How Can Optical Communications Shape the Future of Deep Space Communications? A Survey

With a large number of deep space (DS) missions anticipated by the end of this decade, reliable and high capacity DS communications systems are needed more than ever. Nevertheless, existing DS communications technologies are far from meeting such a goal. Improving current DS communications systems does not only require system engineering leadership but also, very crucially, an investigation of potential emerging technologies that overcome the unique challenges of ultra-long DS communications links. To the best of our knowledge, there has not been any comprehensive surveys of DS communications technologies over the last decade. Free space optical (FSO) technology is an emerging DS technology, proven to acquire lower communications systems size, weight, and power (SWaP) and achieve a very high capacity compared to its counterpart radio frequency (RF) technology, the current used DS technology. In this survey, we discuss the pros and cons of deep space optical communications (DSOC). Furthermore, we review the modulation, coding, and detection, receiver, and protocols schemes and technologies for DSOC. We provide, for the very first time, thoughtful discussions about implementing orbital angular momentum (OAM) and quantum communications (QC) for DS. We elaborate on how these technologies among other field advances, including interplanetary network, and RF/FSO systems improve reliability, capacity, and security and address related implementation challenges and potential solutions. This paper provides a holistic survey in DSOC technologies gathering 200+ fragmented literature and including novel perspectives aiming to setting the stage for more developments in the field.Comment: 17 pages, 8 Figure

Transmissão oportunística de informação em redes veiculares

Mestrado em Engenharia Eletrónica e TelecomunicaçõesThe area of wireless communications has been the subject of several research projects over the last years. The persistent need to ”communicate” in various environments, including high mobility, make the use of wirelessbased communications a strong requirement. In this context, VANETs were created, which are networks based on Ad Hoc concept allowing the communication between vehicles and between vehicles and fixed infrastructures, that increase network’s connectivity. VANETs, due to their characteristics, introduce challenges such as shorts connectivity intervals in sparse networks, and also in situations where connectivity can be down for long periods of time. The work of this Dissertation aims to send non-urgent information in a opportunistic way, maximizing the network resources. The DTN’s concept is thus addressed as a solution to the previous described challenges. Two DTN implementations are studied and tested to be incorporated in WAVE devices communicating using the standard IEEE 802.11p for vehicular networks. After several tests, IBR-DTN proved to be the most robust and ”light” implementation to be used in embedded systems, such as the OBUs used in VANETs. Several implementation problems were detected, through several tests, and corrected to be possible to provide the functional integration of IBR-DTN in a real vehicular environment. The set of tests consisted in: two scenarios in the laboratory environment, to better understand IBR-DTN’s operation; and three scenarios in a real testbed with vehicles and fixed stations. The two scenarios tested in laboratory allowed to conclude the good performance of fragmentation process for different connection time intervals and different file sizes, where the connections between the nodes were periodically interrupted. The scenarios performed on the real testbed show that IBR-DTN operates without problems for various velocities using fixed infrastructures and a car, and two cars moving towards each other in different ways. It can be also concluded that, increasing the velocity, the contact time between nodes decreases, contributing to a larger number of fragments needed to send a specific file. The results show also that IBR-DTN has a good response in high mobility environments like VANETs.A área de comunicações sem fios tem sido alvo de vários projectos de investigação ao longo dos últimos anos. A constante necessidade de ”comunicar” nos mais diversos ambientes, incluindo os de alta mobilidade, requerem comunicações sem fios. Neste contexto foram criadas as Vehicular Ad hoc Networks (VANETs), que são redes baseadas no conceito Ad Hoc, permitindo a comunicação entre veículos e entre veículos e infraestruturas fixas, que aumentam a conectividade da rede. As VANETs, devido às suas carecterísticas, apresentam desafios, tais como intervalos curtos de conectividade em cen´arios onde a densidade de nós é reduzida, e também situações onde não existe comunicação durante longos períodos de tempo. O trabalho da presente dissertação tem como objectivo possibilitar o envio de informação não urgente de forma oportunística, rentabilizando todos os recursos da rede. O conceito de Delay/Disrupt Tolerant Network (DTN) é deste modo abordado como solução para os desafios descritos anteriormente. Desta forma, duas implementações de DTN são estudadas e testadas para serem incorporadas em dispositivos Wireless Access in Vehicular Environments (WAVE), comunicando através da norma IEEE 802.11p para redes veiculares. Depois de vários testes realizados, o IBR-DTN mostrou ser a implementação mais robusta e mais ”leve” para ser utilizada em sistemas embutidos, como é o caso das On-Board Units (OBUs) utilizadas nas VANETs. Vários problemas de implementação foram detectados e corrigidos para ser possível integrar o IBR-DTN de forma funcional num ambiente veicular real. O conjunto de testes realizados consistiu em: dois cenários em laboratório para melhor perceber o funcionamento do IBR-DTN; e três cenários numa testbed real com veículos e estações fixas. Os dois cenários testados em laboratório permitem concluir o bom funcionamento do processo de fragmentação para diferentes tempos de ligação e diferentes tamanhos de ficheiros, onde as ligações entre os nós eram interrompidas periodicamente. Os cenários testados na testbed real mostram que o IBR-DTN funciona sem problemas usando várias velocidades com estações fixas e um carro, e com dois carros dirigindo-se em sentido contrário um ao outro. Permitem também concluir que, aumentando a velocidade, diminui o tempo de contacto entre os nós traduzindo-se num maior número de fragmentos para enviar um determinado ficheiro. Os resultados mostram também que o IBR-DTN tem uma boa resposta em ambientes de alta mobilidade como as VANETs

