Jiang Libin. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 84-87). / Abstracts in English and Chinese. / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Motivations and Contributions --- p.1 / Chapter 1.2 --- Related Works --- p.3 / Chapter 1.3 --- Organization of the Thesis --- p.4 / Chapter Chapter 2 --- Background --- p.6 / Chapter 2.1 --- IEEE 802.11 --- p.6 / Chapter 2.1.1 --- Basics of 802.11 Standard --- p.6 / Chapter 2.1.2 --- Types of Networks --- p.10 / Chapter 2.1.3 --- Automatic Repeat request (ARQ) in 802.11b --- p.11 / Chapter 2.2 --- Hidden- and Exposed-node Problems --- p.15 / Chapter Chapter 3 --- Physical Interference Constraints and Protocol Constraints --- p.19 / Chapter 3.1 --- Protocol-independent Physical Interference Constraints --- p.19 / Chapter 3.2 --- Protocol-specific Physical Interference Constraints --- p.21 / Chapter 3.3 --- Protocol Collision-Prevention Constraints in 802.11 --- p.22 / Chapter 3.3.1 --- Transmitter-Side Carrier-Sensing Constraints --- p.22 / Chapter 3.3.2 --- Receiver-Side Carrier Sensing Constraints --- p.24 / Chapter Chapter 4 --- Formal Definitions of EN and HN Using a Graph Model --- p.27 / Chapter Chapter 5 --- Selective Disregard of NAVs (SDN) --- p.36 / Chapter 5.1 --- SDN. I - Turning off Physical Carrier Sensing and Using Receiver Restart Mode --- p.38 / Chapter 5.2 --- SDN.II - Selective Disregard of NAV (SDN) --- p.38 / Chapter 5.3 --- SDN.III - Constructing s-graph using Power Exchange Algorithm (PE) --- p.40 / Chapter Chapter 6 --- EN and Its Impact on Scalability --- p.42 / Chapter 6.1 --- Validation of SDN by NS-2 Simulations --- p.43 / Chapter 6.2 --- Scalability of SDN --- p.46 / Chapter 6.3 --- Non-Scalability of 802.11 --- p.47 / Chapter Chapter 7 --- Hidden-node Free Design (HFD) --- p.51 / Chapter 7.1 --- HFD for IEEE 802.11 Basic Access Mode --- p.52 / Chapter 7.1.1 --- HFD for basic access mode --- p.52 / Chapter 7.1.2 --- Proof of the HN-free property --- p.56 / Chapter 7.2 --- HFD for IEEE 802.11 RTS/CTS Access Mode --- p.59 / Chapter Chapter 8 --- Performance Evaluation of HFD --- p.62 / Chapter 8.1 --- HFD for Basic Access Mode --- p.62 / Chapter 8.2 --- HFD for RTS/CTS Access Mode --- p.64 / Chapter Chapter 9 --- Combination of SDN and HFD --- p.68 / Chapter Chapter 10 --- Conclusion --- p.75 / Appendices --- p.78 / References --- p.84

