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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Semi-Persistent Medium Access Control Protocols for Wireless Sensor Networks

Guennoun, Mouhcine January 2014 (has links)
Wireless Sensor Networks (WSNs) are dense clusters of sensor nodes, made up of small, intelligent, resource-constrained wireless devices that are deployed to monitor a specific phenomenon in a certain field. The sensor nodes can be constrained by limited power supply, memory capacity and/or processing capabilities, which means that the design of WSNs requires all algorithms and protocols to be lightweight and efficient, and use as little power as possible. The Medium Access Control (MAC) protocol in WSNs, defined by the IEEE 802.15.4 standard, employs the Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) algorithm to control the nodes contending for access to the communication medium. Though the performance of this protocol has been studied extensively, and several improvements to its backoff counter, superframe format and contention-free period (CFP) features have been proposed, very few studies have addressed improving the Clear Channel Assessment (CCA) feature. In this thesis, we study the impact of increasing the value of the contention window beyond the standard value of 2, on the performance of the MAC protocol. We propose a semi-persistent MAC protocol that is a hybrid form of 802.11 and 802.15.4, to achieve a favorable performance that can serve a broad range of applications over the IEEE 802.15.4-based WSNs. We build an analytical model of the proposed protocol based on Markov chain modelling and derive the analytical expressions of the performance metrics, which we then validate against the simulation result sets generated by our in-house built simulation framework. We prove analytically that the probability of collision of the semi-persistent MAC is lower than that of the standard protocol. Based on our theoretical and simulated models, we show that incorporating the semi-persistent feature into existing MAC protocols leads to significant improvement of the performance metrics, including the probability of collision, throughput, energy consumption, transmission delay and reliability, particularly for networks with a large number of sensor nodes.
2

Analysis and design of efficient medium access control schemes for vehicular ad-hoc networks

Han, Chong January 2012 (has links)
In this dissertation, analysis and design of the efficient Medium Access Control (MAC) sub-layer schemes are considered for Vehicular Ad hoc Networks (VANE~s). The contributions of this study are three-fold. First, an analytical model based on Markov chain is developed in order to investigate the performance of the MAC sub-layer of the IEEE 802.11p for vehicular communications. The results indicate that single channel MAC sub-layers may not be adequate for the future Intelligent Transportation Systems (ITS). The analytical model is validated with the results from simulation-based analysis. Performance analysis based on simulations is given on MAC metrics such as throughput, access delay, packet delivery. Second, a multi-channel MAC protocol is proposed and comprehensively analyzed in terms of channel utilizing and Quality of service (QoS) differentiation for dense VANETs. It is demonstrated that the proposed scheme, namely Asynchronous Multichannel MAC with Distributed TDMA (AMCMACD), improves the system performance in terms of throughput, packet delivery rate, collision rate on service channels, load balancing, and service differentiation for dense vehicular networks. Third, to cope with the interference from contention with neighbours within two hops in large-scale networks, a Large-scale Asynchronous Multichannel MAC (LS-AMCMAC) is proposed. The proposed scheme outperforms other benchmark multichannel MAC schemes in large-scale networks, in terms of throughput, channel utilization, dissemination of emergency messages, and the collision rates on control and service channels.
3

PROPOSAL OF POLLED-CSMA PROTOCOL FOR AN AD HOC SCADA SYSTEM

Yuxiang, Yang, Xiaoyu, Dang, Tingxian, Zhou 10 1900 (has links)
International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California / In this paper we introduce an ad hoc distributed supervisory control and data acquisition (SCADA) system, which covers two types of traffic, a small number of slave stations with stream traffic and much larger number of slave stations with burst traffic. To improve the system’s real-time transmission and channel allocation efficiency, a MAC scheme, named Polled-CSMA, is designed by integrating the merits of polling and CSMA protocols. Extensive analysis shows that this MAC protocol meets the system’s QOS requirements.
4