Ereignisbasierte Software-Architektur für Verzögerungs- und Unterbrechungstolerante Netze

Continuous end-to-end connectivity is not available all the time, not even in wired networks. Delay- and Disruption-Tolerant Networking (DTN) allows devices to communicate even if there is no continuous path to the destination by replacing the end-to-end semantics with a hop-by-hop store-carry-and-forward approach. Since existing implementations of DTN software suffer from various limitations, this work presents the event-driven software architecture of IBR-DTN, a lean, lightweight, and extensible implementation of a networking stack for Delay- and Disruption-Tolerant Networking. In a comprehensive description of the architecture and the underlying design decisions, this work focuses on eliminating weaknesses of the Bundle Protocol (RFC 5050). One of these is the dependency on synchronized clocks. Thus, this work takes a closer look on that requirement and presents approaches to bypass that dependency for some cases. For scenarios which require synchronized clocks, an approach is presented to distribute time information which is used to adjust the individual clock of nodes. To compare the accuracy of time information provided by each node, this approach introduces a clock rating. Additionally, a self-aligning algorithm is used to automatically adjust the node's clock rating parameters according to the estimated accuracy of the node's clock. In an evaluation, the general portability of the bundle node software is proven by porting it to various systems. Further, a performance analysis compares the new implementation with existing software. To perform an evaluation of the time-synchronization algorithm, the ONE simulator is modified to provide individual clocks with randomized clock errors for every node. Additionally, a specialized testbed, called Hydra, is being developed to test the implementation of the time-synchronization approach in real software. Hydra instantiates virtualized nodes running a complete operating system and provides a way to test real software in large DTN scenarios. Both the simulation and the emulation in Hydra show that the algorithm for time-synchronization can provide an adequate accuracy depending on the inter-contact times.Eine kontinuierliche Ende-zu-Ende-Konnektivität ist nicht immer verfügbar, nicht einmal in drahtgebundenen Netzen. Verzögerungs- und unterbrechungstolerante Kommunikation (DTN) ersetzt die Ende-zu-Ende-Semantik mit einem Hop-by-Hop Store-Carry-and-Forward Ansatz und erlaubt es so Geräten miteinander zu kommunizieren, auch wenn es keinen kontinuierlichen Pfad gibt. Da bestehende DTN Implementierungen unter verschiedenen Einschränkungen leiden, stellt diese Arbeit die ereignisgesteuerte Software-Architektur von IBR-DTN, eine schlanke, leichte und erweiterbare Implementierung eines Netzwerk-Stacks für Verzögerungs- und unterbrechungstolerante Netze vor. In einer umfassenden Beschreibung der Architektur und den zugrunde liegenden Design-Entscheidungen, konzentriert sich diese Arbeit auf die Beseitigung von Schwächen des Bundle Protocols (RFC 5050). Eine davon ist die Abhängigkeit zu synchronisierten Uhren. Daher wirft diese Arbeit einen genaueren Blick auf diese Anforderung und präsentiert Ansätze, um diese Abhängigkeit in einigen Fällen zu umgehen. Für Szenarien die synchronisierte Uhren voraussetzen wird außerdem ein Ansatz vorgestellt, um die Uhren der einzelnen Knoten mit Hilfe von verteilten Zeitinformationen zu korrigieren. Um die Genauigkeit der Zeitinformationen von jedem Knoten vergleichen zu können, wird eine Bewertung der Uhren eingeführt. Zusätzlich wird ein Algorithmus vorgestellt, der die Parameter der Bewertung in Abhängigkeit von der ermittelten Genauigkeit der lokalen Uhr anpasst. In einer Evaluation wird die allgemeine Portabilität der Software zu verschiedenen Systemen gezeigt. Ferner wird bei einer Performance-Analyse die neue Software mit existierenden Implementierungen verglichen. Um eine Evaluation des Zeitsynchronisationsalgorithmus durchzuführen, wird der ONE Simlator so angepasst, dass jeder Knoten eine individuelle Uhr mit zufälligem Fehler besitzt. Außerdem wird eine spezielle Testumgebung namens Hydra vorgestellt um eine echte Implementierung des Zeitsynchronisationsalgorithmus zu testen. Hydra instanziiert virtualisierte Knoten mit einem kompletten Betriebssystem und bietet die Möglichkeit echte Software in großen DTN Szenarien zu testen. Sowohl die Simulation als auch die Emulation in Hydra zeigen, dass der Algorithmus für die Zeitsynchronisation eine ausreichende Genauigkeit in Abhängigkeit von Kontakthäufigkeit erreicht

