In this dissertation work, we have investigated the main research thrust of decentralized coordination of workflows over web services. To address distributed workflow coordination, first we have developed “Web Coordination Bonds” as a capable set of dependency modeling primitives that enable each web service to manage its own dependencies. Web bond primitives are as powerful as extended Petri nets and have sufficient modeling and expressive capabilities to model workflow dependencies. We have designed and prototyped our “Web Service Coordination Management Middleware” (WSCMM) system that enhances current web services infrastructure to accommodate web bond enabled web services. Finally, based on core concepts of web coordination bonds and WSCMM, we have developed the “BondFlow” system that allows easy configuration distributed coordination of workflows. The footprint of the BonFlow runtime is 24KB and the additional third party software packages, SOAP client and XML parser, account for 115KB.

We propose a simple, passive and deployable approach for fingerprinting traffic on the wired side as a solution for three critical stealth attacks in wireless networks. We focus on extracting traces of the 802.11 medium access control (MAC) protocol from the temporal arrival patterns of incoming traffic streams as seen on the wired side, to identify attacker behavior. Attacks addressed include unauthorized access points, selfish behavior at the MAC layer and MAC layer covert timing channels. We employ the Bayesian binning technique as a means of classifying between delay distributions. The scheme requires no change to the 802.11 nodes or protocol, exhibits minimal computational overhead and offers a single point of discovery. We evaluate our model using experiments and simulations.

Over the past few decades, wireless local area networks (WLANs) have been widely deployed for data communication in indoor environments such as offices, houses, and airports. In order to fairly and efficiently use the unlicensed frequency band that Wi-Fi devices share, the devices follow a set of channel access rules, which is called a wireless medium access control (MAC) protocol. It is known that wireless devices following the 802.11 standard MAC protocol, i.e. the distributed coordination function (DCF), suffer from packet collisions when multiple nodes simultaneously transmit. This significantly degrades the throughput performance. Recently, several studies have reported access techniques to reduce the number of packet collisions and to achieve a collision free WLAN. Although these studies have shown that the number of collisions can be reduced to zero in a simple way, there have been a couple of remaining issues to solve, such as dynamic parameter adjustment and fairness to legacy DCF nodes in terms of channel access opportunity. Recently, In-Band Full Duplex (IBFD) communication has received much attention, because it has significant potential to improve the communication capacity of a radio band. IBFD means that a node can simultaneously transmit one signal and receive another signal in the same band at the same time. In order to maximize the performance of IBFD communication capability and to fairly share access to the wireless medium among distributed devices in WLANs, a number of IBFD MAC protocols have been proposed. However, little attention has been paid to fairness issues between half duplex nodes (i.e. nodes that can either transmit or receive but not both simultaneously in one time-frequency resource block) and IBFD capable nodes in the presence of the hidden node problem.

