2,113 research outputs found
Performance Modelling and Analysis of Weighted Fair Queueing for Scheduling in Communication Networks. An investigation into the Development of New Scheduling Algorithms for Weighted Fair Queueing System with Finite Buffer.
No full textAnalytical modelling and characterization of Weighted Fair Queueing (WFQ) have recently
received considerable attention by several researches since WFQ offers the minimum
delay and optimal fairness guarantee. However, all previous work on WFQ has
focused on developing approximations of the scheduler with an infinite buffer because of
supposed scalability problems in the WFQ computation.
The main aims of this thesis are to study WFQ system, by providing an analytical WFQ
model which is a theoretical construct based on a form of processor sharing for finite
capacity. Furthermore, the solutions for classes with Poisson arrivals and exponential
service are derived and verified against global balance solution.
This thesis shows that the analytical models proposed can give very good results under
particular conditions which are very close to WFQ algorithms, where accuracy of
the models is verified by simulations of WFQ model. Simulations were performed with
QNAP-2 simulator. In addition, the thesis presents several performance studies signifying
the power of the proposed analytical model in providing an accurate delay bounds to
a large number of classes.
These results are not able to cover all unsolved issues in the WFQ system. They represent
a starting point for the research activities that the Author will conduct in the future. The
author believes that the most promising research activities exist in the scheduler method
to provide statistical guarantees to multi-class services. The author is convinced that
alternative software, for example, on the three class model buffer case, is able to satisfy
the large number of buffer because of the software limitation in this thesis. While they can
be a good topic for long-term research, the short-medium term will show an increasing
interest in the modification of the WFQ models to provide differentiated services.Ministry of Higher Educatio
Integrating student self-assessment and feedback in e-learning applications. A proposed educational model.
No full textThere is a large demand for the use of e-learning tools to support student learning, in
the form of distance or blended learning. The need for e-learning environment that encourages
learners to learn independently or in groups in virtual settings is crucial. Some
e-learning environments provide repositories of `resources¿. They neither facilitate a strategy
for learning or teaching, nor they guide students through the resources, and tutors in
constructing their courses.
E-learning environments need to incorporate pedagogical practices which support and
allow students to learn by removing any barriers that might inhibit their learning. Therefore,
one of the most important aspects in developing e-learning environments is defining
appropriate models where technology and pedagogy are integrated.
This thesis provides such a framework for developing e-learning applications; it aims to
make it easier for tutors to implement their lesson content and engage learners to achieve
the course objectives. The proposed model incorporates constructive alignment, assessment
and feedback and unlike other e-learning environments guides the tutor to construct
lessons and help learners to use effective learning environment. Furthermore, the thesis
investigates on how supported learning can help students adapt to the different approaches
to learning. The empirical work undertaken investigates the role of constructing a well
designed self-assessment and feedback unit within a learning environment
A rod-linear cascade model for emulating rotor-stator interaction noise in turbofans: A numerical study
No full textThis manuscript presents a rod-linear cascade model for emulating rotor-stator interaction noise. The model is intended as a test platform for studying noise mitigation techniques for a turbofan fan stage, while it also extends the classical rod-airfoil configuration by considering a row of blades based on realistic geometrical details. The rod-linear cascade model consists of a rod positioned upstream of a 7-blade linear cascade, such that the rod wake impinges onto the central blade. The rod is scaled to obtain a fundamental shedding frequency equal to the first blade passing frequency of the NASA-Glenn Source Diagnostics Test (SDT)fan stage at approach condition. The cascade blade profile is also based on the OGV of the SDT sampled at 90% of the radial span. Subsequently, numerical simulations are performed using lattice-Boltzmann Method on a computational setup comprised of a contraction and a test section enclosing the rod-linear cascade model. The integral length scales of the rod wake and the mean loading of the central blade have been found to be in good agreement with the trends observed in the SDT fan stage. The primary noise sources are localized at the central blade leading edge, although noise propagation to the far-field is influenced by additional diffraction by the other blades. Furthermore, the acoustic-blade row interaction causes intense pressure fluctuation within the inter-blade channels, including in those that are not directly affected by the rod wake.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Wind Energ
Culturally aligned security in banking. A system for rural banking in Ghana.
