To investigate how physics students perceived the sudden shift to online learning at the beginning of COVID-19 pandemic, 18 semistructured interviews were conducted with university students in Austria, Croatia, and Germany. Based on the interviews, a questionnaire was developed and data from N ¼ 578 physics students from five universities in Germany, Austria, and Croatia were gathered. In this paper, we report how students perceived synchronous and asynchronous physics lessons, how their perception correlates with their self-organization skills, which activities and teaching methods were perceived as helpful, and what are the implications for future physics courses. The most common advantages of synchronous course elements reported by students were the possibility to immediately ask questions, the feeling of community and interaction with other students, and the defined daily structure, whereas the most common advantages of asynchronous course elements reported were flexible time management and the possibility to watch videos at their own pace. The data indicate a correlation between preference for synchronous courses and their general self-organization, so instructors should be aware of this connection when planning future courses. Face-to-face lectures at university were perceived as the most helpful course element, followed by the recorded lectures from the instructor and the group work on the assignments, projects, and problems with other students. Furthermore, our results suggest that most students would in the future like to preserve the upload of learning materials and recorded video of the lectures in addition to classroom lectures. Overall, the results of this study suggest that both synchronous and asynchronous course elements should be combined in future online and in-person physics courses.

Les dernières décennies ont donné lieux à d'énormes progrès dans la simulation numérique des phénomènes physiques. D'une part grâce au raffinement des méthodes de discrétisation des équations aux dérivées partielles. Et d'autre part grâce à l'explosion de la puissance de calcul disponible. Pourtant, de nombreux problèmes soulevés en ingénierie tels que les simulations multi-physiques, les problèmes d'optimisation et de contrôle restent souvent hors de portée. Le dénominateur commun de ces problèmes est le fléau des dimensions. Un simple problème tridimensionnel requiert des centaines de millions de points de discrétisation auxquels il faut souvent ajouter des milliers de pas de temps pour capturer des dynamiques complexes. L'avènement des supercalculateurs permet de générer des simulations de plus en plus fines au prix de données gigantesques qui sont régulièrement de l'ordre du pétaoctet. Malgré tout, cela n'autorise pas une résolution ``exacte'' des problèmes requérant l'utilisation de plusieurs paramètres. L'une des voies envisagées pour résoudre ces difficultés est de proposer des représentations ne souffrant plus du fléau de la dimension. Ces représentations que l'on appelle séparées sont en fait un changement de paradigme. Elles vont convertir des objets tensoriels dont la croissance est exponentielle $n^d$ en fonction du nombre de dimensions $d$ en une représentation approchée dont la taille est linéaire en $d$. Pour le traitement des données tensorielles, une vaste littérature a émergé ces dernières années dans le domaine des mathématiques appliquées.Afin de faciliter leurs utilisations dans la communauté des mécaniciens et en particulier pour la simulation en mécanique des fluides, ce manuscrit présente dans un vocabulaire rigoureux mais accessible les formats de représentation des tenseurs et propose une étude détaillée des algorithmes de décomposition de données qui y sont associées. L'accent est porté sur l'utilisation de ces méthodes, aussi la bibliothèque de calcul texttt{pydecomp} développée est utilisée pour comparer l'efficacité de ces méthodes sur un ensemble de cas qui se veut représentatif. La seconde partie de ce manuscrit met en avant l'étude de l'écoulement dans une cavité entraînée à haut nombre de Reynolds. Cet écoulement propose une physique très riche (séquence de bifurcation de Hopf) qui doit être étudiée en amont de la construction de modèle réduit. Cette étude est enrichie par l'utilisation de la décomposition orthogonale aux valeurs propres (POD). Enfin une approche de construction ``physique'', qui diffère notablement des développements récents pour les modèles d'ordre réduit, est proposée. La connaissance détaillée de l'écoulement permet de construire un modèle réduit simple basé sur la mise à l'échelle des fréquences d'oscillation (time-scaling) et des techniques d'interpolation classiques (Lagrange,..). / Numerical simulation has experienced tremendous improvements in the last decadesdriven by massive growth of computing power. Exascale computing