Ng Ping-chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 71-72). / Abstracts in English and Chinese. / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Overview and Motivation --- p.1 / Chapter 1.2 --- Background of Offered Load Control --- p.2 / Chapter 1.3 --- Background of Stability Control --- p.3 / Chapter 1.4 --- Organization of the Thesis --- p.4 / Chapter Chapter 2 --- Performance Problems and Solutions --- p.6 / Chapter 2.1 --- Simulation Set-up --- p.6 / Chapter 2.2 --- High Packet-Drop Rate --- p.7 / Chapter 2.3 --- Re-routing Instability --- p.8 / Chapter 2.3.1 --- Hidden-Node Problem --- p.8 / Chapter 2.3.2 --- Ineffectiveness of Solving Hidden-Node Problem with RTS/CTS …… --- p.9 / Chapter 2.4 --- Solutions to High-Packet Loss Rate and Re-routing Instability --- p.10 / Chapter 2.4.1 --- Link-Failure Re-routing --- p.11 / Chapter 2.4.2 --- Controlling Offered Load --- p.13 / Chapter 2.5 --- Verification of Simulation Results with Real-life Experimental Measurements --- p.14 / Chapter Chapter 3 --- Offered Load Control --- p.16 / Chapter 3.1 --- Capacity Limited by the Hidden-node and Exposed-node Problems --- p.16 / Chapter 3.1.1 --- Signal Capture --- p.18 / Chapter 3.1.2 --- Analysis of Vulnerable Period induced by Hidden Nodes --- p.20 / Chapter 3.1.3 --- Analysis of Vulnerable Period induced by Exposed Nodes --- p.21 / Chapter 3.1.4 --- Sustainable Throughput --- p.22 / Chapter 3.2 --- Capacity Limited by Carrier Sensing Property --- p.23 / Chapter 3.3 --- Numerical Results --- p.26 / Chapter 3.4 --- General Throughput Analysis of a Single Multi-hop Traffic Flow --- p.29 / Chapter 3.5 --- Throughput Analysis on Topologies with Variable Distances between Successive Nodes --- p.31 / Chapter Chapter 4 --- Discussions of Other Special Cases --- p.33 / Chapter 4.1 --- A Carrier-sensing Limited Example --- p.33 / Chapter 4.2 --- A Practical Solution to Improve Throughput --- p.34 / Chapter Chapter 5 --- Achieving Fairness in Other Network Topologies --- p.36 / Chapter 5.1 --- Lattice Topology --- p.36 / Chapter Chapter 6 --- Stability Control --- p.39 / Chapter 6.1 --- Ad-hoc routing protocols --- p.39 / Chapter 6.2 --- Proposed scheme --- p.40 / Chapter 6.2.1 --- Original AODV --- p.41 / Chapter 6.2.2 --- AODV with Proposed Scheme --- p.42 / Chapter 6.2.2.1 --- A Single Flow in a Single Chain of Nodes --- p.43 / Chapter 6.2.2.2 --- Real-break Case --- p.44 / Chapter 6.3 --- Improvements --- p.45 / Chapter Chapter 7 --- Impacts of Data Transmission Rate and Payload Size --- p.48 / Chapter 7.1 --- Signal Capture --- p.48 / Chapter 7.2 --- Vulnerable region --- p.50 / Chapter Chapter 8 --- Performance Enhancements in Multiple Flows --- p.53 / Chapter 8.1 --- Impacts of Re-routing Instability in Two Flow Topology --- p.53 / Chapter 8.2 --- Impacts of Vulnerable Periods in Multiple Flow Topologies --- p.55 / Chapter 8.2.1 --- The Vulnerable Period induced by Individual Hidden-terminal Flow --- p.57 / Chapter 8.2.2 --- The Number of Hidden-terminal Flows --- p.58 / Chapter 8.2.3 --- Correlation between Hidden-terminal Flows --- p.60 / Chapter Chapter 9 --- Conclusion --- p.63 / Chapter Appendix A: --- General Throughput Analysis of a Single Multi-hop Traffic Flow --- p.67 / Chapter A.l --- Capacity Limited by Hidden-node and Exposed-Node --- p.67 / Chapter A.1.1 --- Sustainable Throughput --- p.68 / Chapter A.2 --- Capacity Limited by Carrier Sensing Property --- p.68 / Bibliography --- p.71

