We present an algorithm to store data robustly in a large, geographically distributed network by means of localized regions of data storage that move in response to changing conditions. For example, data might migrate away from failures or toward regions of high demand. The PersistentNode algorithm provides this service robustly, but with limited safety guarantees. We use the RAMBO framework to transform PersistentNode into RamboNode, an algorithm that guarantees atomic consistency in exchange for increased cost and decreased liveness. In addition, a half-life analysis of RamboNode shows that it is robust against continuous low-rate failures. Finally, we provide experimental simulations for the algorithm on 2000 nodes, demonstrating how it services requests and examining how it responds to failures.

Thesis (MScIng)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008. / M.Phil. in Electronic Technolgoy. Includes bibliographical references (p. 53-54).

Στη διπλωματική εργασία μελετάμε τη δυνατότητα συν-λειτουργίας των HAPs και των ad-hoc δικτύων. Μέσα από τη θεωρητική μελέτη και τις προσομοιώσεις προσπαθούμε να μελετήσουμε τις επιδόσεις ενός τέτοιου υβριδικού συστήματος, όσον αφορά τη δρομολόγηση και την αξιοπιστία στη μετάδοση της πληροφορίας. Αρχικά, προχωράμε σε μια θεωρητική μελέτη των στρατοσφαιρικών πλατφόρμων (HAPs) και των ad-hoc δικτύων. Αναφέρουμε δυνατότητες, τεχνολογίες και εφαρμογές των δύο αυτών τηλεπικοινωνιακών τύπων τεχνολογίας. Στη συνέχεια, παρουσιάζουμε το τηλεπικοινωνιακό σενάριο πάνω στο οποίο στηρίχθηκε η προσομοίωση και, τέλος, παρουσιάζουμε αναλυτικά τα αποτελέσματα που προέκυψαν από την πειραματική διαδικασία με μορφή πινάκων και διαγραμμάτων καθώς και τα συμπεράσματα στα οποία καταλήγουμε βάσει των μετρήσεών μας. / --

Vehicular ad hoc networks (VANETs) provide opportunities to exchange traffic information among vehicles allowing drivers to not only adjust their routes but also prevent possible collisions. Due to the criticality of exchanged information, message authentication which will not expose the privacy of vehicles is required. The majority of current authentication schemes for VANETs depend primarily on public-key cryptography which brings extra overhead in terms of delay and requires infrastructure support for certificate verification. Symmetric-key based techniques can be more efficient, but they introduce significant key maintenance overheads. Herein, by considering the natural group behavior of vehicle communications, we propose an efficient and lightweight symmetric-key based authentication scheme for VANETs based on group communication. Expanding the protocol's flexibility, we also propose an extension which integrates certain benefits of asymmetric-key techniques. We analyze the security properties of our proposed schemes to show there applicability when there is little to no infrastructure support. In addition, the proposed protocol was implemented and tested with real-world vehicle data. Simulation results confirmed the efficiency in terms of delay with respect to other proposed techniques.

With the envisioned age of Internet of Things (IoTs), different aspects of Intelligent Transportation System (ITS) will be linked so as to advance road transportation safety, ease congestion of road traffic, lessen air pollution, improve passenger transportation comfort and significantly reduce road accidents. In vehicular networks, regular exchange of current position, direction, speed, etc., enable mobile vehicle to foresee an imminent vehicle accident and notify the driver early enough in order to take appropriate action(s) or the vehicle on its own may take adequate preventive measures to avert the looming accident. Actualizing this concept requires use of shared media access protocol that is capable of guaranteeing reliable and timely broadcast of safety messages. This dissertation investigates the use of Network Coding (NC) techniques to enrich the content of each transmission and ensure improved high reliability of the broadcasted safety messages with less number of retransmissions. A Code Aided Retransmission-based Error Recovery (CARER) protocol is proposed. In order to avoid broadcast storm problem, a rebroadcasting vehicle selection metric η, is developed, which is used to select a vehicle that will rebroadcast the received encoded message. Although the proposed CARER protocol demonstrates an impressive performance, the level of incurred overhead is fairly high due to the use of complex rebroadcasting vehicle selection metric. To resolve this issue, a Random Network Coding (RNC) and vehicle clustering based vehicular communication scheme with low algorithmic complexity, named Reliable and Enhanced Cooperative Cross-layer MAC (RECMAC) scheme, is proposed. The use of this clustering technique enables RECMAC to subdivide the vehicular network into small manageable, coordinated clusters which further improve transmission reliability and minimise negative impact of network overhead. Similarly, a Cluster Head (CH) selection metric F(j) is designed, which is used to determine and select the most suitably qualified candidate to become the CH of a particular cluster. Finally, in order to investigate the impact of available radio spectral resource, an in-depth study of the required amount of spectrum sufficient to support high transmission reliability and minimum latency requirements of critical road safety messages in vehicular networks was carried out. The performance of the proposed schemes was clearly shown with detailed theoretical analysis and was further validated with simulation experiments.