Long time ago, there were developed methods which can allow fast exchange of information at the longest distance possible. Until recent time, the possibilities of long way communications were very limited. There were technological and financial limitations mainly. With telegraph, telephone and the newest – computer networks invention, the telecommunication services became cheaper and much more comprehensive. With accession and high scale spread of internet, the role of communications is much more important. The most actual trend is mobile internet and connected multimedia networks and their instant accessibility from anywhere. On the first side there are classical telecommunication networks as GSM, UMTS, on the other side there is very strong alternative in shape of WiMAX and WiFi networks combination. For the usage of multimedia services in that networks is very important to handle their prefferization over other traffic, which is very difficult task even on wireless media. For the finding of new processes and methods, which can allow it, there is, at fist, the need for highly accurate and authentic mathematical models. In this doctoral thesis is mapped actual state of the art and proposed the new mathematical model of Distribution Coordination Function, which is much-frequent used as access method in 802.11 networks, Wi-Fi.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: In recent years the demand for bandwidth has dramatically increased because of new applications for data and multimedia, and wireless technology has prevailed as a prominent technology for data connectivity, especially for home, office and last mile services. As wireless communications are dependant upon spectrum availability, which is communal, this scarce commodity in communication has to be used as efficiently as possible. Some aspects of this requirement are addressed in this project. We chose the IEEE 802.11b standard for this particular investigation because of its widespread use, the vast amount of applicable literature, the variety of software simulation tools and the ease with which equipment can be obtained. The IEEE 802.11 standard specified the Point Coordination Function as the de- terministic protocol. Recently research into this aspect has stagnated, and it was the purpose of this project to investigate how existing infrastructure networks could be improved by optimising some modes of the 802.11 protocol. The investigation also hoped to determine when to change between Distributed Coordination Function (DCF) and Point Coordination Function (PCF), and to provide an adaptive protocol to do so. This thesis presents mathematical models for the operation of DCF and PCF modes, which is compared with results from a network simulator (ns2), for theoretical veri- fication. A protocol is also proposed to dynamically switch between DCF and PCF, to harness the advantages they present. / AFRIKAANSE OPSOMMING: Die afgelope paar jaar het die aanvraag na bandwydte dramaties verhoog as gevolg van nuwe toepassings vir data en multimedia, en draadlose tegnologie het voorgekom as ’n dominante tegnologie vir data konnektiwiteit, veral vir die huis, kantoor en laaste myl dienste. Omdat draadlose kommunikasie afhanklik is van spektrum beskikbaarheid, wat gemeenskaplik is, moet hierdie skaars kommoditeit in kommunikasie so effektief moontlik gebruik word. Sekere aspekte van die vereiste sal in die tesis ondersoek word. Dit is besluit om die IEEE 802.11b standard vir die spesifieke ondersoek te gebruik as gevolg van die wye toepassing, die groot hoeveelheid beskikbare literatuur, die verskeidenheid simulasie sagteware en die gemak waarmee die toerusting bekom kan word. Die IEEE 802.11 standaard spesifiseer the Punt Koordinasie Funksie (PCF) as die deterministiese protokol vir die betrokke standaard. Onlangs het navorsing oor hierdie aspek gestagneer, en dit is die doel van die projek om te ondersoek hoe bestaande infrastruktuur netwerke moontlik verbeter kan word deur optimering van sekere modusse van die 802.11 protokol. Die ondersoek hoop ook om te bepaal wanneer die oorgang van die Distrubusie Koordinasie Funksie (DCF) en Punt Ko- ordinasie Funksie sal plaasvind, en om ’n dienooreenstemmende protokol te on- twikkel. Die tesis verskaf wiskundige modelle vir die werking van die DCF en PCF modusse, wat vergelyk word met resultate uit ’n netwerk simulator (ns2), vir teoretiese ver- ifikasie. ’n Protokol word ook voorgestel om dinamies te wissel tussen DCF and PCF, om die voordele wat die protokolle verskaf te gebruik.

Ce travail concerne la commande des systèmes multi-agent d’ordre fractionnaire utilisant une topologie de communication fixe. Premièrement, la production en formation avec atténuation absolue et retard de communication est étudiée. Pour cela, une loi de commande et des conditions suffisantes sont proposées. Toutefois, dans certains scénarios, il est souhaitable que tous les agents atteignent la formation souhaitée tout en se déplacent en groupe, au lieu d’un rendez-vous à un point fixe. Ce cas sera traité en étudiant la production en formation avec atténuation relative et retard de communication. Troisièmement, la poursuite par consensus des systèmes avec un état de référence variable dans le temps est étudiée. Une loi de commande commune et une seconde basée sur la prédiction d’erreur sont proposées, et le problème du consensus est résolu quand le graphe de communication contient un arbre dirigé. Il a été prouvé que la convergence du système est plus rapide en utilisant la loi basée sur la prédiction d’erreur plutôt que celle de commande commune. Enfin, les lois de commande ci-dessus sont étendues au cas de la poursuite en formation. En effet, dans de nombreux cas, l'information peut être envoyée à partir d'un état de référence vers les agents voisins uniquement et non pas à l’ensemble des agents. Afin de résoudre ce problème, une loi de commande est proposée afin de résoudre le problème du consensus avec un état de référence constant. Puis, deux lois de commande sont proposées afin de résoudre le problème du consensus avec un état de référence variant dans le temps. Ces lois sont étendues pour résoudre le problème de la poursuite en formation / This thesis focuses on the distributed coordination of fractional-order multi-agent systems under fixed directed communication graph. Firstly, formation producing with absolute damping and communication delay of fractional-order multi-agent systems is studied. A control law is proposed and some sufficient conditions are derived for achieving formation producing. However, in some scenarios, it might be desirable that all agents achieve formation and move as a group, instead of rendezvous at a stationary point. Therefore, secondly, formation producing with relative damping and communication delay is considered. Thirdly, consensus tracking of fractional-order multi-agent systems with a time-varying reference state is studied. A common control law and a control law based on error predictor are proposed, and it is shown that the control laws are effective when a communication graph has directed spanning trees. Meanwhile, it is proved that the convergence of systems is faster using the control law based on error predictor than by the common one. Finally, the above control laws are extended to achieve formation-tracking problems. In fact, in many cases information can be sent from a reference state to only its neighbor agents not to all the agents. In order to solve the above problem, an effective control law is given to achieve consensus with a constant reference state. Then, an effective general control law and an effective particular one are proposed to achieve consensus with a time-varying reference state. Furthermore, the above control laws are extended to achieve the formation tracking problems