Implementation of the FlexRay Protocol Module

Ko, Chao-jen 24 July 2009 (has links)
FlexRay communications system protocol is necessary specification of the future digitizing motor vehicle. Though the principal standard of specifications had established in 2005, little correlative commodities have used in the motor vehicle now. In this paper, I implement the hardware circuit of MAC Layer in the Flexray system. In the system, six major sub-modules: Controller Host Interface (CHI) is connecting with the Host processor interface, Protocol Operation Controller (POC) is controlling all sub-modules after receiving the orders from CHI, Clock Synchronization (CS) is synchronizing between all nodes, Media Access Control (MAC) is dividing slot and arranging the style of transmission data, Coding and Decoding (CODEC) is connecting the Physical Layer, and Frame and Symbol Processing (FSP) is detecting the error of receiving data. Because the system increases the function of synchronization between Nodes in the FlexRay system, it can transmit high data rate in the Time Division Multiple Access system (TDMA). Because the transmitter is two channels, it has the characteristic of high fault tolerance. The two characteristics meet the requirements that transmitting data is high data rate and stabilize.
5

Mindfulness

Björkegren, Adam January 2015 (has links)
No description available.
6

The celebrated antiquary Dubhaltach Mac Fhirbhisigh (c. 1600-1671) : his lineage, life and learning /

Ó Muraíle, Nollaig. January 1996 (has links)
Texte remanié de: Diss. Ph. D.--Dublin--National university of Ireland, 1991. Titre de soutenance : The Background, life and writings of Dubhaltach Mac Fhirbhisigh (d. 1671). / Bibliogr. p. 381-408. Index.
7

Information retrieval and gathering : an experimental prototype for Mac OS X : undergraduate dissertation

Gonzalez-George, Victor. January 2006 (has links)
Thesis (Ph.D)-University of Bath, 2006. / Title appears on item as: Undergraduate dissertation: Information retrieval and gathering: An experimental prototype for Mac OS X. Includes bibliographical references.
8

Vergleich des C-MAC®-Videolaryngoskopes mit der konventionellen Laryngoskopie bei der endotrachealen Intubation im Rahmen von Rapid Sequence Inductions / Comparison of the C-MAC®-Video laryngoscope versus conventional laryngoscopy for the endotracheal intubation when performing a rapid sequence induction