Transmissão oportunística de informação em redes veiculares

Mestrado em Engenharia Eletrónica e TelecomunicaçõesThe development in telecommunications and particularly in wireless communications has been one of the most striking features of the contemporary world. The globalization only has been possible thanks to the evolution of communication technologies which increasingly have allowed to satisfy the constant people's needs of being "always connected" whatever the environment where they are. Concerning the evolution of technologies, vehicular networks have been one of the areas of great interest. This interest has been manifested both in research and in the development of the automotive industry that has produced innovative vehicles which are more and more equipped with new technologies. It is expected that communication in vehicular networks enable not only the communication between vehicles, but also a more comfortable and safe driving, making the user's experience of this type of networks richer and stimulating. The specific characteristics of vehicular networks, namely the high mobility, unpredictable routes, dynamic topology and the consequent and constant loss of connectivity, have been a challenge that has motivated studies to find solutions to these limitations. The work carried out for this dissertation is in the area of Vehicular Ad-hoc Networks (VANETs) and it is based on the Delay and Disruption Tolerant Networks (DTNs). With this project, identified as "Opportunistic Transmission of Information in Vehicular Networks", we aim to study the communication and transmission of information in these networks which do not allow communication without delays and disruptions. For this purpose it is studied the performance of DTN mechanisms in these networks. In this work it is used the implementation IBR-DTN to test DTN in VANETs. This implementation showed, in previous works, to be the one that presents the best performance comparing it with other existing implementations. The study involved, in an initial phase, reading and analyzing the implementation code so that it was possible to add instructions that allowed to observe the behavior of the implementation in the several tests carried out, as well as the correction of the bugs in the implementation. In the first phase, in laboratory, with fixed nodes and in a controlled environment, several scenarios were created to simulate the possible situations a node can meet: direct transfer with and without delay, indirect transfer (multi-hop) and indirect transfer with delay which corresponds to the store and transport of the bundles (set of information) until the next node. From the analysis of the collected information and observing the corresponding graphs, it was possible to observe that the implementation was working properly in the vehicles equipment for communication. Still in laboratory it was built an heterogeneous network with several devices (servers, NetRiders, Single Board Computers (SBCs), tablet, Raspberry Pi e Macbook) to show the integration of the IBR-DTN implementation and its extension in different equipments. During this test several files were sent among these devices, which were correctly received in the nodes previously defined as destination nodes. After testing and checking that everything was working properly in the laboratory, the same implementation was transferred to a testbed with 25 vehicles and 3 fixed infrastructures in Leixões harbor. In this testbed several DTN routing protocols were tested in order to check which of them showed better performance in the delivery rate of the bundles and of the collected information (the log files were also delivered via DTN) from the On-Board Units (OBUs) to the server, located in the Internet. The routing protocol with static routes to the Road Side Units (RSUs) proved a better efficiency compared to the other protocols. This was due to the the fact that this network is well covered with RSUs, and there is no