Survivability in Time-varying Networks

Time-varying graphs are a useful model for networks with dynamic connectivity such as vehicular networks, yet, despite their great modeling power, many important features of time-varying graphs are still poorly understood. In this paper, we study the survivability properties of time-varying networks against unpredictable interruptions. We first show that the traditional definition of survivability is not effective in time-varying networks, and propose a new survivability framework. To evaluate the survivability of time-varying networks under the new framework, we propose two metrics that are analogous to MaxFlow and MinCut in static networks. We show that some fundamental survivability-related results such as Menger's Theorem only conditionally hold in time-varying networks. Then we analyze the complexity of computing the proposed metrics and develop several approximation algorithms. Finally, we conduct trace-driven simulations to demonstrate the application of our survivability framework to the robust design of a real-world bus communication network

Functional brain networks before the onset of psychosis : a prospective fMRI study with graph theoretical analysis

Individuals with an at-risk mental state (ARMS) have a risk of developing a psychotic disorder significantly greater than the general population. However, it is not currently possible to predict which ARMS individuals will develop psychosis from clinical assessment alone. Comparison of ARMS subjects who do, and do not, develop psychosis can reveal which factors are critical for the onset of illness. In the present study, 37 patients with an ARMS were followed clinically at least 24 months subsequent to initial referral. Functional MRI data were collected at the beginning of the follow-up period during performance of an executive task known to recruit frontal lobe networks and to be impaired in psychosis. Graph theoretical analysis was used to compare the organization of a functional brain network in ARMS patients who developed a psychotic disorder following the scan (ARMS-T) to those who did not become ill during the same follow-up period (ARMS-NT) and aged-matched controls. The global properties of each group's representative network were studied (density, efficiency, global average path length) as well as regionally-specific contributions of network nodes to the organization of the system (degree, farness-centrality, betweenness-centrality). We focused our analysis on the dorsal anterior cingulate cortex (ACC), a region known to support executive function that is structurally and functionally impaired in ARMS patients. In the absence of between-group differences in global network organization, we report a significant reduction in the topological centrality of the ACC in the ARMS-T group relative to both ARMS-NT and controls. These results provide evidence that abnormalities in the functional organization of the brain predate the onset of psychosis, and suggest that loss of ACC topological centrality is a potential biomarker for transition to psychosis

Contributions to modeling, structural analysis, and routing performance in dynamic networks

