Analysis of a correlated queue in a communication system

A family of queues is studied for which the service time B_n of customer n and the interarrival time I_n between customers n-1 and n exhibit some sort of proportionality. The focus is on dependencies that arise naturally in the context of communication systems, where the finite speed of the communication links constrains the amount of data that can be received in a given time interval. The simple case of a deterministic proportionality relation between the service time of a customer and its preceding interarrival time is considered. This is extended to allow the addition of an independent, generally distributed overhead to the service time of each customer. Several models that capture the ON-OFF behavior of communication links in packet networks are considered. In all cases, expressions for the delay experienced by a packet in the system are provided. Numerical examples illustrate the impact of dependencies through comparison with less accurate models. The results should be of relevance to environments other than communication as well     

Selective-repeat ARQ: the joint distribution of the transmitter andthe receiver resequencing buffer occupancies

A selective-repeat automatic-repeat request (SR ARQ) system model in which packets arrive at the transmitter according to a general renewal process is analyzed. The overall delay of a packet in a system that operates under the SR ARQ protocol consists of the queuing delay at the transmitter and the resequencing delay at the receiver. The joint distribution of the buffer occupancies at the transmitter and at the receiver is derived, and the tow types of delay are compared using numerical examples     

Spectral methods on triangles and other domains

This article shows how to obtain multidimensional spectral methods as a warped product of one-dimensional spectral methods, thus generalizing direct (tensor) products. This generalization includes the fast Fourier transform. Applications are given for spectral approximation on a disk and on a triangle. The use of the disk spectral method for simulating the Navier-Stokes equations in a periodic pipe is detailed. The use of the triangle method in a spectral element scheme is discussed. The degree of approximation of the triangle method is computed in a new way, which favorably compares with the classical approximation estimates.     

Oil-shale gasification by concentrated sunlight: An open-loop solar chemical heat pipe

Gasification of oil shales followed by catalytic reforming can yield synthesis gas, which is easily transportable and may also be used for producing liquid fuels. We report here on experiments performed at the Weizmann Institute's Solar Central Receiver involving the use of concentrated solar energy as the heat source for the gasification. A number of dual-axis tracking heliostat mirrors provided primary concentration, and compound parabolic concentrators (CPCs) were used for secondary concentration. In addition to being the first experimental study on this particular type of chemical storage of concentrated sunlight, new aspects of this work include demonstrations of the following:

1. (i)significantly higher synthesis gas yields than previously reported;

2. (ii) marked improvements in conversion efficiency in transparent reactors, as opposed to opaque reactors, both as fluidized beds, primarily due to better heat transfer;

3. (iii) a substantial increase in deliverable energy compared to the conventional retorting of oil shales, and considerable reduction in the resulting spent shale.

     

Two types of test-anxious students: Support for an information processing model.

The purpose of this research was to further investigate the information processing model suggested by Benjamin, McKeachie, Lin, and Holinger (1981) in explaining the poor academic performance of highly test-anxious students. Whereas previous studies pointing to deficits in the organization stage of processing mainly relied on self-reports, the first study of this research used a technique that enabled us to measure objectively the organization of course material by students in a nonevaluative situation. Results indicated that highly test-anxious students showed a disadvantage in organization of course material, in comparison with other students. In the second study, we investigated whether types of highly test-anxious students can be differentiated by their information processing skills. The first type includes those with good study habits who do not have problems in encoding and organizing the information but, rather, have a major problem in retrieval for a test. The second type includes those with poor study habits who have problems in all stages of processing. Such a distinction was supported by results comparing performance of these types of highly test-anxious students in an evaluative and a nonevaluative situation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Test of the cultural bias hypothesis: Some Israeli findings.