has beenachieved this year and will allow solving ever more complex problems. But suchlarge systems produce colossal amounts of data which leads to its own difficulties.Moreover, many engineering problems such as multiphysics or optimisation andcontrol, require far more power that any computer architecture could achievewithin the current scientific computing paradigm. In this thesis, we proposeto shift the paradigm in order to break the curse of dimensionality byintroducing decomposition and building reduced order models (ROM) for complexfluid flows.This manuscript is organized into two parts. The first one proposes an extendedreview of data reduction techniques and intends to bridge between appliedmathematics community and the computational mechanics one. Thus, foundingbivariate separation is studied, including discussions on the equivalence ofproper orthogonal decomposition (POD, continuous framework) and singular valuedecomposition (SVD, discrete matrices). Then a wide review of tensor formats andtheir approximation is proposed. Such work has already been provided in theliterature but either on separate papers or into a purely applied mathematicsframework. Here, we offer to the data enthusiast scientist a comparison ofCanonical, Tucker, Hierarchical and Tensor train formats including theirapproximation algorithms. Their relative benefits are studied both theoreticallyand numerically thanks to the python library texttt{pydecomp} that wasdeveloped during this thesis. A careful analysis of the link between continuousand discrete methods is performed. Finally, we conclude that for mostapplications ST-HOSVD is best when the number of dimensions $d$ lower than fourand TT-SVD (or their POD equivalent) when $d$ grows larger.The second part is centered on a complex fluid dynamics flow, in particular thesingular lid driven cavity at high Reynolds number. This flow exhibits a seriesof Hopf bifurcation which are known to be hard to capture accurately which iswhy a detailed analysis was performed both with classical tools and POD. Oncethis flow has been characterized, emph{time-scaling}, a new ``physics based''interpolation ROM is presented on internal and external flows. This methodsgives encouraging results while excluding recent advanced developments in thearea such as EIM or Grassmann manifold interpolation.

Angetrieben durch die Akkretion von Materie in ein super massives Schwarzes Loch in ihrem Zentrum, stellen aktive Galaxien die stärksten und beständigsten Strahlungsquellen im Universum dar. Ihre elektromagnetische Emission kann sich bis in den Gammastrahlenbereich ausbreiten. Das Ziel dieser Arbeit ist, diese Mechanismen und die Orte jenseits der hoch energetischen Emission zu charakterisieren. Dafür werden die Observationen von zwei Aktiven Galaxien im Bereich von hunderten von GeV verwendet, welche mit den Cherenkov Teleskopen MAGIC aufgenommen wurden. Die physikalische Interpretation wird durch Beobachtungen mit dem Fermi Gamma-ray Space Teleskop und durch Multiwellenlängendaten unterstützt. Es werden zwei Aktive Galaxien mit Jet untersucht: PKS 1510-089 und NGC 1275. Die MAGIC Teleskope, welche PKS 1510-089 seit 2012 immer wieder beobachten, detektieren eine signifikante Emission über dutzende von Observationsstunden, was auf schwache aber kontinuierliche Gammastrahlung aus dieser Quelle hinweist. NGC 1275 zeigte in der Periode von September 2016 bis Februar 2017 einen großen Ausbruch im Gammerstrahlenbereich: MAGIC zeichnete eine Variabilität in der Größenordnung von wenigen Stunden und die erstmalige Emission von TeV Photonen. Aus beiden untersuchten Quellen ist ersichtlich, dass die Kombination von Daten aus verschiedenen Instrumenten die physische Diskussion entscheidend beeinflusst. Der Übergang zu zugänglichen und interoperablen Daten wird zu einem zwingenden Thema für Gammastrahlenastronomen, und diese Arbeit stellt das technische Bestreben dar, standardisierte hochrangige Daten für Gammastrahleninstrumente zu erzeugen. Ein Beispiel für eine zukünftige Analyse, die einheitliche High-Level-Daten von einem Gammastrahlensatelliten und vier Cherenkov-Teleskopen kombiniert, wird vorgestellt. Der neue Ansatz, der vorgeschlagen wird, führt die Datenanalyse durch und verbreitet die Ergebnisse, wobei nur Open-Source-Ressourcen verwendet werden. / Powered by the accretion of matter to a supermassive black hole, active galactic nuclei constitute the most powerful and persistent sources of radiation in the universe, with emission extending in the gamma-ray domain. The aim of this work is to characterise the mechanisms and sites beyond this highly-energetic radiation employing observations of two galaxies at hundreds of GeV, conducted with the MAGIC imaging Cherenkov telescopes. The physical interpretation is supported with observations by the Fermi Gamma-ray Space Telescope and with