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A Distributed Virtual Environment (DVE) system provides a shared virtual environment where physically separated users can interact and collaborate over a computer network. More simultaneous DVE users could result in intolerable system performance degradation. We address the three major challenges to improve DVE scalability: effective DVE system performance measurement, understanding the controlling factors of system performance/quality and determining the consequences of DVE system changes. We propose a DVE Scalability Engineering (DSE) process that addresses these three major challenges for DVE design. DSE allow us to identify, evaluate, and leverage trade-offs among DVE resources, the DVE software, and the virtual environment. DSE has three stages. First, we show how to simulate different numbers and types of users on DVE resources. Collected user study data is used to identify representative user types. Second, we describe a modeling method to discover the major trade-offs between quality of service and DVE resource usage. The method makes use of a new instrumentation tool called ppt. ppt collects atomic blocks of developer-selected instrumentation at high rates and saves it for offline analysis. Finally, we integrate our load simulation and modeling method into a single process to explore the effects of changes in DVE resources. We use the simple Asteroids DVE as a minimal case study to describe the DSE process. The larger and commercial Torque and Quake III DVE systems provide realistic case studies and demonstrate DSE usage. The Torque case study shows the impact of many users on a DVE system. We apply the DSE process to significantly enhance the Quality of Experience given the available DVE resources. The Quake III case study shows how to identify the DVE network needs and evaluate network characteristics when using a mobile phone platform. We analyze the trade-offs between power consumption and quality of service. The case studies demonstrate the applicability of DSE for discovering and leveraging tradeoffs between Quality of Experience and DVE resource usage. Each of the three stages can be used individually to improve DVE performance. The DSE process enables fast and effective DVE performance improvement. / Ph. D.

Dans les jeux multijoueurs, où plusieurs joueurs participent à une même partie et communiquent entre eux à travers un réseau, les joueurs peuvent avoir des visions différentes du jeu en raison des délais de communication. Ceci engendre des incohérences encore plus importantes sur les réseaux mobiles tels que les réseaux 3G où les délais de communication peuvent atteindre plusieurs secondes. Des algorithmes doivent donc être mis en place pour maintenir la cohérence entre les joueurs. Ces algorithmes sont assez complexes et difficiles à programmer et mettre au point. Le but de cette thèse est de proposer une architecture logicielle qui découple la conception du jeu, de ses besoins de communication en proposant une infrastructure de communication qui pourra être interchangeable en fonction des technologies ; cette infrastructure prend la forme originale d'un composant de synchronisation. Ainsi, les conséquences des propriétés physiques du transport des informations pourront être plus ou moins masquées en intégrant dans cette infrastructure les algorithmes adéquats. / Multiplayer games users' have increased since the widespread use of the internet. Withthe arrival of rich portable devices and faster cellular wireless networks, multiplayer games on mobile phones and PDAs are becoming a reality. For multiplayer games to be playable, they should be highly interactive, fair and should have a consistent state for all the players. Because of the high wireless network latency and jitters, the issue of providing interactive games with consistent state across the network is non-trivial. In this thesis, we propose different approaches for achieving consistency in mobile multiplayer games in the face of high latency and large and variable jitters. Although absolute consistency is impossible to achieve because information takes time to travel from one place to another, we exploit the fact that strong consistency is not always required in the virtual world and can be relaxed in many cases. Our proposed approach uses the underlying network latency and the position of different objects in the virtual world to decide when to relax consistency and when to apply strong consistency mechanisms. We evaluate our approach by implementing these algorithms in J2ME based games played on mobile phones. The algorithms for consistency mechanism are very complex and are often intermixed with the game core logic's code, which makes it hard to program a game and to change its code in the future. We propose to separate the consistency mechanisms from the game logic and put them in a distributed component responsible for both consistency maintenance and communication over the network. We call this reusable component a Synchronization Medium.

The number and types of input devices and related interaction technique types are growing rapidly. Innovative input devices such as game controllers are no longer used just for games, propriety consoles and specific applications, they are also used in many distributed virtual environments, especially the so-called serious virtual environments. In this dissertation a distributed, service based framework is presented to offer context-sensitive interaction interoperability that can support mapping between input devices and suitable application tasks given the attributes (device, applications, users, and interaction techniques) and the current user context without negatively impacting performances of large scale distributed environments. The mapping is dynamic and context sensitive taking into account the context dimensions of both the virtual and real planes. What device or device component to use, how and when to use them depend on the application, task performed, the user and the overall context, including location and presence of other users. Another use of interaction interoperability is as a testbed for input devices, and interaction techniques making it possible to test reality based interfaces and interaction techniques with legacy applications. The dissertation provides a description how the framework provides these affordances and a discussion of motivations, goals and the addressed challenges. Several proof of the concept implementations were developed and an evaluation of the framework performance (in terms of system characteristics) demonstrates viability, scalability and negligible delays. / Ph. D.