Cette thèse apporte des contributions à la modélisation, compréhension ainsi qu’à la communication efficace d’information dans les réseaux dynamiques peuplant la périphérie de l’Internet. Par réseaux dynamiques, nous signifions les réseaux pouvant être modélisés par des graphes dynamiques dans lesquels noeuds et liens évoluent temporellement. Dans la première partie de la thèse, nous proposons un nouveau modèle de mobilité - STEPS - qui permet de capturer un large spectre de comportement de mobilité humains. STEPS mets en oeuvre deux principes fondamentaux de la mobilité humaine : l’attachement préférentiel à une zone de prédilection et l’attraction vers une zone de prédilection. Nous proposons une modélisation markovienne de ce modèle de mobilité. Nous montrons que ce simple modèle paramétrique est capable de capturer les caractéristiques statistiques saillantes de la mobilité humaine comme la distribution des temps d’inter-contacts et de contacts. Dans la deuxième partie, en utilisant STEPS, nous analysons les propriétés comportementales et structurelles fondamentales des réseaux opportunistes. Nous redéfinissons dans le contexte des réseaux dynamiques la notion de structure petit monde et montrons comment une telle structure peut émerger. En particulier, nous montrons que les noeuds fortement dynamiques peuvent jouer le rôle de ponts entre les composants déconnectés, aident à réduire significativement la longueur du chemin caractéristique du réseau et contribuent à l’émergence du phénomène petit-monde dans les réseaux dynamiques. Nous proposons une façon de modéliser ce phénomène sous STEPS. À partir d’un réseau dynamique régulier dans lequel les noeuds limitent leur mobilité à leurs zones préférentielles respectives. Nous recablons ce réseau en injectant progressivement des noeuds nomades se déplaçant entre plusieurs zones. Nous montrons que le pourcentage de tels nœuds nomades est de 10%, le réseau possède une structure petit monde avec un fort taux de clusterisation et un faible longueur du chemin caractéristique. La troisième contribution de cette thèse porte sur l’étude de l’impact du désordre et de l’irrégularité des contacts sur la capacité de communication d’un réseau dynamique. Nous analysons le degré de désordre de réseaux opportunistes réels et montrons que si exploité correctement, celui-ci peut améliorer significativement les performances du routage. Nous introduisons ensuite un modèle permettant de capturer le niveau de désordre d’un réseau dynamique. Nous proposons deux algorithmes simples et efficaces qui exploitent la structure temporelle d’un réseau dynamique pour délivrer les messages avec un bon compromis entre l’usage des ressources et les performances. Les résultats de simulations et analytiques montrent que ce type d’algorithme est plus performant que les approches classiques. Nous mettons également en évidence aussi la structure de réseau pour laquelle ce type d’algorithme atteint ses performances optimum. Basé sur ce résultat théorique nous proposons un nouveau protocole de routage efficace pour les réseaux opportunistes centré sur le contenu. Dans ce protocole, les noeuds maintiennent, via leurs contacts opportunistes, une fonction d’utilité qui résume leur proximité spatio-temporelle par rapport aux autres noeuds. En conséquence, router dans un tel contexte se résume à suivre le gradient de plus grande pente conduisant vers le noeud destination. Cette propriété induit un algorithme de routage simple et efficace qui peut être utilisé aussi bien dans un contexte d’adressage IP que de réseau centré sur les contenus. Les résultats de simulation montrent que ce protocole superforme les protocoles de routage classiques déjà définis pour les réseaux opportunistes. La dernière contribution de cette thèse consiste à mettre en évidence une application potentielle des réseaux dynamiques dans le contexte du « mobile cloud computing ». En utilisant les techniques d’optimisation particulaires, nous montrons que la mobilité peut augmenter considérablement la capacité de calcul des réseaux dynamiques. De plus, nous montrons que la structure dynamique du réseau a un fort impact sur sa capacité de calcul. ABSTRACT : This thesis contributes to the modeling, understanding and efficient communication in dynamic networks populating the periphery of the Internet. By dynamic networks, we refer to networks that can be modeled by dynamic graphs in which nodes and links change temporally. In the first part of the thesis, we propose a new mobility model - STEPS - which captures a wide spectrum of human mobility behavior. STEPS implements two fundamental principles of human mobility: preferential attachment and attractor. We show that this simple parametric model is able to capture the salient statistical properties of human mobility such as the distribution of inter-contact/contact time. In the second part, using STEPS, we analyze the fundamental behavioral and structural properties of opportunistic networks. We redefine in the context of dynamic networks the concept of small world structure and show how such a structure can emerge. In particular, we show that highly dynamic nodes can play the role of bridges between disconnected components, helping to significantly reduce the length of network path and contribute to the emergence of small-world phenomenon in dynamic networks. We propose a way to model this phenomenon in STEPS. From a regular dynamic network in which nodes limit their mobility to their respective preferential areas. We rewire this network by gradually injecting highly nomadic nodes moving between different areas. We show that when the ratio of such nomadic nodes is around 10%, the network has small world structure with a high degree of clustering and a low characteristic path length. The third contribution of this thesis is the study of the impact of disorder and contact irregularity on the communication capacity of a dynamic network. We analyze the degree of disorder of real opportunistic networks and show that if used correctly, it can significantly improve routing performances. We then introduce a model to capture the degree of disorder in a dynamic network. We propose two simple and efficient algorithms that exploit the temporal structure of a dynamic network to deliver messages with a good tradeoff between resource usage and performance. The simulation and analytical results show that this type of algorithm is more efficient than conventional approaches. We also highlight also the network structure for which this type of algorithm achieves its optimum performance. Based on this theoretical result, we propose a new efficient routing protocol for content centric opportunistic networks. In this protocol, nodes maintain, through their opportunistic contacts, an utility function that summarizes their spatio-temporal proximity to other nodes. As a result, routing in this context consists in following the steepest slopes of the gradient field leading to the destination node. This property leads to a simple and effective algorithm routing that can be used both in the context of IP networks and content centric networks. The simulation results show that this protocol outperforms traditional routing protocols already defined for opportunistic networks. The last contribution of this thesis is to highlight the potential application of dynamic networks in the context of "mobile cloud computing." Using the particle optimization techniques, we show that mobility can significantly increase the processing capacity of dynamic networks. In addition, we show that the dynamic structure of the network has a strong impact on its processing capacity