Nam Chung Ho. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 68-71). / Abstracts in English and Chinese. / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Overview and Motivation --- p.1 / Chapter 1.2 --- Organization of the Thesis --- p.5 / Chapter Chapter 2 --- Background --- p.6 / Chapter 2.1 --- Basics of 802.11 Standard --- p.6 / Chapter 2.1.1 --- Distributed Coordination Function (DCF) / Point Coordination Function (PCF) --- p.7 / Chapter 2.1.2 --- RTS/CTS --- p.8 / Chapter 2.2 --- Types of Networks --- p.9 / Chapter 2.3 --- Automatic Repeat request (ARQ) in 802.11b --- p.11 / Chapter 2.3.1 --- Importance of Link-layer ARQ in Wireless Networks --- p.12 / Chapter 2.3.2 --- MAC Algorithm of 802.11b Standard --- p.13 / Chapter 2.3.3 --- Modified MAC algorithm in 802.11b commercial products --- p.14 / Chapter 2.4 --- Automatic Adjustment of Radio Data Rate in Commercial 802.11b Products --- p.15 / Chapter Chapter 3 --- Head-of-Line Blocking in Access Points --- p.17 / Chapter 3.1 --- Cause of HOL blocking in 802.11b --- p.17 / Chapter 3.1.1 --- Calculation of Worst-Case Service Time for Packet at Head of Queue --- p.19 / Chapter 3.2 --- Simulation Settings --- p.21 / Chapter 3.2.1 --- Propagation Models Available in NS2 --- p.21 / Chapter 3.2.2 --- Variables of Shadowing Model --- p.25 / Chapter 3.3 --- Simulation Results on UDP --- p.26 / Chapter 3.4 --- Experimental Results on UDP --- p.28 / Chapter 3.5 --- Simulation Results on TCP --- p.32 / Chapter 3.6 --- Experimental Results on TCP --- p.34 / Chapter 3.7 --- Possible Solutions of HOL Blocking Problem --- p.35 / Chapter 3.7.1 --- Weakening Link-layer ARQ in 802.11b --- p.36 / Chapter 3.7.2 --- Effectiveness of ARQ in 802.11b --- p.37 / Chapter 3.7.2.1 --- Set-up for Network Experiments --- p.38 / Chapter 3.7.2.2 --- Results and Analysis --- p.39 / Chapter 3.7.3 --- Virtual Queuing --- p.45 / Chapter Chapter 4 --- Study of Uplink Traffic --- p.50 / Chapter 4.1 --- Poor Pulling Down the Rich --- p.51 / Chapter 4.2 --- Signal Capturing Effect --- p.53 / Chapter 4.2.1 --- Mathematical Analysis of Signal Capturing Effect --- p.55 / Chapter Chapter 5 --- Packet Loss Patterns in 802.11b WLAN --- p.61 / Chapter 5.1 --- """Random Loss"" vs ""Bursty Loss""" --- p.61 / Chapter 5.2 --- Experimental Evaluation --- p.62 / Chapter Chapter 6 --- Conclusion --- p.65 / Bibliography --- p.68

Avec le déploiement de la technologie WiFi ces dernières années, les réseaux locaux sans fil connaissent un grand succès auprès des institutions et les réseaux ad hoc suscitent un réel intérêt auprès de la communauté de R&D. Parallèlement, avec l'émergence des services multimédias dans les réseaux mobiles, des travaux pour l'introduction de la qualité de service dans les réseaux ad hoc ont été proposés. Les études existantes sont souvent basées sur des hypothèses limitées et inadaptées aux propriétés des réseaux ad hoc. Dans cette thèse, nous proposons d'abord un protocole de routage avec différenciation de terminaux qui maximise les liens sans fil de haute qualité. Le but d'une telle approche est de prendre en compte l'hétérogénéité des nœuds dans les réseaux ad hoc et de supporter les situations où des terminaux mobiles peuvent accepter ou refuser la fonction de routage. Notre proposition apporte une solution aux variations des capacités des liens sans fil en routant les paquets de préférence à travers les routeurs collaboratifs ayant une grande capacité de transmission dans le but de maintenir une meilleure qualité de lien (grand débit) des routes dans le réseau. Nous prouvons ensuite la nécessité d'une gestion multicouches de la qualité de service dans un tel environnement. Cela permet de définir une stratégie de QoS en plusieurs couches de communication dans le but de prendre en considération les contraintes liées aux spécificités des réseaux mobiles ad hoc. Une combinaison des mécanismes de qualité de service au niveau IP et MAC (IEEE 802.11) est étudiée. De plus le principe de routage avec différenciation de terminaux décrit ci-dessus a été combiné avec la gestion de la qualité de service IP et MAC. Un autre volet de la thèse traite l'aspect de contrôle de topologie dans les réseaux ad hoc. Il consiste à contrôler la morphologie du réseau en utilisant la mobilité d'un ensemble de routeurs dédiés. Le principe est d'utiliser positivement la mobilité, qui est habituellement subie dans le réseau, afin d'améliorer les performances de ce dernier. Ainsi, des stratégies de déploiement des routeurs dédiés sont étudiées (dans un réseau ad hoc autonome ou interconnecté à une infrastructure) dans le but d'offrir une meilleure connectivité ou pour assurer un meilleur support de la qualité de service des applications temps réel. Pour les deux stratégies, le problème a été formulé comme un problème de programmation linéaire entière mixte. Cette thèse a été réalisée dans le cadre du projet ITEA Ambience. Dans ce contexte, nous avons entre autres contribué à la réalisation d'une plate-forme, qui illustre un exemple d'applications des réseaux ad hoc où des mécanismes décrits dans les parties précédentes sont utilisés afin d'améliorer les performances du système et répondre aux besoins des utilisateurs mobiles, dans un contexte hétérogène sécurisé. Mots-clés : Qualité de service, IEEE 802.11, Différenciation de services, Hétérogénéité des nœuds, Routage ad hoc, Contrôle de topologie, Routeurs dédiés, Connectivité.