Lausmann, Michael January 2018 (has links) (PDF)
Die suffiziente Atemwegssicherung ist eine der wichtigsten Aufgaben des Anästhesisten. Die endotracheale Intubation mit dem konventionellen Laryngoskop nach Macintosh stellt aktuell den Goldstandard dar. Gerade bei aspirationsgefährdeten Patienten sollte die Intubation zügig, aber dennoch schonend und sicher erfolgen. Die Rapid Sequence Induction als Sonderform der Narkoseeinleitung ist hier das Mittel der Wahl. Bei Vorliegen eines schwierigen Atemweges kann diese aber massiv erschwert sein. Eine relativ neue Alternative abseits der konventionellen Laryngoskopie bietet hier die Anwendung der Videolaryngoskopie. Das C-MAC der Firma Karl Storz ist ein Vertreter dieser Videolaryngoskope und erst seit wenigen Jahren auf dem Markt erhältlich. Basierend auf bisherigen Studien sollte nun im Rahmen der klinischen Anwendung des C-MAC bei Blitzintubationen eine etwaige Überlegenheit im Vergleich mit der direkten Laryngoskopie evaluiert werden. Um diese Frage weiter zu klären, wurden in dieser Studie Daten von 60 Patienten erhoben, die vor Elektivoperationen in der Augen- und HNO-Heilkunde eine RSI benötigten. Jeweils 30 Patienten wurden randomisiert, entweder mit Hilfe des C-MAC-Videolaryngoskopes oder des konventionellen Laryngoskopes nach Macintosh, intubiert. Der Intubation mit dem Videolaryngoskop vorgeschaltet war immer eine direkte Laryngoskopie. Primärer Endpunkt der Studie war der Intubationserfolg. Bezüglich dieses Endpunktes zeigte sich keine Überlegenheit des C-MAC, da der Intubationserfolg in beiden Gruppen exakt gleich war. Es gelangen 96,7 % der Intubationen jeweils im ersten Versuch. Hinsichtlich der Intubationsdauer bestanden keine Vorteile durch die Verwendung der Videolaryngoskopie. In der Versuchsgruppe benötigten die Intubationen im Mittel 33,2 Sekunden und damit 15,3 Sekunden länger als in der Kontrollgruppe, in der die Intubationen im Durchschnitt 17,9 Sekunden in Anspruch nahmen. Es zeigte sich eine klinisch relevante Überlegenheit des C-MAC bezüglich der Klassifikation nach Cormack und Lehane, also eine Verbesserung der Glottisvisualisierung. Aufgrund der primär besseren Visualisierung der Stimmbandebene wurden signifikant weniger Optimierungshilfen benötigt. Einschränkend ist hierbei zu erwähnen, dass die Tubuspassage trotz optimaler Sichtverhältnisse massiv erschwert sein kann, da in der Videolaryngoskopie die orale nicht mit der pharyngealen und der laryngealen Achse übereinstimmen muss und somit eine starke Divergenz zwischen der visuellen und der anatomischen Achse zu Intubationsproblemen führen kann. Der Schwierigkeitsgrad der Intubationen war bei Anwendung des C-MAC geringer als in der Kontrollgruppe, was sich somit in niedrigeren Werten bezüglich der Intubation Difficulty Scale widerspiegelte. Es bestanden keine signifikanten Unterschiede hinsichtlich der Vermeidung von, durch die Intubation verursachten, Schleimhautblutungen und postoperativen Beschwerden. Die beiden Laryngoskopietechniken hatten keinen unterschiedlichen Einfluss auf die hämodynamischen Parameter der Probanden. Insgesamt sollten beide Methoden als gleichwertig betrachtet werden. Das C-MAC stellt somit eine nützliche Alternative bei der RSI dar. Aufgrund der besseren Visualisierungsmöglichkeiten eignet sich die Videolaryngoskopie besonders beim schwierigen Atemweg. / Airway management is one of the most important skills for an Anesthetist to master. The endotracheal intubation using a Macintosh Blade for conventional laryngoscopy is the gold standard for airway management. In patients at high risk for pulmonary aspiration the intubation should be completed quickly as possible, but also without causing injury. Rapid Sequence Induction is an anesthesia induction technique designed to facilitate rapid tracheal intubation in these patients. In patients with a (suspected) difficult airway this can be a real challenge. A relatively new approach to provide an alternative to the conventional direct laryngoscopy is the use of the video laryngoscopy. The C-MAC Blade produced by the company KARL STORZ is an example of the video laryngoscope and has been available for a few years. Based on recent studies this study compares the use of the C-MAC versus conventional laryngoscopy in a clinical setting for the RSI to evaluate if an eventual superiority is present. To investigate this question, we studied the data of 60 Patients receiving elective Ear/Nose/Throat or Ocular surgery who required a rapid sequence induction. We randomly allocated the patients into 2 groups who where either intubated with help of the CMAC Videolaryngoscope or conventionally with Macintosch blade. The primary endpoint of the study was a successful endotracheal intubation. Regarding the primary endpoint the study showed no advantage for the Videolaryngoscope group, as the rate of successful endotracheal intubation was exactly the same in both groups. In 96,7% the first intubation attempt was successful. Considering the length of time it took to intubate the patient there was also no advantage for the Videolaryngoscope. In the CMAC group the time required to intubate was on average 33,2 seconds and therefore 15,3 seconds longer than the control group where the intubation took on average 17,9 seconds. It could be shown that there is a clinically relevant improvement in the classification system of Cormack-Lehane when using CMAC, as well as better view of the glottis. As a result, a significant reduction of the number of tools to optimize the view were needed. It must be said though, that placing the endotracheal tube can be more difficult, even though there is improved laryngeal imaging. This is due to the fact that with video laryngoscopy the oral axis does not line up with the pharyngeal and laryngeal axis resulting in a strong divergence of the visual and anatomical axis which can lead to intubation difficulties. The difficulty rating the endotracheal intubation was lower in the CMAC group as compared to the control group which correlated with the low rating in the difficulty scale. There was no significant difference in the avoidance of mucosal bleeding or postoperative complaints due to the endotracheal intubation. Both laryngoscope techniques showed no difference on the hemodynamic parameters of the subjects. In conclusion both methods can be seen as equivalent. The CMAC is a useful alternative when performing a rapid sequence induction. As a result of the better visualization of the glottis the video laryngoscope can be a useful tool in patients with
9