Improving Link Prediction in Intermittently Connected Wireless Networks by Considering Link and Proximity Stabilities

Several works have outlined the fact that the mobility in intermittently connected wireless networks is strongly governed by human behaviors as they are basically human-centered. It has been shown that the users' moves can be correlated and that the social ties shared by the users highly impact their mobility patterns and hence the network structure. Tracking these correlations and measuring the strength of social ties have led us to propose an efficient distributed tensor-based link prediction technique. In fact, we are convinced that the feedback provided by such a prediction mechanism can enhance communication protocols such as opportunistic routing protocols. In this paper, we aim to bring out that measuring the stabilities of the link and the proximity at two hops can improve the efficiency of the proposed link prediction technique. To quantify these two parameters, we propose an entropy estimator in order to measure the two stability aspects over successive time periods. Then, we join these entropy estimations to the tensor-based link prediction framework by designing new prediction metrics. To assess the contribution of these entropy estimations in the enhancement of tensor-based link prediction efficiency, we perform prediction on two real traces. Our simulation results show that by exploiting the information corresponding to the link stability and/or to the proximity stability, the performance of the tensor-based link prediction technique is improved. Moreover, the results attest that our proposal's ability to outperform other well-known prediction metrics.Comment: Published in the proceedings of the 13th IEEE International Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM), San Francisco, United States, 201

Towards efficacy and efficiency in sparse delay tolerant networks

The ubiquitous adoption of portable smart devices has enabled a new way of communication via Delay Tolerant Networks (DTNs), whereby messages are routed by the personal devices carried by ever-moving people. Although a DTN is a type of Mobile Ad Hoc Network (MANET), traditional MANET solutions are ill-equipped to accommodate message delivery in DTNs due to the dynamic and unpredictable nature of people\u27s movements and their spatio-temporal sparsity. More so, such DTNs are susceptible to catastrophic congestion and are inherently chaotic and arduous. This manuscript proposes approaches to handle message delivery in notably sparse DTNs. First, the ChitChat system [69] employs the social interests of individuals participating in a DTN to accurately model multi-hop relationships and to make opportunistic routing decisions for interest-annotated messages. Second, the ChitChat system is hybridized [70] to consider both social context and geographic information for learning the social semantics of locations so as to identify worthwhile routing opportunities to destinations and areas of interest. Network density analyses of five real-world datasets is conducted to identify sparse datasets on which to conduct simulations, finding that commonly-used datasets in past DTN research are notably dense and well connected, and suggests two rarely used datasets are appropriate for research into sparse DTNs. Finally, the Catora system is proposed to address congestive-driven degradation of service in DTNs by accomplishing two simultaneous tasks: (i) expedite the delivery of higher quality messages by uniquely ordering messages for transfer and delivery, and (ii) avoid congestion through strategic buffer management and message removal. Through dataset-driven simulations, these systems are found to outperform the state-of-the-art, with ChitChat facilitating delivery in sparse DTNs and Catora unencumbered by congestive conditions --Abstract, page iv

Geomorphic responses to natural and human disturbances in a mountain catchment at multiple temporal and spatial scales

