Modelling IP Mobility

We study a highly simplified version of the proposed mobility support in version 6 of Internet Protocols (IP). We concentrate on the issue of ensuring that messages to and from mobile agents are delivered without loss of connectivity. We provide three models, of increasingly complex nature, of a network of routers and computing agents that are interconnected via the routers: the first is without mobile agents and is treated as a specification for the next two; the second supports mobile agents, and the third additionally allows correspondent agents to cache the current location of a mobile agent. Following a detailed analysis of the three models to extract invariant properties, we show that the three models are related by a suitable notion of equivalence based on barbed bisimulation. Finally, we report on some experiments in simulating and verifying finite state versions of our model.     

RVGEN: a tool for generation of random variates

A number of applications in simulation and software testing require random number generation, both uniform and non-uniform. Although libraries are available for random number generation, there is no user-friendly tool to enable the user to use and build random number generators. This paper presents the RVGEN tool, developed at TRDDC. Using the tool, the software developer can design a random number generator specific to a particular pattern. The tool takes specifications at a high level and also partly in C++, and generates C++ code for a number of user-friendly functions. These include setting and getting of random variate parameters, GUIs for inputting parameters, validation of parameters, parameter input and output from a file, generation of random variate class declarations, generation of seeds for multiple streams, and testing of the random variate generator. The tool provides a number of classical, general-purpose and empiric distributions by default.     

A discrete event approach to the command sequence replanning and control in a teleprogramming system

A command sequence replanning and control method, which enables the slave system to autonomously recover from error conditions, is proposed in a telerobot system. A task model described as a form of the controlled Petri net (CPN) is used as a prior knowledge for the slave system to carry out the given task successfully without the operator's aid even in unexpected error conditions. The CPN model incorporates the contact states and transitional motions between them that possibly exist in the task execution process. The motion command is automatically generated from the master system and transmitted to the slave whenever the contact state changes in the master model. Referring to the CPN model and the given motion commands, the slave system detects if the actual contact state is unexpected and then plans the recovery path from the unexpected state. The feasibility of the command sequence replanning and control algorithm is verified through an example to perform a simple part‐mating task. © 2001 John Wiley & Sons, Inc.     

Mammalian abasic site base excision repair. Identification of the reaction sequence and rate-determining steps

Base excision repair (BER) is one of the cellular defense mechanisms repairing damage to nucleoside 5'-monophosphate residues in genomic DNA. This repair pathway is initiated by spontaneous or enzymatic N-glycosidic bond cleavage creating an abasic or apurinic-apyrimidinic (AP) site in double-stranded DNA. Class II AP endonuclease, deoxyribonucleotide phosphate (dRP) lyase, DNA synthesis, and DNA ligase activities complete repair of the AP site. In mammalian cell nuclear extract, BER can be mediated by a macromolecular complex containing DNA polymerase beta (beta-pol) and DNA ligase I. These two enzymes are capable of contributing the latter three of the four BER enzymatic activities. In the present study, we found that AP site BER can be reconstituted in vitro using the following purified human proteins: AP endonuclease, beta-pol, and DNA ligase I. Examination of the individual enzymatic steps in BER allowed us to identify an ordered reaction pathway: subsequent to 5' "nicking" of the AP site-containing DNA strand by AP endonuclease, beta-pol performs DNA synthesis prior to removal of the 5'-dRP moiety in the gap. Removal of the dRP flap is strictly required for DNA ligase I to seal the resulting nick. Additionally, the catalytic rate of the reconstituted BER system and the individual enzymatic activities was measured. The reconstituted BER system performs repair of AP site DNA at a rate that is slower than the respective rates of AP endonuclease, DNA synthesis, and ligation, suggesting that these steps are not rate-determining in the overall reconstituted BER system. Instead, the rate-limiting step in the reconstituted system was found to be removal of dRP (i.e. dRP lyase), catalyzed by the amino-terminal domain of beta-pol. This work is the first to measure the rate of BER in an in vitro reaction. The potential significance of the dRP-containing intermediate in the regulation of BER is discussed.     