<p>Wireless communication is being developed in the last years day by day, there are several standards that talks about it. We are going to go through the IEEE standard 802.11 which talks about wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Looking this more carefully we will study MAC specifications and its environment. </p><p>The work that ISY department at Institute of Technology of Linkoping University has proposed is to design a MAC sublayer implementation for WLANs using C language programming and testing it with the test environment called “test bench”. This test bench will simulate LLC sublayer and PHY layer, in this way, our MAC implementation will has to interact with it. Therefore we will simulate a wireless network where we are going to have a short number of stations and we are going to look at carefully the MAC sublayer response in an ad hoc network.</p>

We consider an online learning scenario in which the learner can make predictions on the basis of a fixed set of experts. The performance of each expert may change over time in a manner unknown to the learner. We formulate a class of universal learning algorithms for this problem by expressing them as simple Bayesian algorithms operating on models analogous to Hidden Markov Models (HMMs). We derive a new performance bound for such algorithms which is considerably simpler than existing bounds. The bound provides the basis for learning the rate at which the identity of the optimal expert switches over time. We find an analytic expression for the a priori resolution at which we need to learn the rate parameter. We extend our scalar switching-rate result to models of the switching-rate that are governed by a matrix of parameters, i.e. arbitrary homogeneous HMMs. We apply and examine our algorithm in the context of the problem of energy management in wireless networks. We analyze the new results in the framework of Information Theory.

The rapid growth in the functionality of Wi-Fi networking in recent years has benefited academic environments. Consistent with their role as centers of innovation academic institutions have an interest in facilitating as much mobile, computer networking functionality as possible to parties of varying levels of affiliation, while also assuring confidentiality and integrity of communications. Providing secure yet functional Wi-Fi access to guests and affiliates in an academic environment presents significant challenges. Academic institutions have taken a wide variety of approaches to this problem. This study presents and analyzes data gathered from semi-structured telephone interviews with employees focused on computer networking and security in academic environments regarding their institutions’ approaches toward striking a balance between security and functionality. The results are summarized, conclusions are presented, and solutions to common problems are reviewed. Finally, remaining significant research questions are presented and explored.

Covert channels are becoming more popular as security risks grow in networks. One area that is promising for covert channels is wireless networks, since many use a collision avoidance scheme such as carrier sense multiple access with collision avoidance (CSMA/CA). These schemes often introduce randomness in the network, which provides good cover for a covert timing channel. In this thesis, we use the 802.11 standard as an example to demonstrate a wireless covert channel. In particular, most 802.11 configurations use a distributed coordinated function (DCF) to assist in communications. This DCF uses a random backoff to avoid collisions, which provides the cover for our covert channel. Our timing channel provides great improvements on other recent covert channels in the field of throughput, while maintaining high accuracy. We are able to achieve throughput over 8000 bps using Covert DCF, or by accepting a throughput of 1800 bps we can achieve higher covertness and 99% accuracy as well.