This thesis is concerned with analytical modeling of Wireless Local Area Networks (WLANs) that are based on IEEE 802.11 Distributed Coordination Function (DCF). Such networks are popularly known as WiFi networks. We have developed accurate analytical models for the following three network scenarios: (S1) A single cell WLAN with homogeneous nodes and Poisson packet arrivals, (S2) A multi-cell WLAN (a) with saturated nodes, or (b) with TCP-controlled long-lived downloads, and (S3) A multi-cell WLAN with TCP-controlled short-lived downloads. Our analytical models are simple Markovian abstractions that capture the detailed network behavior in the considered scenarios. The insights provided by our analytical models led to two applications: (i) a faster “model-based'” simulator, and (ii) a distributed channel assignment algorithm. We also study the stability of the network through our Markov models. For scenario (S1), we develop a new approach as compared to the existing literature. We apply a “State Dependent Attempt Rate'” (SDAR) approximation to reduce a single cell WLAN with non-saturated nodes to a coupled queue system. We provide a sufficient condition under which the joint queue length Markov chain is positive recurrent. For the case when the arrival rates into the queues are equal we propose a technique to reduce the state space of the coupled queue system. In addition, when the buffer size of the queues are finite and equal we propose an iterative method to estimate the stationary distribution of the reduced state process. Our iterative method yields accurate predictions for important performance measures, namely, “throughput'”, “collision probability” and “packet delay”. We replace the detailed implementation of the MAC layer in NS-2 with the SDAR contention model, thus yielding a ``model-based'' simulator at the MAC layer. We demonstrate that the SDAR model of contention provides an accurate model for the detailed CSMA/CA protocol in scenario (S1). In addition, since the SDAR model removes much of the details at the MAC layer we obtain speed-ups of 1.55-5.4 depending on the arrival rates and the number of nodes in the single cell WLAN. For scenario (S2), we consider a restricted network setting where a so-called “Pairwise Binary Dependence” (PBD) condition holds. We develop a first-cut scalable “cell-level” model by applying the PBD condition. Unlike a node- or link-level model, the complexity of our cell-level model increases with the number of cells rather than with the number of nodes/links. We demonstrate the accuracy of our cell-level model via NS-2 simulations. We show that, as the “access intensity” of every cell goes to infinity the aggregate network throughput is maximized. This remarkable property of CSMA, namely, “maximization of aggregate network throughput in a distributed manner” has been proved recently by Durvy et al. (TIT, March, 2009) for an infinite linear chain of nodes. We prove it for multi-cell WLANs with arbitrary cell topology (under the PBD condition). Based on this insight provided by our analytical model we propose a distributed channel assignment algorithm. For scenario (S3), we consider the same restricted network setting as for scenario (S2). For Poisson flow arrivals and i.i.d. exponentially distributed flow sizes we model a multi-cell WLAN as a network of processor-sharing queues with state-dependent service rates. The state-dependent service rates are obtained by applying the model for scenario (S2) and taking the access intensities to infinity. We demonstrate the accuracy of our model via NS-2 simulations. We also demonstrate the inaccuracy of the service model proposed in the recent work by Bonald et al. (SIGMETRICS 2008) and identify the implicit assumption in their model which leads to this inaccuracy. We call our service model which accurately characterizes the service process in a multi-cell WLAN (under the PBD condition) “DCF scheduling” and study the “stability region” of DCF scheduling for small networks with single or multiple overlapping “contention domains”.