multi-wavelength data. Two peculiar jetted galaxies are studied: PKS 1510-089 and NGC 1275. The first source, monitored by MAGIC since 2012, presents a significant emission over tens of observation hours, in what appears to be a low but persistent gamma-ray state. The second source has instead shown, in the period between September 2016 and February 2017, a major outburst in its gamma-ray activity with variability of the order of few hours and emission of TeV photons. The broad band emission of jetted galaxies is commonly modelled with the radiative processes of a population of electrons accelerated in the jet. While PKS 1510-089 conforms to this scenario, modelling the gamma-ray outburst of NGC 1275 requires placing the acceleration and radiation of electrons close to the event horizon of the black hole. From both the sources studied it is evident that the combination of data from different instruments critically drives the physical discussuion. Moving towards accessible and interoperable data becomes a compelling issue for gamma-ray astronomers and this thesis presents the technical endeavour to produce standardised high-level data for gamma-ray instruments. An example of a future analysis combining uniformed high-level data from a gamma-ray satellite and four Cherenkov telescopes is presented. The novel approach proposed performs the data analysis and disseminates the results making use only of open-source assets.

La majeure partie des communications mondiales est transportée par des systèmes transocéaniques à fibre optique. Il est estimé que d'ici 2020 le trafic de données atteindra 4.3 ZB par an. Afin de faire face à cette demande, différentes technologies sont actuellement étudiées pour augmenter la capacité des systèmes de transmission très longue distance. Avec l'avènement des circuits intégrés à haute vitesse, des formats de modulation avancés et des techniques de traitement de signal numérique (DSP) peuvent être utilisés pour maximiser l'efficacité spectrale de transmission. Par ailleurs, la capacité des systèmes modernes est fortement limitée par les effets non-linéaires de type Kerr dans la fibre. Ainsi, la première partie de ce travail est axée sur l’étude de la performance et des gains réalisables par des techniques DSP à faible complexité pour mitiger les effets non-linéaires monocanal. En outre, l’utilisation des formats de modulation multiniveaux à haute efficacité spectrale au-delà de 16QAM a pris de l'ampleur pour augmenter le débit de transmission des systèmes, notamment avec l’introduction des formats QAM avec mise en forme probabiliste (PCS-QAM), plus performants que les formats QAM classiques. La deuxième partie de ce travail présente donc une comparaison théorique ainsi qu’expérimentale du format PCS-64QAM avec d’autres formats à haute efficacité spectrale pour les distances transatlantiques. La mise en œuvre d’un format PCS-64QAM conçu pour les distances transpacifiques est également abordée. Enfin, la dernière partie de ce travail concrétise les résultats des travaux menés dans les deux sections précédentes en présentant plusieurs records de transmission / Global data traffic is expected to reach up to 4.3 ZB per year by 2020. With the majority of the global communications being transported on submarine point-to-point fiber-optic systems, different cutting-edge technologies have been under research to cope with this unprecedented traffic growth. Continuous advances in high-speed integrated circuits have allowed the use of advanced modulation formats and digital signal processing (DSP) techniques to maximize the transmission spectral efficiency. With mitigation of fiber linear effects efficiently carried out by DSP with relative low-complexity, the capacity of modern fiber optic systems rests limited by fiber nonlinearities. To this extent, in the first part of this work, the performance and achievable benefits of low-complexity DSP techniques aiming to mitigate fiber Kerr nonlinear effects are investigated. Besides nonlinear compensation techniques, the use of multi-level modulation formats beyond 16QAM and high symbol rate channels have gained momentum to increase the system spectral efficiency. One of the major breakthroughs in the recent years, has been the introduction of QAM-based probabilistic constellation shaping (PCS-QAM), which has proven to outperform regular QAM formats. In this sense, in the second part of this work, the practical achievable rate increase brought by PCS-QAM for transoceanic distances is investigated. A theoretical and experimental comparison with other high-capacity formats is performed, and the design of a PCS-QAM for trans-Pacific distances is addressed. Finally, in the last section, several transmission records using the two above techniques are reported