The use of 802.11-based wireless mesh networks (WMNs) as an alternative network backbone technology is growing rapidly. The primary advantages of this approach are ease of deployment and lower cost. However, such networks typically exhibit poor fairness properties, often starving nodes if they are too many hops distant from the gateway. Researchers have shown a growing interest in this problem in recent years. Many solutions proposed amount to some level of source rate control, either by policing directly at the source, or via TCP congestion control reacting to a gateway-enforced rate limit. However, there has been limited study on the effectiveness of source rate control. In this thesis we first demonstrate that source rate control can only partially solve the fairness issue in 802.11-based WMNs, with some routers experiencing an undesirable degree of unfairness, which we call structural unfairness. We then identify the four necessary factors that cause structural unfairness. If we can eliminate or reduce any one of these conditions, we can eliminate or ameliorate the unfairness problem. We first investigate two techniques to improve 802.11 MAC scheduling: fixing the contention window and packet spacing at every router node, both means achievable with commodity 802.11 hardware. We show that the combination of these mechanisms provides a significant gain in fairness. We also perform case studies using another three techniques, channel re-assignment, routing changes, and careful router placement, to remove or reduce other necessary conditions. We demonstrate that these techniques, whenever applicable, can eliminate the unfairness problem entirely at times, or at least improve the situation.

Nous avons assisté ces dernières années à la montée en puissance des réseaux locaux sans fil ou encore Wi-Fi, qui sont en passe de devenir l'une des principales solutions de connexion pour de nombreuses entreprises. Le marché du sans fil se développe rapidement dès lors que les entreprises constatent les gains de productivité qui découlent de la disparition des câbles. Avec cette évolution rapide de ce type dématérialisé de réseaux, les exigences en termes de sécurité deviennent de plus en plus sévères. De ce fait, beaucoup de travaux et d'efforts ont été consentis ces dernières années afin d'aboutir à des solutions pour sécuriser ces réseaux. Toutefois, des vulnérabilités persistent encore et il est toujours possible de monter des attaques plus ou moins facilement. Notamment, contre le dernier né des protocoles de sécurité Wi-Fi, à savoir WPA2, qui bien qu'étant plus robuste sur le plan conceptuel que les générations précédentes, fait face à un problème majeur, celui de son incompatibilité matérielle avec les précédents protocoles. En effet, WPA2 exige de nouveaux équipements matériels, ce qui constitue un surcoût économique énorme pour les entreprises ayant déjà déployé des équipements Wi-Fi d'anciennes générations. Dans ce mémoire, nous élaborons une synthèse exhaustive de toutes les attaques qui ciblent les réseaux Wi-Fi. Cette synthèse comprend une classification des attaques par rapport aux standards de sécurité ainsi que l'illustration des détails de leur mise en œuvre. Outre le volet conceptuel et théorique, nous abordons également le volet pratique et montrons sa richesse. Nous proposons également une nouvelle approche architecturale de sécurisation des réseaux Wi-Fi dans l'entreprise. Notre proposition prend en compte l'hétérogénéité des équipements et des standards de sécurité supportés. Cette nouvelle architecture a le mérite d'offrir une grande flexibilité ainsi qu'une sécurité renforcée par rapport aux approches traditionnelles. Pour élaborer cette solution sécurisée, nous nous sommes basés principalement sur la différenciation à plusieurs niveaux (standard de sécurité supporté, communauté d'utilisateurs, nature de trafic). Ces niveaux de différenciation offrent la granularité nécessaire pour permettre une meilleure gestion du réseau et un meilleur contrôle d'accès aux ressources, ce qui améliore la sécurité du réseau Wi-Fi en particulier et du système d'information de l'entreprise dans son ensemble. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Wi-Fi, sécurité, attaque, architecture sécurisée, différenciation.