Analysis and optimisation of the HIPERLAN Channel Access Contention Scheme

This paper analyses the HIPERLAN channel access mechanism. Both a purely mathematical and a simulation model are built in order to evaluate the two successive phases of the contention scheme. Several optimisations, based on the choice of parametric values, are proposed, evaluated and compared.     

Spectrally stable Er-fiber laser for application in phase-sensitive optical time-domain reflectometry

A high degree of frequency stability in an Er-fiber laser with the Fabry-Perot configuration was achieved using a long (25-km) optical-feedback loop. The frequency-drift rate was determined to be <1 MHz/min by observing temporal fringes in an unbalanced Mach-Zehnder interferometer. Instantaneous linewidth measured with a delayed self-heterodyne interferometer was less than the 3-kHz resolution limit. The laser was utilized in an optical time-domain reflectometry experiment to detect and locate a time-varying phase perturbation applied to an optical fiber.     

The feasibility of smoking bans on psychiatric units

Cryptococcal meningitis has a high mortality rate of central nervous infection. The patients usually die of the disease itself, or complications from increased intracranial pressure. Early diagnosis and treatment, including surgical drainage, will improve the results. In this series, twenty-one patients with high intracranial pressure (ICP > 300 mmH2 O) are presented. Fourteen received implantation of Ommaya reservoir to aspirate cerebrospinal fluid (CSF) for relief of symptoms of ICP. Meanwhile 4 of these 14 patients also received intraventricular injection of amphotericin B because of poor response to systemic drugs. Another seven patient received systemic drug therapy only. Survival during therapy occurred in 11 of 14 patients in the surgical group, compared with only 1 of 7 patients treated by drug therapy alone (P = 0.019). In the 14 patients who received implantation of an Ommaya reservoir, there was one complication of CSF leakage when the reservoir ruptured because of repeated aspiration. For patients with cryptococcal meningitis with high ICP, early implantation of an Ommaya reservoir will improve the survival rate.     

An on-line grain size measuring system for grain-oriented steels

It is important to monitor grain- oriented steel during production so that regions of small, loss- inducing grains can be detected. These are a rare occurrence, but need to be eliminated to prevent a stripe of small grains from passing unnoticed. On- line full- strip- width power loss measurements may overlook stripes of small grains. A system for detecting small grains is described.     

Grain-boundary segregation and precipitation of boron in 0.2 percent carbon steels

The segregation and precipitation of boron have been studied in two 0.2C−0.6Mn0.−5Mo steels containing (nominally) 10 and 50 ppm B. After heating to 1260 δC, samples were air-cooled to 870 °C and then held for times between 0 and 5000 seconds. Additional samples were heated to 1260 °C, air-cooled to 900 °C, reduced in thickness by 50 pct, and then held for various times at 870 °C, as above. The distributions of boron under these various conditions were investigated qualitatively by an autoradiographic technique. In both steels, segregation and/or precipitation of boron at austenite grain boundaries was detected under all conditions examined. Precipitation of iron borocarbide particles occurred along austenite grain boundaries in the 50 ppm B steel during cooling to the holding temperature of 870 °C, while in the 10 ppm B steel, such precipitation occurred only after long times at 870 °C. Mechanical properties of single-pass-rolled samples were measured after tempering to assess the effects of borocarbide precipitation on notch toughness. Such precipitation lowered the Charpy upper shelf energy and increased the transition temperature.     

Effect of H2O2 addition mode on enzymatic removal of phenol from wastewater in the presence of polyethylene glycol

The effect of batch and semi-batch additions of the stoichiometric amount of H₂O₂ on enzymatic removal of phenol has been studied over the phenol concentration range of 1-10 mmol/L. It is found that the ratio between the maximum H₂O₂ concentration during the reaction and the initial horseradish peroxidase concentration ((H₂O₂)_max/(HRP)_o) controls the rate of phenol removal by horseradish peroxidase in the presence of the additive, polyethylene glycol. An optimum range of this ratio is determined to be between 10 and 25 μmol/U. Empirical models are proposed to predict the values of (H₂O₂)_max/(HRP)_o required to treat given phenol concentrations under different modes for adding H₂O₂.     

Millisecond pulsars: nature's most stable clocks

The author describes the role pulsars might play in time and frequency technology. Millisecond pulsars are rapidly rotating neutron stars: some 20 km in diameter, 1.4 times as massive as the Sun, and spinning as fast as several thousand radians per second. Radio noise generated in a pulsar's magnetosphere by a highly beamed process is detectable over interstellar distances, as a periodic sequence of pulses. High-precision comparisons between pulsar time and terrestrial atomic time show that over intervals of several years, some millisecond pulsars have fractional stabilities comparable to those of the best atomic clocks. The author briefly reviews the physics of pulsars, discusses the techniques of pulsar timing measurements, and summarizes the results of careful studies of pulsar stabilities     

Role of Specimen Geometry in the Effect of Fiber Orientation on the Thermal Conductivity of a Uniaxial Carbon-Fiber-Reinforced Aluminoborosilicate Glass-Matrix Composite

A study concerning the effect of fiber orientation on the thermal conductivity of a uniaxial carbon-fiber-reinforced borosilicate glass was conducted. For thin specimens, and thick specimens with specimen sides cut parallel to the fiber direction, the dependence of thermal conductivity on fiber orientation showed excellent agreement with theoretical behavior predicted for a composite infinite in extent. In contrast, the thermal conductivity data for rectangular thick specimens fell well below the data for the thin and angled specimens. For fiber orientation of 45° and higher, at which the heat was directed toward the side of the specimen rather than across, the thermal conductivity values showed excellent agreement with theoretical behavior predicted for a finite composite strip with insulated sides.