Os avanços tecnológicos apontam para uma renovação da infraestrutura atual de comunicações ópticas, de modo a torná-la adequada à operação dentro dos novos paradigmas das redes, em que a elasticidade e eficiência espectrais se aliam à alta capacidade de transmissão. Sob o ponto de vista do planejamento de uma operadora de telecomunicação, é desejável que a substituição de equipamentos ocorra de forma mais gradual e que a operação da infraestrutura atual seja otimizada para acomodar a demanda por alta capacidade sem requerer a construção de uma infraestrutura completamente nova. Neste contexto, esta tese investiga estratégias de otimização combinando técnicas que utilizam múltiplas taxas de transmissão (MLR &#8211 Mixed Line Rate) e múltiplos formatos de modulação (MMF &#8211 Multiple Modulation Formats) com foco em um cenário condizente a realidade brasileira, com taxa de transmissão de 10 Gbit/s modulados com a técnica não retorna a zero (NRZ &#8211 non return to zero), migrando para taxa de 40 e/ou 100 Gbit/s. São analisados os benefícios proporcionados com o uso de redes MLR-MMF em comparação com a substituição da rede legada por apenas uma taxa (SLR &#8211 single line rate). A infraestrutura da camada física considerada é uniforme e pode transportar sinais de 10/40/100 Gbit/s, apenas os tipos de transponders são diferentes. Para analisar o desempenho da rede descrita é proposto um modelo de integração entre as ferramentas MatLab e OptiSystem. A primeira é utilizada para definir a rota e a segunda para simular a propagação do sinal óptico na fibra ao longo da rota escolhida. Os valores de taxa de erro de bit (BER &#8211 Bit Error Rate) e a relação sinal ruído óptica (OSNR &#8211 Optical Signal Noise Rate) são armazenados em um banco de dados para uso posterior. Dessa forma diversas topologias podem ser analisadas. Em uma rede real esse tipo de informação pode ser obtido por meio de técnicas de monitoramento óptico (OPM &#8211 Optical Performance Monitoring). Por esse motivo, também são investigados e testados experimentalmente dois métodos de monitoramento da OSNR: i) anulamento da polarização para obter OSNR e ii) monitor multiparâmetro baseado em modulação da polarização em baixa frequência. A partir das análises, concluiu-se que o uso de redes MLR-MMF pode ser uma alternativa para equilibrar o consumo energético e a probabilidade de bloqueio (PB). A opção por esse tipo de rede ajuda na economia energética e mantém a PB em níveis aceitáveis. Além disso, o uso de monitoramento óptico auxilia na obtenção de informações usadas para controlar e gerenciar eficientemente recursos de rede cumprindo os requisitos de qualidade de serviço. / Technological advances point out to an upgrade on the current optical communication infrastructure in order to adequate it to the new networks paradigms, where elasticity and spectral efficiency are associated to the high capacity transmission. From the perspective of a telecom operator planning, it is desirable that the equipment replacement occurs as gradually as possible and that the operation of the current infrastructure is optimized to accommodate the demand for high capacity without requiring the build-upof a completely new infrastructure. In this context, this thesis investigates optimization strategies combining MLR (mixed line rate) and MMF (multiple modulation formats) techniques focusing on a scenario similar to the Brazilian reality, with a transmission rate of 10 Gbit/s NRZ modulated, switching to a rate of 40 and/or 100 Gbit/s. The benefits provided by the use of MMF-MLR network (non return to zero)s compared to the replacing based on a legacy network by only one rate (SLR single line rate) are discussed. The infrastructure of the physical layer is considered uniform and can carry signals 10/40/100 Gbit/s, only the transponder configurations are different. To analyze the network performance we proposed an integration of MATLAB and OptiSystem tools. The first is used to define the route and the second to simulate the propagation of the optical signal in the fiber along the chosen route. The BER and OSNR values are stored in a database for later use. In this way several topologies can be analyzed. In a real network such information may be obtained by optical monitoring techniques (OPM). For this reason, two methods for monitoring OSNR are also investigated: i) the polarization nulling and ii) multiparameter monitor based on modulation of polarization at low frequency. From the analysis it was concluded that the use of MMF-MLR schemes can be an alternative to balance the energy consumption and the blocking probability (BP). The option for this type of network saves energy and helps maintaining BP within acceptable levels. Furthermore, the use of optical monitoring allows the gathering of information used to control and efficiently manage network resources meeting the requirements for quality of service.