relation between the historic of contacts and the probability that the vehicles will meet again in the future.O desenvolvimento na área das telecomunicações e, mais particularmente, nas comunicações sem-fios tem sido um dos traços mais marcantes do mundo actual. A globalização só tem sido possível graças à evolução dos meios de comunicação que cada vez mais permitem satisfazer a constante necessidade das pessoas estarem sempre ligadas, qualquer que seja o ambiente em que se encontrem. As redes veiculares têm sido uma das áreas de elevado interesse na evolução das tecnologias. Esse interesse tem-se manifestado tanto ao nível da investigação como ao nível do desenvolvimento da indústria automóvel que tem produzido veículos cada vez mais equipados com novas tecnologias. Prevê-se que a comunicação em redes veiculares permitam não só a comunicação entre os veículos, mas também uma condução mais confortável e segura, tornando a experiência dos utilizadores deste tipo de redes mais rica e estimulante. As características específicas das redes veiculares, nomeadamente a elevada mobilidade, rotas imprevisíveis, topologia dinâmica e a consequente e constante perda de conectividade, tornam-se um desafio que tem motivado estudos no sentido de se encontrarem soluções para essas limitações. O trabalho desenvolvido para esta dissertação insere-se na área das Vehicular Ad-hoc NETworks (VANETs) e baseia-se nas Delay and Disruption Tolerant Networks (DTNs). Com este projecto, identificado como "Transmissão Oportunística de Informação em Redes Veiculares", pretende-se estudar a comunicação e envio de informação nas redes que permitem uma comunicação com atrasos e disrupções. Para o efeito é estudado o desempenho de mecanismos de DTN nestas redes. Neste trabalho é utilizada a implementação IBR-DTN para testar DTN nas redes veiculares. Esta implementação mostrou, em trabalhos anteriores, ser aquela que apresenta melhor desempenho face a outras que existem. O estudo envolveu, numa fase inicial, a leitura e analise de código da implementação para que fosse possível adicionar instruções que permitissem observar o comportamento da implementação nos diversos testes realizados, bem como a correcção de erros da implementação. Na primeira fase, em laboratório, com nos fixos e num ambiente controlado, foram realizados vários cenários que mostram as situações possíveis que um nó pode encontrar: transferência direta com e sem atraso, transferência indirecta (multi-hop) e transferência indirecta com atraso que corresponde ao armazenamento e transporte dos bundles (conjunto de informação) até ao próximo nó. A partir da analise da informação recolhida e observação dos gráficos obtidos foi possível verificar o correcto funcionamento da implementação nos equipamentos de comunicação entre veículos. Ainda em laboratório foi construída uma rede heterogénea com diversos dispositivos (servidores, NetRiders, Single Board Computers (SBCs), tablet, Raspberry Pi e Macbook) com o objectivo de mostrar a integração da implementação IBR-DTN e as suas extensões em diferentes equipamentos. Neste teste foram enviados ficheiros entre estes dispositivos, os quais foram recebidos correctamente nos nos definidos como destino. Depois de testar e certificar que tudo funcionava em laboratório, a mesma implementação foi transferida para uma testbed com 25 veículos e 3 infraestruturas fixas, no porto de Leixões. Nesta testbed foram testados diversos protocolos de encaminhamento DTN de forma a verificar qual apresentava melhor desempenho na taxa de entrega dos bundles e da informação recolhida (os ficheiros de log foram também entregues através de DTN) das On-Board Units (OBUs) para o servidor, localizado na Internet. O protocolo com rotas estáticas para as Road Side Units (RSUs) demonstrou uma melhor eficiência em relação aos restantes devido ao facto de esta rede estar bem coberta e de não existir uma relação entre o histórico de contactos e a probabilidade de os veículos se encontrarem novamente