Aquesta tesis es centra en l’estudi de la transferència d’aigua i sediments des de les àrees font de sediments fins les zones de sedimentació a múltiples escales temporals (des de dades amb una freqüència de cinc minuts fins informació de tot un segle) i espacials (des de vessants fins a conques), i les seves implicacions en la morfologia de la llera de la conca de l’Alt Cinca, una conca de muntanya localitzada al vessant sud dels Pirineus. A escala de micro-conca, les dades d’alta resolució obtingudes durant 5 anys en dos badlands contrastats (0.3 ha cadascun) revelen com la pluja controla els processos erosius associats a l’escolament superficial, m’entre que les baixes temperatures tenen una relació significativa amb els processos de moviments en massa. La morfometria d’aquestes superfícies, conjuntament amb la cobertura vegetal són factors clau que determinen els principals processos geomorfològics i els associats canvis topogràfics. Els principals processos observats han estat seqüències d’Erosió i Sedimentació (Cutting and Filling) i Moviments Gravitacionals (Mass Wasting). Tot i que els badlands tenen un paper important en la producció de sediments, el balanç de sediments de la conca del riu Soto (10 km2) indica que aquestes superfícies no sempre controlen l’exportació de sediments a la sortida de petites conques de muntanya amb caràcter intermitent. Aquest fet és degut a la fluctuació de la connectivitat funcional de la xarxa de drenatge causada per la freqüència i magnitud dels polsos d’aigua i sediments durant crescudes sobtades o flashy. La xarxa de drenatge actua com a font i zona d’emmagatzematge de sediments i es clau per entendre les marcades diferències que hi ha en la proporció del sediment que s’exporta en relació a la producció o també anomenat Sediment Delivery Ratio. Els canvis en els usos de sòl en moltes conques de muntanya constatats des de els anys 50 del segle XX tenen un efecte directe en la generació d’escolament (quantitat i magnitud) i en la producció de sediments. La majoria de la superfície de la conca de l’Alt Cinca (1565 km2) ha sofert aforestació, que ha resultat en una reducció de la connectivitat sedimentaria estructural. La construcció de terrasses afecta la connectivitat molt més que els canvis en la coberta del sòl. Les terrasses generalment redueixen la connectivitat degut a l’establiment de zones planes entre mig de zones en pendent. Contràriament, a una escala més local, les terrasses poden incrementar la connectivitat degut a la convergència de flux produïda per les pròpies estructures, o per la caiguda d’aquestes degut a l’abandonament. La construcció de carreteres modifica el pendent i la xarxa de drenatge, fet que comportarà canvis en la connectivitat estructural, que a la vegada poden afectar els processos erosius en les zones veïnes. Així, els fluxos d’aigua i sediments a la conca de l’Alt Cinca han estat àmpliament modificats durant l’últim segle, amb implicacions directes en la forma de la llera. A més a més, impactes locals derivats de les extraccions d’àrids, construcció d’esculleres i embasaments també han condicionat la dinàmica sedimentaria d’aquest riu, amb un impacte directe sobre la morfologia. Aquesta situació ha generat una metamorfosis de la morfologia del riu en la part baixa de l’Alt Cinca (12 km), canviant d’un patró trenat, molt dinàmic, a un patró més estable amb una tendència cap a canal únic. Els resultats obtinguts indiquen que el tram d’estudi està assolint un nou equilibri morfo-sedimentari imposat pels canvis en els fluxos d’aigua i sediments ocorreguts en el darrer segle, incloent les pertorbacions antròpiques que han modificat la geometria del canal i les característiques morfològiques de la llera. S’han observat un total de tres fases en la seva evolució: abans de l’any 1927, el tram d’estudi es trobava en una situació de quasi-equilibri majoritàriament condicionada per les riuades. Entre l’any 1927 i 2012 el riu s’ha ajustat a les diferents pertorbacions que ha tingut a múltiples escales temporals i espacials. Aquest ajust s’ha portat a terme mitjançant dos canvis contrastats en les característiques morfològiques. Finalment, després de l’any 2012, els resultats indiquen que el riu pot haver assolit de nou un nou equilibri, ajustant-se als fluxos d’aigua i sediment imposats i a la nova configuració del canal. Aquesta tesis presenta innovadors mètodes quantitatius per a l’estudi de la producció de sediments i la transferència entre els diferents compartiments de les conques fluvials. La principal novetat en la majoria