Ciências Empresariais / Master Programme in Management Sciences / A dissertação que se apresenta enquadra-se na problemática da identificação dos factores determinantes para as vendas no mercado da distribuição, com enfoque no sector do retalho. O espaço em área de venda, identificado pelos retalhistas como o seu recurso mais dispendioso, assume um papel importante como factor explicativo das vendas, sendo dada relevância à sua influência ao longo de toda a investigação. Por outro lado, procura-se avaliar a importância do espaço e de outros factores para diferentes categorias de produtos e segmentos de loja. Este estudo serve-se de um exemplo representativo de lojas que operam no mercado português, pertencentes a um dos principais grupos retalhistas existentes, para avaliar esta problemática Relativamente à metodologia adoptada no estudo do grupo de retalho português recorreu-se a modelos econométricos - regressão linear, segmentação e modelos de mistura de regressão, usando informação da empresa e das suas insígnias. Concluiu-se que o espaço é um factor determinante para as vendas, cuja importância varia entre lojas, grupos de lojas e categorias de produtos. Estes resultados são importantes na tomada de decisão do retalhista, na medida em que contribuem para a realização de estudos de simulação de lojas, prevendo os efeitos das variações de vendas provocados por alterações de espaço. Os factores relacionados com o preço face à concorrência, rendimento per capita da população alvo e qualidade no atendimento também foram analisados e inferiu-se sobre a sua importância relativa. Os modelos de segmentação adoptados foram também úteis, porque permitiram identificar grupos homogéneos de lojas, facultando ao gestor informação útil para a definição de políticas direccionadas a estes grupos. / The following dissertation covers the field of identification of the determinant sales factors at the distribution market, with the emphasis on the retail sector. The sales area, which is considered by retailers as the highest investment outlay point, assumes an important role as explanatory sales factor and gets a relevant attention throughout the research according to its influential character. On the other hand, the ambition has been set to evaluate the importance of the retail sales area and of the other factors concerning various product categories and various segments of stores. This study operates with a representative example of stores working in the Portuguese market and belonging to one of the existing local retail groups, with the objective of analysing this problematic. As for the methodology applied to the study of the Portuguese retail group, the econometric models were chosen linear regression, segmentation, regression mixture models, survey information of the company in question and its store formats. To sum up, the sales area is a determinant sales factor, which importance varies between the stores, groups of stores and product categories. These results are relevant to the retail's decisions since they facilitate the realization of underlying studies predicting the effects of the sale variations provoked by sales area alteration. The factors related to the price contest in face of the competitors, the income per capita of the population as well as the service quality were analysed and studied its relative importance. The applied segmentation models proved to be useful since they allowed identify homogeneous store groups, delivering valuable information to the managers that facilitates the definition of particular attitude policies regarding these groups

.