Grain size effects on the dielectric properties of ferroelectric Bi2VO5.5 ceramics

Dielectric properties and microstructural characteristics of ferroelectric bismuth vanadate (Bi₂VO_5.5) ceramics exhibiting grain sizes of 7, 10, 20 and 25 m have been studied. Microstructural studies indicate the presence of ferroelectric 90° domain patterns on the surface as well as in the bulk of the coarse-grained ceramics. The dielectric constant and the loss tangent both at room temperature and in the vicinity of the Curie temperature have been found to increase with increasing grain size. The Curie temperature (725 K) is found to shift slightly (by about 7 K) towards higher temperatures as the grain size increases (7–25 m). The magnitude of the dielectric anomaly around 725 K is found to be higher for coarse-grained ceramics. The dielectric constant and the loss have been found to decrease with increase in frequency (1–100 kHz) for all the ceramics studied. The increase in dielectric constant with increasing grain size is attributed to a decrease in thickness of the relatively more insulating grain boundary layer.     

Impedance analysis of Pb<Subscript>2</Subscript>Sb<Subscript>3</Subscript>LaTi<Subscript>5</Subscript>O<Subscript>18</Subscript> ceramic

Polycrystalline sample of Pb₂Sb₃LaTi₅O₁₈, a member of tungsten- bronze (TB family, was prepared using a high temperature solid- state reaction technique. XRD analysis indicated the formation of a single-phase orthorhombic structure. The dielectric studies revealed the diffuse phase transition and the transition temperature was found to be at 52° C. Impedance plots were used as tools to analyse the sample behaviour as a function of frequency. Cole-Cole plots showed Debye relaxation. The activation energy was estimated to be 0·634 eV from the temperature variation of d.c. conductivity. The nature of variation of d.c. conductivity with temperature suggested NTCR behaviour.     

Metallurgical investigation of prematurely failed hot-strip mill work-rolls: Some microstructural observations

A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M₃C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol% was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking, which ultimately induced premature spalling of hot-strip mill work-rolls.     

Premature failure of work-rolls in tandem mill: Some microstructural revelations

The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life. Image analysis of roll microstructures indicated that small carbide size, large carbide volume fraction, and high carbide count were characteristic of higher-life rolls, and that large carbide size, low carbide volume fraction, and less carbide density were typical of lower-life rolls. The carbides in both types of microstructure were M₇C₃ type.     

Analysis of temperature and pressure changes in liquefied natural gas (LNG) cryogenic tanks

Liquefied natural gas (LNG) is being developed as a transportation fuel for heavy vehicles such as trucks and transit buses, to lessen the dependency on oil and to reduce greenhouse gas emissions. The LNG stations are properly designed to prevent the venting of natural gas (NG) from LNG tanks, which can cause evaporative greenhouse gas emissions and result in fluctuations of fuel flow and changes of fuel composition. Boil-off is caused by the heat added into the LNG fuel during the storage and fueling. Heat can leak into the LNG fuel through the shell of tank during the storage and through hoses and dispensers during the fueling. Gas from tanks onboard vehicles, when returned to LNG tanks, can add additional heat into the LNG fuel. A thermodynamic and heat transfer model has been developed to analyze different mechanisms of heat leak into the LNG fuel. The evolving of properties and compositions of LNG fuel inside LNG tanks is simulated. The effect of a number of buses fueled each day on the possible total fuel loss rate has been analyzed. It is found that by increasing the number of buses, fueled each day, the total fuel loss rate can be reduced significantly. It is proposed that an electric generator be used to consume the boil-off gas or a liquefier be used to re-liquefy the boil-off gas to reduce the tank pressure and eliminate fuel losses. These approaches can prevent boil-off of natural gas emissions, and reduce the costs of LNG as transportation fuel.     

Understanding the Heat Transfer and Solidification of Plasma-Sprayed Yttria-Partially Stabilized Zirconia Coatings

A variety of microstructures have been observed in plasma-sprayed yttria-partially stabilized zirconia (YSZ) thermal barrier coatings. Control of the coating microstructures requires a good understanding of the heat transfer and solidification during the process. This article presents a quantitative analysis of heat transfer and solidification of plasma-sprayed YSZ splats. The analysis is based on a simple heat transfer and solidification model that solves a one-dimensional moving boundary problem with consideration of melt undercooling prior to solidification and nonequilibrium crystalline growth kinetics at the moving interface. The solidification morphology is first assumed to be planar, and the stability of the planar interface is examined against the absolute stability velocity calculated from the linear stability theory. Examining the temperature distribution in a solidifying YSZ zirconia splat indicates that a large positive temperature gradient exists in front of the interface, which leads to a stable planar interface and a segregation-free columnar structure, agreeing well with experimental observation. The model also finds that a low interface velocity results from poor heat transfer, which leads to a formation of cells and, therefore, the segregation of yttria. A steady-state dendrite tip growth model is then employed to calculate the radius of the cell tips and thus the cell spacings, which is then compared with experimental observations.