dels capítols de la tesis recau en l’elevada resolució, de les dades obtingudes, tant temporal com espacial. Els resultats obtinguts en aquesta tesi permeten entendre millor el funcionament dels sistemes fluvials i la seva evolució, aspectes claus per donar suport en la millora i gestió de conques hidrogràfiques de muntanya.Esta tesis se centra en el estudio de la transferencia de agua y sedimentos desde las áreas fuente de sedimentos hasta las áreas de sedimentación a múltiples escales temporales (desde datos con una frecuencia de cinco minutos hasta información de un siglo) y espaciales (desde laderas hasta cuencas), y sus implicaciones en la morfología del cauce en la Cuenca del Alto Cinca (vertiente sur de los Pirineos). A escala de micro-cuenca, los datos de alta resolución obtenidos durante 5 años en dos badlands contrastados (0.3 ha cada uno) revelan como la lluvia controla los procesos erosivos asociados a la escorrentía superficial, mientras que las bajas temperaturas tienen una relación significativa con los procesos de movimientos en masa. La morfometría de estas superficies, conjuntamente con la cobertura vegetal, son factores clave que determinan los principales procesos geomorfológicos y los cambios topográficos asociados. Los principales procesos observados han sido secuencias de Erosión y Sedimentación (Cutting and Filling) y Movimientos Gravitacionales (Mass Wasting). Pese a que los badlands tienen un papel importante en la producción de sedimentos, el balance de sedimentos de la cuenca del rio Soto (10 km2) indica que estas superficies no siempre controlan la exportación de sedimentos a la salida de pequeñas cuencas de montaña con carácter intermitente. Esto principalmente se debe a la fluctuación de la conectividad funcional de la red de drenaje causada por la frecuencia y magnitud de los pulsos de agua y sedimentos durante las crecidas súbitas o flashy. La red de drenaje actúa como fuente y zona de almacenamiento de sedimentos, y es clave para entender las marcadas diferencias que hay en la proporción de sedimento que se exporta en relación a la producción o Sediment Delivery Ratio. Los cambios en los usos del suelo en muchas cuencas de montaña constatados desde los años 50 del siglo XX tienen un efecto directo en la generación de escorrentía (cantidad y magnitud) y en la producción de sedimentos. La mayoría de la superficie de la cuenca del Alto Cinca (1565 km2) ha sufrido forestación, que ha resultado en una reducción de la conectividad sedimentaria estructural. La construcción de terrazas afecta la conectividad mucho más que los cambios en la cobertura del suelo. A una escala más local, las terrazas pueden incrementar la conectividad debido a la convergencia de flujo producida por las propias estructuras, o por la caída de estas debido a su abandono. La construcción de carreteras modifica la pendiente y la red de drenaje, hecho que comporta cambios en la conectividad estructural, que a su vez pueden afectar a los procesos erosivos en las zonas vecinas. Así, los flujos de agua y sedimento en la cuenca del Alto Cinca han estado ampliamente modificados durante el último siglo, con implicaciones directas en la morfología del canal. Además, impactos locales derivados de las extracciones de áridos, construcción de escolleras y embalses también han condicionado la dinámica sedimentaria de este río, con un impacto directo sobre la morfología. Esta situación ha generado una metamorfosis de la morfología del río en la parte baja del Alto Cinca (12 km), cambiando de un patrón trenzado, muy dinámico, a un patrón más estable con una tendencia hacia el canal único. Los resultados obtenidos indican que el tramo de estudio está alcanzando un nuevo equilibrio morfo-sedimentario impuesto por los cambios en los flujos de agua y sedimentos ocurridos durante el último siglo, incluyendo las perturbaciones antrópicas que han modificado la geometría del canal y las características morfológicas del cauce. Se han observado un total de tres fases en su evolución: antes de 1927, el tramo de estudio se encontraba en una situación de casi-equilibrio, mayoritariamente controlada por las crecidas. Entre el año 1927 y 2012 el río se ha ajustado a las diferentes perturbaciones que ha tenido a múltiples escalas temporales y espaciales. Este ajuste se ha llevado a cabo mediante dos cambios contrastados en las características morfológicas. Finalmente, después del año 2012, los resultados indican que el río puede haber alcanzado un nuevo equilibrio, ajustándose a los flujos de agua y sedimento impuestos