Television is increasingly both global and local. Those television industries discussed in this thesis transact in an extensive neo-network of flows in talents, financing, and the latest forms of popular culture. These cities attempt to become media capitals but their status waxes and wanes, depending on their success in exporting their Asian media productions. What do marital arts dramas, interactive game-shows, children's animation and teenage idol soap operas from East Asian television industries have in common? Through the systematic use of TV formatting strategies, these television genres have become the focus for indigenous cultural entrepreneurs located in the East Asian cities of Hong Kong, Singapore and Taipei to turn their local TV programmes into tradable culture. This thesis is a re-consideration of the impact of media globalisation on Asian television that re-imagines a new global media order. It suggests that there is a growing shift in perception and trade among once-peripheral television industries that they may be slowly de-centring Hollywood's dominance by inserting East Asian popular entertainment into familiar formats or cultural spaces through embracing global yet local cultures of production. While TV formats like Survivor, Millionaire, Big Brother and American Idol have become profitable and powerful franchises globally, in East Asia, the size of TV format trade is actually eclipsed by the regional trade in East Asian popular cultural commodities from martial arts novels and films, manga and romantic fiction, to popular music. These commodities have become the source of remaking local television culture into tradable cultures as local TV programmes use formatting practices to circulate within their region. The many faces of formatting in television are explored through four case studies - from Hong Kong (TVB's Heaven Sword and Dragon Sabre), Singapore (Robert Chua Productions' Everyone Wins, Peach Blossom Media's Tomato Twins) and Taipei (Comic Ritz Production's Meteor Garden). Conceptualised as Asian media productions, these TV programmes are sites for examining individual agency, the network flows of popular culture and structural changes of their respective broadcasting fields. This thesis argues that TV formatting practices can become a currency for neo-networked media producers to create a medium of cultural exchange that sets up the possibility for a common market for cultural trade in East Asia. However, the ease with which TV formatting practices and re-sale of TV programmes are copied lower barriers for competition and often this tends toward over production. Over-exposure kills many new genres of production and discourages investment in the research and development component of creating TV formats for trade. Change in East Asian television industries is also aided by media conglomeration, global access through satellite TV, the Internet and increasingly digital entertainment, media de-regulation and pro-development policies. A number of factors and conditions that accompany the rise of TV formatting in East Asia (such as the role of independents vis-a-vis big local players, the emergence of copyright issues and marketing celebrities) contribute to the innovations that result from adapting formatting practices to local contexts, and suggest how each city's television industry attempts to address the rise of tradable cultural commodities that are increasingly made for pan-Asian consumption.

La consommation d'énergie des centres de données est un enjeu majeur. Leurs communications internes représentent près du quart de cette consommation. Les technologies de commutation optique ont en principe une meilleure efficacité énergétique que les solutions actuelles. Ce travail porte sur les réseaux optiques en mode paquet pour des centres de données de petite et moyenne taille. Il s'est déroulé dans le cadre du projet EPOC (Energy Proportional and Opportunistic Computing) qui s'intéresse à la réduction de la consommation d'énergie d'un centre de données alimenté partiellement par des sources renouvelables. Une hypothèse clé est l'absence d'un réseau de stockage dédié réduisant ainsi la consommation des interconnexions. Par contre, afin de pouvoir éteindre certains serveurs selon la charge de travail et l'énergie disponible, le débit doit être de l'ordre de 100 Gbit/s. Après un état de l'art des réseaux optiques pour centre de données nous choisissons une solution reposant sur une infrastructure entièrement passive et des émetteurs rapidement accordables en longueur d'onde, proposée récemment dans la littérature (POPI).Nous étudions les limitations dues aux composants (pertes d'insertion, plage d'accord et espacement des canaux). Nous proposons une extension (E-POPI) qui permet d'augmenter le nombre de serveurs connectés en utilisant plusieurs plages de longueurs d'onde. Pour les centres de données de plus grande taille nous proposons un réseau à deux étages (intra- et inter-racks) opérant respectivement dans les bandes C et L, POPI+. La