Ad-hoc on-demand networks have received a significant interest in the literature. Many routing schemes for such networks focus on finding the optimal path. In this work, however, we consider the routing problem from the viewpoint of sustaining QoS (quality of service) requirement. Some algorithms such as MP-DSR already have considered this problem in terms of the end-to-end reliability requirement. While the MP-DSR algorithm is capable of resolving the issue to a certain extent, it only considers disjoint paths between a pair of source and destination nodes. In reality due to mobility of nodes there may not exist such disjoint paths. In this work the proposed approach is independent of the nature of the paths, and yet it achieves the required reliability to satisfy QoS requirement.

Made available in DSpace on 2014-06-12T15:52:15Z (GMT). No. of bitstreams: 1 license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2009 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / O surgimento de interfaces de rede sem-fio de baixo custo no mercado e o crescimento na demanda por dispositivos móveis (como Smartphones, PDAs, Internet Tablets e Laptops) permitiram a criação de cenários onde serviços de rede para usuários móveis possam existir sem nenhuma infra-estrutrutura pré-configurada. No entanto, a interoperabilidade entre tais redes, que são dinâmicas e heterogêneas, é atualmente objeto de pesquisa. Várias pesquisas na área de redes ad hoc sem-fio tem focado em uma única tecnologia sem-fio, baseada no padrão IEEE 802.11, onde os nós da rede são vistos de maneira plana (2D), ou seja, como elementos homogêneos, identificados apenas por endereços IP, não levando em consideração seus perfis de hardware e tecnologias de rede. Desta forma, pesquisas envolvendo mais de uma tecnologia de rede encontram-se em estágios iniciais. Novas propostas são necessárias para estes cenários, que são cada vez mais comuns, envolvendo múltiplos dispositivos com múltiplas interfaces de rede (multi-homed). Este trabalho propõe o protocolo de roteamento 3D, direcionado a cenários onde há heterogeneidade de dispositivos e tecnologias de rede. O objetivo do protocolo de roteamento proposto é prover mecanismos para a interoperabilidade de redes ad hoc heterogêneas, considerando outra dimensão de informações, aqui denominada de terceira dimensão (3D), que consiste em agregar mais informações, como informações de contexto, recursos dos dispositivos e interfaces de rede, ao processo de roteamento. Para isto, o protocolo considera os seguintes aspectos fundamentais: o processo de bootstrapping da rede heterogênea e dos nós, a construção e disseminação de informações de ciência de contexto entre os nós, e a atribuição de papéis específicos para determinados nós da rede. A avaliação do protocolo é feita através de experimentos em um test-bed real, utilizando um protótipo da implementação do protocolo, num cenário composto de dispositivos móveis como Smartphones OpenMoko, Internet Tablets N810 da Nokia e Laptops, possuindo tecnologias Bluetooth e 802.11, executando versões embarcadas do sistema operacional Linux

No infrastructure, no centralized administration and self-configuration are the main characteristics of MANETs. The primary motivation of MANET deployment is to increase portability, mobility and flexibility. However, this mobility causes an unpredictable change in topology and makes routing more difficult. Many routing algorithms have been proposed and tested over the last few years in order to provide an efficient routing in Ad Hoc networks. In this report we will show our conducted study with AODV (reactive), DSDV (proactive) and ZRP (hybrid) routing protocols. The performance of routing protocols have been evaluated carefully by analyzing the affects of changing network parameters such as, number of nodes, velocity, pause time, workload and flows on three performance metrics: packet delivery ratio, routing cost and average end- to- end delay. All the simulation work has been conducted in NS2. Our simulation results show that AODV gives better performance in all designed simulation models in terms of packets delivery ratio. DSDV shows the second best performance. Performance of ZRP is found average.

Unmanned Aerial Vehicles (UAVs) recently enabled a myriad of new applications spanning domains from personal entertainment and industrial inspection, to criminal surveillance and forest monitoring. A combination of sensor collection, wireless communication and path planning between multiple distributed agents is the natural way to support applications. Several small UAVs working collaboratively can rapidly provide extended reach, at low cost, and efficiently stream sensor information to operators on a ground station. A significant amount of previous work has addressed each of these topics independently, but in this dissertation we propose a holistic approach for joint coordination of networking and topology (placement of mobile nodes). Our thesis is that this approach improves user-interactive control of UAVs for live-streaming applications in terms of throughput, delay and reliability. In order to defend these claims, this dissertation begins by experimentally evaluating and modeling the wireless link between two UAVs, under different conditions. Due to limited link range, and the need for wide-area operation, the model is extended to encompass a multi-hop topology. We show that the performance of such networks using COTS devices is typically poor, and solutions must rely on coordination of network protocol and topology, simultaneously. At the network layer, we introduce a novel Time-division Multiple Access (TDMA) scheme called Distributed Variable Slot Protocol that relies on adaptive slot-length. We prove its convergence as well as its meliorated performance experimentally validated, namely 50% higher packet delivery. In terms of network topology, we show that without node placement control overall performance of the network is severely penalized, due to natural link asymmetries. We propose a novel protocol, named Dynamic Relay Placement, that is able to do both online link quality model-estimation and in a distributed fashion decide the best location for each network node, increasing throughput by 300%. Finally, we demonstrate the end-to-end system in a multi-vehicle monitoring mission. We show that coordination of multiple UAVs increases the sensor sampling rate up to 7 times in wide areas when compared to a naive approach. This work considers environmental constraints such as wind, as well as the intrinsic limitations of the vehicles such as maximum acceleration.