The major aim of this study was to examine the cross-cultural validity of the test bias contention, with particular concern for possible sociocultural group differences in the construct and predictive validity of college entrance scholastic aptitude tests in Israel. The analyses were based on the test scores of 1,538 Israeli college student candidates of varying ethnic group membership, applying for admission to a major Israeli campus. The psychometric properties of the test battery were compared by ethnic group via a variety of internal (factor structure, reliability, etc.) and external (predictive validity, homogeneity of regression, etc.) test bias criteria. On the whole, the data provided little evidence for differential construct or predictive validity of aptitude test scores as a function of ethnic group membership, thus lending a greater deal of generality to previous research on test bias, generally negating the cultural bias hypothesis. (34 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dynamics of catalytic reactions and reactors

Heterogeneous catalytic reactions and catalytic reactors are known to exhibit complex dynamic behavior and significant progress has been made, in the past two decades, in understanding this complexity using the tools of nonlinear dynamics and advanced experimental methods. This article presents an overview of complex dynamic behavior in heterogeneous catalysis and points out their intriguing nature along with practical implications. The issues discussed are the source of oscillatory behavior, classification of complex motions, transitions to chaotic solutions and spatiotemporal patterns in reactors of various geometries.     

The multi-track angiography catheter: a new tool for complex catheterisation in congenital heart disease

Autogenous vein grafts are currently the number one choice in microsurgical practice for managing vessel length discrepancies. A significant percentage of these grafts is used in the venous system. Allografts may serve as a viable alternative source of vein grafts. Few studies in the past have described the use of cryopreserved vein grafts (1 mm in diameter) in the venous system. In the present study, we cryopreserved the femoral veins of rats and transplanted these across rat species (ACI to Lewis) as interpositional vein grafts into a defect in the femoral vein. Short-term patencies (21 days), expressions of systemic antigenicity, histology, and scanning electron microscopic studies were performed. We obtained 100% patencies with fresh control veins and cryopreserved veins at 21 days.     

Biosynthesis of medium-chain triacylglycerols and phospholipids by HepG-2 cells

In an attempt to understand the metabolism by the liver of fatty acids (FA) of different chain length, we have studied the incorporation of [1-¹⁴C]-labeled C₂, C₈, C₁₀, C₁₂, and C₁₆ into cellular lipids by HepG-2 cells. Over 90% of the radiolabeled FA were detected in phospholipids (PL) and triacylglycerols (TAG). The incorporation of C₁₂ and C₁₆ was three to four times higher than that of C₈ and C₁₀ (and reached 35 nmoles per mg protein after 1.5 h). The radioactivity of C₂, C₈, and C₁₀ was recovered mainly in PL. C₁₂ and C₁₆ were incorporated at approximately equal amounts into PL and TAG. The radioactivity of both C₂ and C₈ was recovered exclusively in long-chain FA, suggesting oxidation of C₈ into C₂ units prior to FA synthesis. C₁₀ likewise yielded mainly long-chain FA. However 10% of unchanged C₁₀ was found in PL and up to 30% in TAG. ¹⁴C–C₁₂ was largely incorporated unchanged. Under these conditions, the presence of C₁₀ and C₁₂ in PL and TAG was shown also by gas-liquid chromatography. In the presence of either C₂, C₈, or C₁₀, up to 30% of ¹⁴C-monounsaturated FA were detected in PL and TAG. With C₁₂ and C₁₆, the fraction of ¹⁴C-monounsaturated FA was much smaller suggesting that extensive desaturation occurred during de novo synthesis.     

Intensity-based statistical scorer for tandem mass spectrometry

We describe a new statistical scorer for tandem mass spectrometry. The scorer is based on the probability that fragments with given chemical properties create measured intensity levels in the experimental spectrum. The scorer's parameters are computed using a fully automated procedure. Benchmarking the new scorer on a large set of experimental spectra, we show that it performs significantly better than the widely used cross-correlation scoring algorithm of Eng et al. (Eng, J. K; McKormack, A. L.; Yates, J. R. J. Am. Soc. Mass Spectrom. 1994, 5, 976-989.).