No full textThis thesis is an investigation into the unique rural banking system in Ghana and the role of information systems in fraud control. It presents a robust information security and internal control model to deal with fraud for the banking system. The rural banking industry has been noted for poor internal control leading to fraud. This has resulted in poor performance and even the collapse of some banks. The Focus of the study was on the processes used to deliver banking services.
To design a protection system, a number of rural banks were visited. This was to understand the environment, regulatory regimes and the structure and banking processes of the industry and banks. Systemic vulnerabilities within the industry which could be exploited for fraud were found. The lack of structures like an address system and unreliable identification documents makes it difficult to use conventional identification processes. Also the lack of adequate controls, small staff numbers and the cross organisational nature of some transactions among other cultural issues reduces the ability to implement transaction controls. Twenty fraud scenarios were derived to illustrate the manifestation of these vulnerabilities.
The rural banking integrity model was developed to deal with these observations. This protection model was developed using existing information security models and banking control mechanisms but incorporating the nature of the rural banking industry and culture of its environment. The fraud protection model was tested against the fraud scenarios and was shown to meet the needs of the rural banking industry in dealing with its systemic vulnerabilities. The proposed community-based identification scheme deals with identification weaknesses as an alternative to conventional identity verification mechanisms. The Transaction Authentication Code uses traditional adinkra symbols. Whilst other mechanisms like the Transaction Verification Code design
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internal controls into the banking processes. This deals with various process control weaknesses and avoids human discretion in complying with controls. Object based separation of duties is also introduced as a means of controlling conflicting tasks which could lead to fraud
Improving TCP performance over heterogeneous networks : The investigation and design of End to End techniques for improving TCP performance for transmission errors over heterogeneous data networks.
No full textTransmission Control Protocol (TCP) is considered one of the most important protocols
in the Internet. An important mechanism in TCP is the congestion control
mechanism which controls TCP sending rate and makes TCP react to congestion
signals. Nowadays in heterogeneous networks, TCP may work in networks with some
links that have lossy nature (wireless networks for example). TCP treats all packet
loss as if they were due to congestion. Consequently, when used in networks that
have lossy links, TCP reduces sending rate aggressively when there are transmission
(non-congestion) errors in an uncongested network.
One solution to the problem is to discriminate between errors; to deal with congestion
errors by reducing TCP sending rate and use other actions for transmission
errors. In this work we investigate the problem and propose a solution using an
end-to-end error discriminator. The error discriminator will improve the current
congestion window mechanism in TCP and decide when to cut and how much to
cut the congestion window.
We have identified three areas where TCP interacts with drops: congestion window
update mechanism, retransmission mechanism and timeout mechanism. All of
these mechanisms are part of the TCP congestion control mechanism. We propose
changes to each of these mechanisms in order to allow TCP to cope with transmission
errors. We propose a new TCP congestion window action (CWA) for transmission
errors by delaying the window cut decision until TCP receives all duplicate acknowledgments
for a given window of data (packets in flight). This will give TCP a clear
image about the number of drops from this window. The congestion window size is
then reduced only by number of dropped packets. Also, we propose a safety mechanism
to prevent this algorithm from causing congestion to the network by using
an extra congestion window threshold (tthresh) in order to save the safe area where
there are no drops of any kind. The second algorithm is a new retransmission action
to deal with multiple drops from the same window. This multiple drops action
(MDA) will prevent TCP from falling into consecutive timeout events by resending
all dropped packets from the same window. A third algorithm is used to calculate
a new back-off policy for TCP retransmission timeout based on the network¿s available
bandwidth. This new retransmission timeout action (RTA) helps relating the
length of the timeout event with current network conditions, especially with heavy
transmission error rates.
The three algorithms have been combined and incorporated into a delay based
error discriminator. The improvement of the new algorithm is measured along with
the impact on the network in terms of congestion drop rate, end-to-end delay, average
queue size and fairness of sharing the bottleneck bandwidth. The results show that
the proposed error discriminator along with the new actions toward transmission
errors has increased the performance of TCP. At the same time it has reduced the
load on the network compared to existing error discriminators. Also, the proposed
error discriminator has managed to deliver excellent fairness values for sharing the
bottleneck bandwidth.
Finally improvements to the basic error discriminator have been proposed by
using the multiple drops action (MDA) for both transmission and congestion errors.