Delay tolerant network for Navy scenarios: quality-based approach

Mestrado em Engenharia Eletrónica e TelecomunicaçõesThe navy operations involve several participants that work between them with common objectives and usually under challenged communication conditions. There are natural constrains that are imposed by the operation environment, e.g. hilly terrains. There are also artificial constrains that are created by enemy elements which force conditions to affect the navy operation (or other military forces), e.g. intentional jamming. The military often uses proprietary devices to communicate between them. Despite of the effectiveness of these devices, they are expensive and usually offer a limited range of services. However, the recent technological advances allow the proliferation of several mobile devices with wireless communication capabilities and with the value to easily insert new features, but these devices are still not prepared to military networks in terms of communication. Thus, this dissertation proposes to use Delay Tolerant Networks (DTNs) with a new routing protocol Quality-PRoPHET (Q-PRoPHET) able to measure the quality of the wireless links and route the information using the connections with best quality, where the probability of transmission is higher. The Q-PRoPHET uses a quality function to evaluate the quality of the connections and a transitive property to route through multiple hops. This algorithm was implemented in IBR-DTN and it was evaluated in three scenarios that emulate three scenarios observed during the navy tactical operations. Two of these scenarios were tested inside a building and the last one was tested in an external environment using real mobility of the nodes. The obtained results show that Q-PRoPHET has better performance than PRoPHET in terms of delivery ratio, endto-end delay and packets transmission, which are critical parameters for the communication in navy operations.As operações da marinha envolvem vários intervenientes que trabalham entre si com objetivos comuns e frequentemente sob condições de comunicação desafiadoras. Existem constrangimentos naturais que são impostos pelo ambiente da operação, por exemplo, geografia acidentada do terreno. Existem também constrangimentos artificiais que são criados por elementos hostis que forçam condições de modo a prejudicar as operações da marinha (ou outras equipas militares), por exemplo, criação de interferência intencional. Os militares geralmente usam equipamentos de comunicação proprietários para comunicar entre si. Apesar da eficácia destes equipamentos, eles são caros e normalmente oferecem uma gama de serviços limitada. Contudo, os recentes avanços tecnológicos permitiram a proliferação de muitos dispositivos portáteis com capacidade de comunicação sem fios e com o valor de acrescentar novas funcionalidades de formas muito simples, mas estes dispositivos ainda não estão adaptados para as redes militares em termos de comunicação. Esta dissertação propõe usar Redes Tolerantes a Atrasos (DTNs) com um novo protocolo de encaminhamento QualityPRoPHET (Q-PRoPHET) capaz de medir a qualidade das ligações sem-fios e encaminhar a informação pelas ligações de melhor qualidade, onde a probabilidade de sucesso da transmissão é maior. O Q-PRoPHET usa uma função de qualidade para avaliar a qualidade das ligações e uma propriedade transitiva para encaminhamento a múltiplos saltos. Este algoritmo foi implementado no IBR-DTN e foi avaliado em três cenários que emulam três cenários observados durante operações táticas da Marinha. Dois destes cenários foram testados dentro de um edifício e o último foi testado em ambiente exterior, recorrendo a mobilidade real dos nós. Os resultados obtidos mostram que o Q-PRoPHET tem melhor desempenho que o PRoPHET em termos de taxa de entrega, tempo de entrega e transmissão de pacotes, que são parâmetros críticos para as comunicações das operações da marinha

Application of Machine Learning Techniques to Delay Tolerant Network Routing

This dissertation discusses several machine learning techniques to improve routing in delay tolerant networks (DTNs). These are networks in which there may be long one-way trip times, asymmetric links, high error rates, and deterministic as well as non-deterministic loss of contact between network nodes, such as interplanetary satellite networks, mobile ad hoc networks and wireless sensor networks. This work uses historical network statistics to train a multi-label classifier to predict reliable paths through the network. In addition, a clustering technique is used to predict future mobile node locations. Both of these techniques are used to reduce the consumption of resources such as network bandwidth, memory and data storage that is required by replication routing methods often used in opportunistic DTN environments. Thesis contributions include: an emulation tool chain developed to create a DTN test bed for machine learning, the network and software architecture for a machine learning based routing method, the development and implementation of classification and clustering techniques and performance evaluation in terms of machine learning and routing metrics