connexion entre étages se fait via une passerelle transparente dans un sens et opaque dans l'autre. Différentes solutions de contrôle des deux étages sont détaillées.Un des éléments essentiels de faisabilité de ces architectures est de concilier la montée en débit avec les pertes du réseau passif d'interconnexion. Les techniques cohérentes des transmissions longue distance ne sont pas actuellement envisageables pour un centre de données. Nous avons donc étudié les formats PAM 4 et 8, par simulation avec différents débits (jusqu'à 112 Gbit/s et récepteurs (PIN, APD et SOA-PIN) et aussi, expérimentalement, à 12 et 18 Gbit/s. Nous avons développé une méthode de compensation des distorsions générées par les différents composants qui procure un compromis entre précision de correction et temps de calcul.Ces résultats nous permettent de déterminer les pertes d'insertion tolérables. Nous les combinons avec les limitations liées à la plage d'accord des émetteurs et à l'encombrement spectral des canaux occupant des fenêtres multiples de 12,5 GHz pour dimensionner les différentes architectures. Les réseaux POPI, E-POPI et POPI+ permettent respectivement la connexion de 48, 99 et 2352 entités à 112 Gbit/s. Nos évaluations tiennent compte d'une possible dispersion des caractéristiques des principaux composants. / Data-center energy consumption is nowadays a major issue. Intra-data-center networking accounts almost for a quarter of the data-center total power consumption. Optical switching technologies could provide higher power efficiency than current solutions based on electrical-packet switching. This work focuses on optical-packet-switched networks for small- and medium-size data centers. It takes part of the EPOC (Energy-Proportional and Opportunistic Computing) project, which main interest consists on reducing the overall power consumption of a data center partially powered by renewable sources. A key assumption is that our data center does not rely on a dedicated storage network, in order to reduce the consumption of those interconnections. In addition, with the aim of being able to turn off some servers according to the workload and the available energy, the bit rate must be close to 100 Gbit/s. We have chosen, after studying the state of the art of data-center interconnects, a purely passive network architecture based on fast-wavelength-tunable transmitters under the name of POPI.We study POPI's limitations due to its components (insertion loss, tuning range and channel spacing). We then propose an extension called E-POPI that allows to increase the number of connected servers by using several transmission bands. For larger data centers, we propose POPI+, a two-stage infrastructure for intra- and inter-rack communications operating in the C and L bands, respectively. The connection between both stages is done via a transparent gateway in one direction and an opaque one in the other. We discuss different control solutions for both stages.The feasibility of these architectures depends on, among other factors, dealing with bit-rate increasing and power losses of a passive interconnect. Coherent long-distance-transmission techniques are not currently suited to data centers. We therefore studied PAM 4 and 8 modulation formats with direct detection. On one hand, by simulation, with different bit rates (up to 112 Gbit/s) and receivers (PIN, APD and SOA-PIN) and, on the other hand, experimentally, at 12 and 18 Gbit/s. We have developed a method for compensating the distortions generated by the different network components. Our method takes into account a good tradeoff between correction accuracy and computation time.Simulation results allow us to determine the amount of insertion loss that may be supported. We then combine these results with the limitations of transmitters-tuning range and channel spacing using multiple of 12.5 GHz slots for dimensioning the proposed architectures. POPI, E-POPI and POPI+ interconnects allow the connection of 48, 99 and 2352 entities, respectively, at 112 Gbit/s. Our assessments take into account a potential dispersion of the characteristics of the main architecture components.

Made available in DSpace on 2010-04-20T20:51:34Z (GMT). No. of bitstreams: 3 41045.pdf.jpg: 19735 bytes, checksum: 8982b386440c1e32af9686848e0e3c38 (MD5) 41045.pdf: 4547294 bytes, checksum: 970a4994285745a21416ed6bbfb91a6a (MD5) 41045.pdf.txt: 342901 bytes, checksum: 383d58a1ea2d69d2a23310a4cbafa28b (MD5) Previous issue date: 2007-03-22T00:00:00Z / O mercado de baixa renda no Brasil, que engloba a população das classes sócio-econômicas CDE , representa cerca de 77% dos domicílios brasileiros, com renda familiar de até R$ 2.000,00 por mês (10 salários mínimos em 2002), movimentando cerca de R$ 500 bilhões/ ano em consumo, o equivalente a 35% do consumo agregado nacional (IBGE, 2003). Na categoria de alimentos, esse segmento da população é responsável por 53% do consumo nacional, sendo que em categorias como cereais em geral, leite fresco e em pó, fraldas descartáveis e sabão