The results showed improvements in the performance as well as decreases in the
congestion loss rates when compared to a similar error discriminator.Ministry of Higher Education and
King Saud University in Saudi Arabia
Rod Korns
No full textPhoto showing J. Roderic ("Rod") Korns, a historian of western trails and author of "West from Fort Bridger
Understanding migratory flow caused by helicoid wire spacers in rod bundles: An experimental and theoretical study
No full textThe core of a Liquid Metal Fast Breeder Reactor (LMFBR) consists of cylindrical fuel rods that are wrapped by a helicoidally-wound wire spacer to enhance mixing and to prevent damage by fretting. It is known that the liquid metal close to the rod is forced to follow the wires, and that liquid metal further away from the rod crosses the wires (called: migratory flow). This work aims at gaining more insight into the physics behind migratory flow and to provide a model for its bending angle. To this purpose, the flow field in a 7-rods, wire-wrapped, hexagonal bundle with water is studied within the Reynolds number range of 4990–16330 by using Particle Image Velocimetry (PIV). Refraction of the light is minimized by using Fluorinated Ethylene Propylene (FEP), which is a refractive index-matching (RIM) material. These measurements confirm that liquid near the rod follows the helicoid path and bends cross-wise with respect to the wire further away from the rod. A theoretical model for the bending angle of the flow is derived from the Euler equations and shows that the bending is primarily caused by the pressure gradient field induced by the wire. The model shows a very good correspondence with the experimentally obtained PIV data. These findings improve our understanding of the physics at play in rod bundle flows with wrapped wires and can be of assistance in developing practical correlations for frictional pressure losses and heat transfer in such bundles.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.RST/Reactor Physics and Nuclear MaterialsRST/Radiation, Science and Technolog
Interplanetary robots: true stories of space exploration/ Rod Pyle.
No full textIncludes bibliographical references and index.A NASA insider tells the exciting story of robotic space missions to explore the solar system. Exploring the planets has been a goal of America's space program since the dawn of the space race. This insider's perspective examines incredible missions of robotic spacecraft to every corner of our solar system and beyond. Some were flown into glory, while others were planned and relegated to dusty filing cabinets. All were remarkable in their aspirations. Award-winning science writer Rod Pyle profiles both the remarkable spacecraft and the amazing scientists and engineers who made them possible. From the earliest sprints past Venus and Mars to Voyager1's current explorations of the space between the stars, this exciting book sheds new light on ever-more ambitious journeys designed to increase the human reach into the solar system. Drawing on his perspective as a writer for NASA's Jet Propulsion Laboratory, ground zero for NASA's planetary exploration, the author further details plans now in development to look for signs of life on Jupiter's moon Europa, submarines that will dive into the hazy hydrocarbon lakes of Saturn's moon Titan, and intelligent spacecraft that will operate for months without human intervention on Mars and in the outer solar system well into the 2030s. Equally compelling are programs of exploration that were considered but never left the drawing board, such as automobile-sized biology laboratories designed for a Mars landing in the 1960s and plans to detonate atomic bombs on the moon. Complemented by many rarely-seen photos and illustrations, these stories of incredible engineering achievements, daring imaginations, and technological genius will fascinate and inspire.1 online resourc
Corrective force analysis for scoliosis from implant rod deformation
No full textBackground: Scoliosis is a serious disease in which a human spine is abnormally deformed in three dimensions with vertebral rotation. Surgical treatment is attained when the scoliotic spine is corrected into its normal shape by implant rods and screws fixed into the vertebrae. The three-dimensional corrective forces acting at the screws deformed the implant rod during the surgical treatment of scoliosis. The objective of this study was to propose a method to analyze the three-dimensional forces acting at the rod using the changes of implant rod geometry before and after the surgical treatment. Methods: An inverse method based on Finite Element Analysis is proposed. The geometries of implant rod before and after the surgical treatment were measured three dimensionally. The implant rod before the surgical treatment was reconstructed using an elasto-plastic finite element model. The three-dimensional forces were applied iteratively to the rod through the screws such that the rod is deformed the same after the surgical treatment of scoliosis. Findings: The maximum force acting at the screw of each patient ranged from 198 N to 439 N. The magnitude of forces was clinically acceptable. The maximum forces occurred at the lowest fixation level of vertebra of each patient. Interpretation: The three-dimensional forces distribution that deformed the rod can be evaluated using the changes of implant geometry. Although the current clinical cases are still few, this study demonstrated the feasibility of measuring the forces that deformed the implant rod after the surgical treatment of scoliosis
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