y la nueva configuración del canal. Esta tesis presenta innovadores métodos cuantitativos para el estudio de la producción de sedimentos y la transferencia entre los diferentes compartimentos de las cuencas fluviales. La principal novedad en la mayoría de los capítulos de la tesis recae en la elevada resolución de los datos obtenidos, tanto temporal como espacial. Los resultados obtenidos en esta tesis permiten entender mejor el funcionamiento de los sistemas fluviales y su evolución, aspectos clave para dar soporte y apoyo en la mejoría y gestión de cuencas hidrográficas de montaña.This thesis focusses on the study of water and sediment transfer from sources to sinks at multiple temporal (from 5-min data to a century data sets) and spatial (from slope to catchment scales) scales and their implications for channel morphology in the Upper Cinca catchment, a mountain catchment located in the Southern Pyrenees. At the micro-catchment scale, our 5-year High Resolution Data Set of two contrasted badlands (around 0.3 ha each) reveal as rainfall control overland-surface flow processes while low temperatures have a significant relation with mass movement-based processes. Morphometry together with vegetation cover are key factors determining main geomorphic processes and associated topographic changes. Main observed geomorphic processes were Cutting and Filling and Mass Wasting. Although badlands may have an important role on sediment production, the 2-year sediment budget of the Soto catchment (10 km2) indicates that badlands do not always control the export of sediments at the outlet of small intermittent mountain catchments. This is mainly due to the fluctuation of the functional connectivity of the channel network caused by the frequency and magnitude of water and sediment pulses during flashy floods. The channel drainage network acts as sediment source and sink and it is key to understand marked differences in the Sediment Delivery Ration. Land use and cover in many mountain catchments have been modified since the fifties of the 20th century, having a direct effect on runoff and sediment production. Most of the area of the Upper Cinca catchment (1565 km2) has undergone afforestation, which resulted in a decrease of structural sediment connectivity. Terracing affects connectivity much more than changes in land cover. Terraces generally reduce connectivity due to the establishment of flat areas between slopes and, contrarily, locally, may increase connectivity due the convergence produced by the structures or the collapse of terraces due to abandonment. Road construction, however, modify slope and the drainage network, which leads to changes in connectivity that could affect erosional processes in the neighbouring areas. Thus, water and sediment fluxes through the Upper Cinca are spatially and temporal dynamic and have been dramatically modified in the last century, with direct implications on channel morphology. Additionally, localised disturbances such as gravel mining, channel embankments and dams have also impacted on sedimentary dynamics, thus channel morphology. This situation led to a river metamorphosis, changing from a braided pattern to a more static channel towards a wandering pattern. We hypothesise that the lowermost 12-km reach of the Upper Cinca has reaching a new equilibrium imposed by catchment-scale changes of water and sediment fluxes caused by global changes, but also influenced by localised human-disturbances that modify channel geometry and morpho-sedimentary characteristics. Three phases were identified: before 1927, the reach remained in a quasi-equilibrium state imposed, mainly, by water and sediment supply during flood events. During the period 1927-2012 the river adjusted to the disequilibrium imposed by disturbances acting at different temporal and spatial scales, yielding two contrasted channel states. Finally, after 2012, we hypothesise that the river may be reaching again a new equilibrium, adjusting to the imposed water and sediment fluxes and the new channel configuration. This thesis presents some novel quantitative methods for the study of sediment production and transfer between the different compartments of fluvial catchments. The main transversal novelty in all the methods used in each chapter lies in the high resolution of the data obtained. This comprehensive analysis aids at understanding the functioning of the river system and their evolution based on multiple-scale disturbances, which can help to support integrated watershed management practices or plans