QoS provisioning for IEEE 802.11 MAC protocols

Ge, Ye 12 October 2004 (has links)
No description available.
10

Modelagem e simulação para transferência de oxigênio em aeração forçada por bolhas utilizando os métodos MAC e Crank-Nicolson / Modeling and simulation for oxygen transfer in forced aeration bubbles using the MAC and Crank-Nicolson methods

Falanca, Nayara 21 October 2013 (has links)
A aeração artificial em meio líquido visa suprir a baixa concentração de oxigênio dissolvido em ambientes deficientes deste, sendo um processo comumente aplicado na área de tratamento de efluentes por processos aeróbios. Uma forma de produzir aeração artificial é através de difusores submersos no fundo de um tanque com formação de bolhas, que ascendem e transferem seu oxigênio ao longo da coluna dágua. Para simular e melhor entender este processo e sua fluidodinâmica, um modelo inicial simplificado foi proposto, baseado em conceitos teóricos e equações que representem o fenômeno, como a de dispersão de bolhas, velocidades médias e concentração de oxigênio dissolvido. Foi considerado para realização dos cálculos um tanque retangular, com entrada de água a esquerda e saída de água a direita, com superfície sólida ao fundo e superfície livre acima, em contato com o ar. As equações de transporte de massa foram discretizadas no tempo, utilizando a técnica de Crank-Nicolson e no espaço segundo metodologia de diferenças centrais, solucionadas posteriormente por meio de técnicas iterativas do tipo Gauss-Seidel. As equações de transporte de quantidade de movimento foram resolvidas com o método MAC, explícito nas velocidades do fluido e implícito na pressão, para fornecer o campo de velocidade e pressão. A equação de que fornece o perfil de espalhamento das bolhas foi tomada como uma Equação Gaussiana de dispersão bidimensional. A implementação computacional necessária para aplicação das abordagens foi no ambiente de programação MATLAB. O método MAC e a técnica Crank-Nicolson apresentaram resultados satisfatórios para simulações rápidas e noção do comportamento do fluido e da concentração de oxigênio dissolvido. / The artificial aeration in aqueous mean aims to supply low concentration of dissolved oxygen in deficient atmospheres of it. This process is commonly applied in the field of wastewater treatment by aerobic processes. One way to produce artificial aeration is through diffusers submerged at the bottom of a tank with the formation of bubbles, which ascend and transfer its oxygen throughout the water column. To simulate and better understand this process and its fluid dynamics, a simplified initial model was proposed based on theoretical concepts and equations that represent the phenomenon, as the dispersion of bubbles, average speeds and dissolved oxygen concentration. It was considered for the calculations one rectangular tank with the water inlet and water outlet from left to right, with solid surface to the bottom and free surface at the top in contact with air. The mass transport equations were discretized in time, using the Crank-Nicolson technique and in space according to the methodology of central differences, subsequently resolved by Gauss-Seidel technique iterative. The transport equations of momentum were solved with the MAC method, explicit on the velocities of the fluid and implicit on pressure, to afford the velocity field and pressure. The equation which provides the spreading profiles of bubbles was taken as a two-dimensional Gaussian dispersion equation. The computational implementation necessary for application of the approaches was the programming environment MATLAB. The MAC method and Crank- Nicolson technique showed satisfactory results for quick simulations and notion of the behavior of the fluid and the concentration of dissolved oxygen.

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