em pedra são responsáveis por cerca de 80% do consumo no país (LATIN PANEL, 2004). Os pequenos varejistas são os principais responsáveis pelo atendimento a este segmento da população, já que a participação das grandes redes de auto-serviço é relativamente baixa nesse mercado. Os pequenos auto-serviços e o varejo tradicional (padarias, mercearias, empórios) respondem por cerca de 81% das vendas (LATIN PANEL, 2004) e possuem uma penetração superior a 90% no mercado de baixa renda (HILÁRIO 2006). Eles são abastecidos em sua grande maioria de forma indireta pela indústria, que se utiliza de atacadistas e distribuidores para fazerem com que seus produtos cheguem até esses consumidores de menor renda. Esses varejistas possuem características e necessidades distintas dos supermercados de maior porte, dentre as quais pode-se citar: menor capital de giro; menor poder de negociação na hora da compra, em razão de falta de escala; dificuldade de investimento em tecnologia e em treinamento de pessoal (HILÁRIO, 2006) que demandam ações diferenciadas de seus fornecedores, atacadistas e distribuidores. Por meio de uma pesquisa de caráter exploratório, em uma amostra de empresas que atuam no setor varejista e de distribuição, buscou-se identificar, com base no modelo de relacionamentos comprador-vendedor desenvolvido por WILSON (1995), as variáveis ativas mais importantes em cada etapa dos relacionamentos entre o pequeno varejo de baixa renda e os intermediários da indústria, e apontar as estratégias de relacionamento de maior importância na visão dos varejistas. A conclusão do estudo mostra que as estratégias de relacionamento entre atacadistas e pequenos varejistas é ainda muito focada no aspecto transacional, imperando a questão do preço, com baixo envolvimento e compromisso entre os agentes. Embora esta seja, aparentemente, a realidade dos relacionamentos entre atacadistas e pequenos varejistas, existe uma diferenciação nas estratégias de relacionamento entre distribuidores e pequenos varejistas, bem como iniciativas por parte de alguns atacadistas em desenvolverem relacionamentos baseados também em compromisso e confiança. Este parece o caminho a ser trilhado pelas empresas atacadistas e distribuidoras que pretendem criar e manter vantagens competitivas sustentáveis e desenvolver parcerias de sucesso com o pequeno varejo. / The low-income market in Brazil, characterized by the CDE socio-economic classes, represents about 77% of the Brazilian households. This parcel of the Brazilian population earns up to R$ 2.000,00 per month (10 Brazilian minimum wages in 2002), spending about R$ 500 billion per year, corresponding to 35% of the national aggregated consumption (IBGE, 2003). In the category of food products, this segment of the population is responsible for 53% of the national consumption. In other categories like general cereals, fresh and powdered milk, diapers and bar soaps, they are responsible for 80% of the consumption in the country. (Latin Panel, 2004). The small retailers are the main channel in supporting this population segment, since the participation of the bigger chains of supermarkets are relatively lower in this market. The small supermarkets and the traditional retail ( bakeries and grocery stores) respond for about 81% of sales (Latin Panel, 2004) and have a penetration above 90% in the low-income market (Hilario, 2006). They are supplied, indirectly, mostly by the industry, that uses wholesalers and distributors to make their products reach these low-income consumers. These retailers have characteristics and needs others than bigger supermarkets, among that, can be mentioned: lower operating capital; lower negotiation power at purchasing time, due to the low volumes; difficulty to invest in technology and employees training (Hilario, 2006) that demand different actions from suppliers, wholesalers, and distributors. Through a research of exploratory character, in a sample of companies that work in the retailer and wholesaler market, it searched to identify, based in the buyer-seller relationship model developed by WILSON (1995), the most important active variables in each step of the relationships between the small low-income retail and the industry intermediates and point out the most important strategies in the retailers’ sight. The conclusion of the study shows that the relationship strategies, mainly among wholesalers and small retailers, are much focused in the transactional aspect yet, ruling the price aspect, with low involvement and commitment among the agents. Although it is, apparently, the reality of the relationship among wholesalers and small retailers, there is a differentiation in the relationship strategies among distributors and small retailers, and initiatives by the wholesalers to develop relationships also based on commitment and trust. This seems the way to be tracked by the wholesalers and distributors companies that intend to create and maintain sustainable competitive advantages, and develop success partnerships with the small retail.