Rede multi-tecnologia para recolha de dados ambientais através de comunicações oportunistas

mestrado em Engenharia Eletrónica e TelecomunicaçõesO conceito de Smart City surge da combinação do paradigma de Internet of Things (IoT) sobre contextos urbanos aliado à exploração de soluções de Tecnologias de Informação e Comunicação (TIC). O típico cenário de Smart City tem de lidar com desafios, tais como as elevadas quantidades de sensores e geradores de dados, dos quais alguns são colocados em dispositivos de grande mobilidade, visando a recolha e geração de todo o tipo de informações e levando ao aumento do número de dispositivos comunicantes. Esta dissertação foca o desenvolvimento e implementação de uma plataforma heterogénea de sonorização ambiental com o objectivo de servir de infraestrutura para aplicações no âmbito das Smart Cities. Esta pretende tirar proveito da utilização de múltiplas tecnologias de comunicação, nomeadamente tecnologias de longo e curto alcance. Para al em disto, visto que a plataforma visa ambientes urbanos, esta tira proveito de uma rede oportunista e tolerante a atrasos, Delay Tolerant Network (DTN), através de entidades m oveis que circulam pela cidade, nomeadamente bicicletas. Assim sendo, esta dissertação propõe: (1) o desenho e desenvolvimento da rede e dos seus constituintes; (2) uma extensão a um protocolo de controlo de acesso ao meio, Medium Access Control (MAC), para a tecnologia LoRa com o objectivo de o dotar compatível para ambientes de gateways múltiplas; (3) novas estratégias de encaminhamento para a rede tolerante a atrasos, tendo em consideração a topologia e as características apresentadas por esta. As avaliações realizadas permitiram concluir que o protocolo MAC para LoRa em ambientes de gateways múltiplas proposto contribui para um aumento da escalabilidade da rede, bem como para uma melhoria do seu desempenho. Relativamente às estratégias de encaminhamento propostas para a DTN, os testes realizados permitiram avaliar o impacto que cada estratégia tem sobre o comportamento da rede, nomeadamente a taxa de entrega dos pacotes de dados, a sobrecarga da rede, o número de pacotes transmitidos, entre outros. Com estes resultados foi possível perceber as in- suficiências que as funcionalidades propostas têm sobre a solução geral, e identificar as caraterísticas necessárias de uma solução escalável para a recolha de dados massivos num ambiente de IoT.The Smart City concept is the combination of the Internet of Things (IoT) paradigm under an urban context with the exploitation of Information and Communication Technologies (ICT) solutions. The typical Smart City scenario has to deal with an extensive amount of sensors and data generators, some of them placed in high mobile devices, deployed to collect and generate all type of information which will increase the number of communicating machines. This dissertation focuses on the development and implementation of a heterogeneous environmental sensing platform to serve as an infrastructure for Smart City applications. It aims to take advantage of the use of multiple communication technologies, namely long and short range. Being within an urban environment, the platform bene ts from an opportunistic and Delay Tolerant Network (DTN) through mobile entities that travel over the city, such as bicycles. Therefore, this dissertation proposes: (1) the design and development of the network and its elements; (2) an extension to a LoRa Medium Access Control (MAC) protocol in order to endow it with capabilities to operate in multi-gateway environments; and lastly, (3) new forwarding strategies for the opportunistic network that takes into consideration the network topology. The performed evaluations showed that the proposed multi-gateway LoRa MAC protocol contributes to increase the LoRa network scalability, as well as its performance. The performed tests to the proposed DTN forwarding strategies evaluate the impact of each strategy on the network behavior, namely the delivery ratio, network overhead, number of transmitted packets, among others. As a result, it is possible to perceive which are the in- uences introduced by the proposed functionalities on the overall solution, and identify the characteristics of a scalable solution to collect massive data in an IoT environment

Distribuição de conteúdos em redes veiculares usando mecanismos de comunicação tolerantes ao atraso

