a stable benchmark (ÆAWL) and one attached to the peat surface (ÆRWL). From August 2005 until August 2006 the ASL oscillated at one site through a range of 22 cm following AWL fluctuations (in total 47 cm). Consequently, RWL fluctuations were reduced on average to 53% of AWL fluctuations. The strong AWL-ASL relationship was linear for 15 sites with manual measurements. However, eight sites showed significantly higher rates of peatland surface oscillation during the wet season (ie. high AWLs) and thus a non-linear behaviour. Temporary flotation of upper peat layers during the wet season may have caused this non-linear behaviour. On the peatland scale AWL fluctuations (mean 40 cm among sites) were reduced by 30–50% by PSO except for three sites with shallow and dense peat at the peatland margin (7–11%). The reduction of RWL fluctuation was high compared to literature values. The spatial variability of PSO seemed to match well with vegetation patterns rather than peat thickness or bulk density. Sites with large PSO showed high cover of Empodisma minus. Surface level changes exhibited surprisingly hysteretic behaviour subsequent to raised AWLs, when the rise of ASL was delayed. This delay reversed the positive ASL-AWL relationship because the surface slowly rose even though AWL started receding. Hysteresis was more pronounced during the dry season than during the wet season. The observed hysteresis can be sufficiently simulated by a simplistic model incorporating delayed ASL fluctuations. PSO has wide implications for peatland hydrology by reducing RWL fluctuations, which feed back to peat decomposition and plant cover and potentially to (drainable) porosity. Stable RWL also reduce the probability of surface run-off. It is further argued that the gas content of the roots of plants, particularly Empodisma minus, added enough buoyancy to detach the uppermost peat layers resulting in flotation

Microbiological modulation of suspended particulate matter dynamics: A study of biological flocculation in nutrient-enriched waters

The study of suspended particulate matter (SPM) dynamics has conventionally focused on physical and hydrodynamical interactions, with little attention paid on exploring the role of SPM as a micro-ecosystem that sustains a wide diversity of microbial colonies. This thesis puts forth a new paradigm of SPM dynamics that integrates mineral, chemical, and biological components into one framework to emphasize the role of microorganisms in altering the chemistry and structure of SPM, which further affect its transport and deposition. Microbiological modulation of SPM dynamics was investigated in this thesis by coupling experiments with numerical models. Experimental results revealed that the size of biomass-affected SPM was approximately 60% larger and the capacity dimension was 2% lower as compared to biomass-free SPM. In contrast, the average settling velocity was observed to be nearly invariant for all SPM types. It was also found that the probability for SPM to aggregate was highly dependent on SPM shape and surface asperity, suggesting that microorganisms can alter SPM collision and aggregation kinematics through their role in modifying SPM structure and shape. Analyses coupling experimental results and a biogeochemical model further reveal the feedback interactions between minerals, chemicals, and microorganisms. It shows how changes in sediment and water qualities can have impacts on microorganisms that in turn modify SPM characteristics and result in further alteration of sediment and water qualities. This thesis provides an insight into the role played by microorganisms in engineering the architecture and altering the chemistry of SPM, with experimental evidence and simulation results put forth to emphasize that the contributions of nutrients and microorganisms cannot be neglected in modelling and predicting SPM dynamics

Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Interrogating pancreatic islet alpha cell calcium dynamics

Imperial Users onl