Massively multi-user virtual environments (MMVEs)are becoming increasingly popular with millions of users. Typically, commercial implementations rely on client/server architectures for managing the game state and use message passing mechanisms to communicate state changes to the clients. We have developed the Typed Grid Object Sharing (TGOS)service providing data sharing of in-memory data. TGOS aims at simplifying the development of MMVEs by sharing scene graphs in a peer-to-peer way and also data of backend services. Replication is controlled by different consistency models, including restartable transactions combined with optimistic synchronization for strong consistency. In this paper we describe the data centric architecture of the Wissenheim Worlds application and relevant parts of TGOS. Furthermore, we present an evaluation showing the feasibility and efficiency of the proposed approach.

Distributed shared memory (DSM) is a well-known alternative to explicit message passing and remote procedure call. Numerous DSM systems and consistency models have been proposed in the past. The Plurix project implements a DSM operating system (OS) storing data and code, within the DSM. Our DSM storage features a new consistency model (transactional consistency) combining restartable transactions with an optimistic synchronization scheme instead of relying on a hard to use weak consistency model. In this paper we evaluate our system for the first time with a real parallel application, a parallel ray-tracer. The measurements show that our DSM scales quite well for this application even though we are using a strong consistency model.

State-of-the-art middleware like .NET or J2EE offers transparent access to distributed objects and services but lack support for consistency between groups of objects. Distributed Shared Memory (DSM) is an interesting alternative to build distributed applications because it maintains consistency among object groups. Although DSM has been used in the past only for special number crunching programs we expect it to be a good foundation to simplify the development of multi-player games and virtual reality applications. In this paper we present the relevant parts of our transactional DSM operating system (OS). Subsequently, we describe the gaming framework built on top of the DSM system. Finally we evaluate the framework with a sample game.

The Plurix project implements an object-oriented operating system (OS) for PC clusters. Communication is achieved via shared objects in a distributed shared memory (DSM). The consistency of this distributed memory is guaranteed by an optimistic synchronization scheme and restartable transactions. We contend that coupling object orientation with the DSM property allows quick system startup, simplified development of distributed applications and a type-consistent system bootstrapping procedure. The OS (including kernel and drivers) is written in Java using proprietary Plurix Java compiler (PJC) to translate Java source code directly into Intel machine instructions. We briefly illustrate the architecture of our DSM-based OS kernel and the resulting synergies for communication between applications and OS. We present advanced issues of memory management with respect to the DSM-kernel classes and strategies to avoid false-sharing.

Distributed Shared Memory (DSM) is an interesting alternative to build distributed object system. Explicit message passing and remote invocation is replaced by the simple and unified DSM abstraction. The recurrent performance drawbacks of DSM Systems are often caused by expensive distributed locking mechanisms. In response to this our multipurpose Plurix Operating System (OS) implements a Transaction based DSM. Memory consistency is maintained by atomic transactions and optimistic synchronization mechanisms which have been used in database technology in the past. Such a transaction based DSM with optimistic synchronization guarantee a sequential consistent view on the shared data to every mode in the cluster.

Within this paper we simulate the transmission of MPEG-2 transport stream (TS) packets over a wireless ATM network. Based on a finite state radio channel model for the physical layer of a wireless ATM link including the characteristics of the ATM and MAC layer, different packing schemes are evaluated for encapsulating MPEG-2 transport stream packets in ATM adaptation Layer 5 (AAL5) PDUs. We analyze the performance with respect to both delay and visual quality in terms of PSNR based on a calculated cell error ratio (CER) for each given state of the radio model. The statistics show, that the 1TP (one MPEG-2 TS per AAL5 PDU) scheme outperforms all other packing schemes in terms of visual quality. At medium channel quality (38 dB), quality is judged to be good, although the CER may be as high as 25%.

Distributed multimedia applications with real time capabilities will come to the desktop, as soon as quality of service (QoS) capable networks are easy to program for. Therefore, we propose a QoS application programmer interface (API) for the Java language, which provides an abstraction from QoS capable networks and allows a simple specification of QoS parameters. Platform independence is achieved by Java, which also allows easy integration with the World Wide Web if the classes are used within applets. We first describe our architecture, which abstracts from different QoS capable transport protocols and networks and provides easy to use access to QoS networking. We identify different layers within a distributed multimedia application and show how mapping of QoS parameters at different layers can be accomplished by our API. Different performance evaluations show that our architecture can reach up to 90% the performance of native code measured during a stream test and up to 87% measured during a request response test.

ATM enforces traffic contracts-applications agree to send not more data then negotiated and the network tries to provide predictable service. Thus ATM based applications should be easy to develop. For wireless ATM the situation remains not the same. Hand-off to new a access point may result in invalidating the old contract and dropping connections. Applications have to cope with the new situation by e.g. adapting their bandwidth needs. In the context of this paper, we propose to shift the burden of adaptation from the application to a middleware layer, which also acts as a central co-ordinator for requesting and reserving local resources. Media adaptation is performed by using media filters which are controlled by the QoS-providing middleware. A distributed resource negotiation protocol negotiates end-to-end application QoS parameters and uses QoS mappers to derive the necessary filter settings.

Multimedia streams are difficult to transmit in full quality and real-time due to their size and constraints imposed by their timely presentation, Especially in slow channels with variable throughput such as typical Internet connections -media streaming is extremely hard, We introduce a new iterative method for the transmission of media streams.The idea is to match each iteration of the transmission process exactly to the play-out time of the respective media stream, thus allowing synchronous presentation of the stream's contents during the transmission.The quality of the playback improves with the number of completed iterations, from rough during the first pass to full quality at the end.The WebMovie system implements this iterative streaming scheme for video data.After the introduction of the system, performance measurements are discussed and compared to conventional methods of movie transmission.

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An exceptionally compact Pascal Compiler was written. We explain how.