Mestrado em Engenharia Eletrónica e TelecomunicaçõesThe last couple of decades have been the stage for the introduction of new telecommunication networks. It is expected that in the future all types of vehicles, such as cars, buses and trucks have the ability to intercommunicate and form a vehicular network. Vehicular networks display particularities when compared to other networks due to their continuous node mobility and their wide geographical dispersion, leading to a permanent network fragmentation. Therefore, the main challenges that this type of network entails relate to the intermittent connectivity and the long and variable delay in information delivery. To address the problems related to the intermittent connectivity, a new concept was introduced – Delay Tolerant Network (DTN). This architecture is built on a Store-Carry-and-Forward (SCF) mechanism in order to assure the delivery of information when there is no end-to-end path defined. Vehicular networks support a multiplicity of services, including the transportation of non-urgent information. Therefore, it is possible to conclude that the use of a DTN for the dissemination of non-urgent information is able to surpass the aforementioned challenges. The work developed focused on the use of DTNs for the dissemination of non-urgent information. This information is originated in the network service provider and should be available on mobile network terminals during a limited period of time. In order to do so, four different strategies were deployed: Random, Least Number of Hops First (LNHF), Local Rarest Bundle First (LRBF) e Local Rarest Generation First (LRGF). All of these strategies have a common goal: to disseminate content into the network in the shortest period of time and minimizing network congestion. This work also contemplates the analysis and implementation of techniques that reduce network congestion. The design, implementation and validation of the proposed strategies was divided into three stages. The first stage focused on creating a Matlab emulator for the fast implementation and strategy validation. This stage resulted in the four strategies that were afterwards implemented in the DTNs software Helix – developed in a partnership between Instituto de Telecomunicac¸˜oes (IT) and Veniam R , which are responsible for the largest operating vehicular network worldwide that is located in Oporto city. The strategies were later evaluated on an emulator that was built for the largescale testing of DTN. Both emulators account for vehicular mobility based on information previously collected from the real platform. Finally, the strategy that presented the best overall performance was tested on a real platform – in a lab environment – for concept and operability demonstration. It is possible to conclude that two of the implemented strategies (LRBF and LRGF) can be deployed in the real network and guarantee a significant delivery rate. The LRBF strategy has the best performance in terms of delivery. However, it needs to add a significant overhead to the network in order to work. In the future, tests of scalability should be conducted in a real environment in order to confirm the emulator results. The real implementation of the strategies should be accompanied by the introduction of new types of services for content distribution.Nas últimas décadas tem-se assistido à introdução de novas redes de telecomunicações. Entre estas destacam-se as redes veiculares constituídas por todo o tipo de veículos com capacidades de intercomunicação. As redes veiculares têm especificidades singulares face a outro tipo de redes devido à constante mobilidade dos nós e à sua elevada dispersão geográfica. Os principais desafios introduzidos por este tipo de redes prendem-se com a conectividade intermitente e o atraso longo e variado na entrega da informação. Por forma a fazer face aos problemas relacionados com a conectividade intermitente, introduziu-se um novo conceito intitulado de Delay Tolerant Network (DTN). Esta arquitetura assenta num mecanismo de StoreCarry-and-Forward (SCF) por forma a garantir a entrega de informação em situações onde não existe um caminho estabelecido fim-a-fim. As redes veiculares suportam uma multiplicidade de serviços, nos quais se inclui o transporte de informação não-urgente. Desta forma, a utilização de uma DTN para a difusão de informação não-urgente permite ultrapassar os desafios identificados anteriormente. O trabalho realizado foca-se na utilização de DTNs para a disseminação de informação não-urgente. Por forma a operacionalizar esta premissa foram implementadas quatro estratégias distintas: Random, Least Number of Hops First (LNHF), Local Rarest Bundle First (LRBF) e Local Rarest Generation First (LRGF). Todas estas estratégias tem um objetivo comum: disseminar um conteúdo na rede no menor tempo possível minimizando ao máximo o congestionamento da rede. Foram também implementadas e estudadas técnicas para minimizar o congestionamento do meio. A metodologia de desenho, implementação e validação das estratégias propostas foi desenvolvida em três fases. A primeira focou-se na criação de um emulador Matlab para a implementação rápida e validação das estratégias. Dessa primeira fase resultaram quatro estratégias que foram posteriormente implementadas no software de DTNs Helix desenvolvido através de uma parceria entre o Instituto de Telecomunicações (IT) e a Veniam R (responsáveis pela maior rede veicular em operação a nível mundial localizada na cidade do Porto). As estratégias foram depois avaliadas num emulador construído para fazer testes de grande escala. Ambos os emuladores introduzem a mobilidade dos veículos com base em informação recolhida previamente da plataforma real. Por fim a estratégia que apresentou o melhor desempenho foi introduzida e testada numa plataforma real para demonstração de conceito e operacionalidade. Conclui-se que duas das estratégias implementadas (LRBF and LRGF) são passíveis de utilização na rede real garantido uma taxa de entrega significativa. A estratégia LRBF apresentou o melhor desempenho em termos de entrega, no entanto, necessita de adicionar um overhead considerável na rede para funcionar. No futuro devem ser realizados testes de escalabilidade em ambiente real por forma a confirmar os resultados obtidos em ambiente de emulação e real em pequena escala. A implementação real das estratégias deve ser acompanhada pela introdução de novos tipos de serviços para distribuição de conteúdos