We present a resource allocation perspective to Quality of Service in Distributed Virtual Environments. The user of a DVE system will have improved Quality of Service if he/she is allocated the right amount of resources at the right time. Instead of allocating resources on a static basis, we adopt a dynamic need based resource allocation scheme that provides real-time resource allocation. Optimal resource assignments are calculated offline and a neural network is trained with the knowledge of optimal solutions from the offline Operations Research Techniques and it is then used to deliver near-optimal resource allocation decisions in real-time. We also present a case study of network bandwidth allocation and prove the usefulness of the technique. / Master of Science

Αντικείμενο της παρούσας διατριβής υπήρξε αφενός η μελέτη και η αξιολόγηση των υπαρχόντων τεχνικών και αφετέρου ο σχεδιασμός, η υλοποίηση και η αποτίμηση νέων τεχνικών και μηχανικών για τη βελτιστοποίηση της απόδοσης του συστήματος με τη βέλτιστη αξιοποίηση των διαθέσιμων πόρων. / The objective of this dissertation was on the one hand the study and analysis of existing techniques and algorithms for large-scale DVEs and on the other hand the design, implementation and assessment of new techniques, methods and algorithms that can ensure the quality of the provided services and the efficient operation of the system. The algorithms and techniques developed were analysed and evaluated through extensive experiments both for identifying possible weaknesses and for highlighting the applications that can achieve optimum results. More specifically, two approaches have been designed and developed. The first approach is related to the design and development of a partitioning algorithm, that, in contrary to existing approaches, exploits the special characteristic of the virtual world and the impact those characteristics have on users’ behavior in order to predict the needs of the environment, in terms of resources, before they are required. The second approach is related to the design and development of a dynamic management approach that exploits the dynamic nature of DVEs in order to perform load balancing and to ensure performance optimization. During the elaboration of this dissertation, we found out that one of the basic problems when designing and implementing algorithms, methods and techniques for large-scale DVEs is the way that their efficiency could be examined. In most of the cases, the evaluation is based on theoretical models, which often fail to meet the circumstances and situations met in real DVEs. In particular, for supporting largescale DVEs, extended infrastructure is needed in terms of both hardware and software. Due to the fact that both researchers and application designers do not always have access to such extended infrastructure, the assessment and evaluation of developed techniques are extremely difficult. In most of the cases, both application designers and researchers adopt specialized methods for evaluating different techniques, while in other cases simulation tools have been developed from scratch. However, given the fact that the design and implementation is application or technique-specific, the reusability of these tools for different architectures and algorithms is not always successful. To this direction and for overcoming this important limitation, a simulation modeling framework for assessing DVEs performance was designed and implemented. The framework takes into account a number of both generic and special-cause parameters, which can be set on demand by the DVE designers and stake-holders and it is based on transforming system requirements to the concepts of operational management.

Distributed virtual environments (DVEs) have been an active area of research and engineering for more than 20 years. The most widely deployed DVEs are network games such as Quake, Halo, and World of Warcraft (WoW), with millions of users and billions of dollars in annual revenue. Deployed DVEs remain expensive centralized implementations despite significant research outlining ways to distribute DVE workloads. This dissertation shows previous DVE research evaluations are inconsistent with deployed DVE needs. Assumptions about avatar movement and proximity - fundamental scale factors - do not match WoW's workload, and likely the workload of other deployed DVEs. Alternate workload models are explored and preliminary conclusions presented. Using realistic workloads it is shown that a fully decentralized DVE cannot be deployed to today's consumers, regardless of its overhead. Residential broadband speeds are improving, and this limitation will eventually disappear. When it does, appropriate security mechanisms will be a fundamental requirement for technology adoption. A trusted auditing system ('Carbon') is presented which has good security, scalability, and resource characteristics for decentralized DVEs. When performing exhaustive auditing, Carbon adds 27% network overhead to a decentralized DVE with a WoW-like workload. This resource consumption can be reduced significantly, depending upon the DVE's risk tolerance. Finally, the Pairwise Random Protocol (PRP) is described. PRP enables adversaries to fairly resolve probabilistic activities, an ability missing from most decentralized DVE security proposals. Thus, this dissertations contribution is to address two of the obstacles for deploying research on decentralized DVE architectures. First, lack of evidence that research results apply to existing DVEs. Second, the lack of security systems combining appropriate security guarantees with acceptable overhead.