Marshall Space Flight Center Research and Technology Report 2018

Many of NASAs missions would not be possible if it were not for the investments made in research advancements and technology development efforts. The technologies developed at Marshall Space Flight Center contribute to NASAs strategic array of missions through technology development and accomplishments. The scientists, researchers, and technologists of Marshall Space Flight Center who are working these enabling technology efforts are facilitating NASAs ability to fulfill the ambitious goals of innovation, exploration, and discovery

Information-centric communication in mobile and wireless networks

Information-centric networking (ICN) is a new communication paradigm that has been proposed to cope with drawbacks of host-based communication protocols, namely scalability and security. In this thesis, we base our work on Named Data Networking (NDN), which is a popular ICN architecture, and investigate NDN in the context of wireless and mobile ad hoc networks. In a first part, we focus on NDN efficiency (and potential improvements) in wireless environments by investigating NDN in wireless one-hop communication, i.e., without any routing protocols. A basic requirement to initiate informationcentric communication is the knowledge of existing and available content names. Therefore, we develop three opportunistic content discovery algorithms and evaluate them in diverse scenarios for different node densities and content distributions. After content names are known, requesters can retrieve content opportunistically from any neighbor node that provides the content. However, in case of short contact times to content sources, content retrieval may be disrupted. Therefore, we develop a requester application that keeps meta information of disrupted content retrievals and enables resume operations when a new content source has been found. Besides message efficiency, we also evaluate power consumption of information-centric broadcast and unicast communication. Based on our findings, we develop two mechanisms to increase efficiency of information-centric wireless one-hop communication. The first approach called Dynamic Unicast (DU) avoids broadcast communication whenever possible since broadcast transmissions result in more duplicate Data transmissions, lower data rates and higher energy consumption on mobile nodes, which are not interested in overheard Data, compared to unicast communication. Hence, DU uses broadcast communication only until a content source has been found and then retrieves content directly via unicast from the same source. The second approach called RC-NDN targets efficiency of wireless broadcast communication by reducing the number of duplicate Data transmissions. In particular, RC-NDN is a Data encoding scheme for content sources that increases diversity in wireless broadcast transmissions such that multiple concurrent requesters can profit from each others’ (overheard) message transmissions. If requesters and content sources are not in one-hop distance to each other, requests need to be forwarded via multi-hop routing. Therefore, in a second part of this thesis, we investigate information-centric wireless multi-hop communication. First, we consider multi-hop broadcast communication in the context of rather static community networks. We introduce the concept of preferred forwarders, which relay Interest messages slightly faster than non-preferred forwarders to reduce redundant duplicate message transmissions. While this approach works well in static networks, the performance may degrade in mobile networks if preferred forwarders may regularly move away. Thus, to enable routing in mobile ad hoc networks, we extend DU for multi-hop communication. Compared to one-hop communication, multi-hop DU requires efficient path update mechanisms (since multi-hop paths may expire quickly) and new forwarding strategies to maintain NDN benefits (request aggregation and caching) such that only a few messages need to be transmitted over the entire end-to-end path even in case of multiple concurrent requesters. To perform quick retransmission in case of collisions or other transmission errors, we implement and evaluate retransmission timers from related work and compare them to CCNTimer, which is a new algorithm that enables shorter content retrieval times in information-centric wireless multi-hop communication. Yet, in case of intermittent connectivity between requesters and content sources, multi-hop routing protocols may not work because they require continuous end-to-end paths. Therefore, we present agent-based content retrieval (ACR) for delay-tolerant networks. In ACR, requester nodes can delegate content retrieval to mobile agent nodes, which move closer to content sources, can retrieve content and return it to requesters. Thus, ACR exploits the mobility of agent nodes to retrieve content from remote locations. To enable delay-tolerant communication via agents, retrieved content needs to be stored persistently such that requesters can verify its authenticity via original publisher signatures. To achieve this, we develop a persistent caching concept that maintains received popular content in repositories and deletes unpopular content if free space is required. Since our persistent caching concept can complement regular short-term caching in the content store, it can also be used for network caching to store popular delay-tolerant content at edge routers (to reduce network traffic and improve network performance) while real-time traffic can still be maintained and served from the content store