Sascha Kümmel

Design and implementation of a support platform for distributed mobile computing

With the rapid development of mobile computer systems and mobile communication infrastructures, a broad field of distributed mobile computing is enabled. The paper first discusses these developments in closer detail and summarizes the resulting requirements concerning adequate software support. An application scenario of the service engineering area illustrates specific aspects including bandwidth and location management, dynamic configuration, resource heterogeneity, disconnection, and security. Based on these considerations, a generic software support platform for distributed mobile computing is derived. It addresses several of these aspects by providing application-independent and reusable support services. In particular, it offers a framework for organizing distributed mobile applications into manageable domains, it equips mobile stations with enhanced functionality for location, resource and bandwidth management, and it uses industry standard RPC communication facilities for enhanced portability. The design, implementation and use of the support platform is illustrated based on a specific part of the application, a mobile multimedia e-mail system. Experiences and implementation aspects in this context are particularly emphasized.

Protocol support for optimized, context sensitive request/response communication over connection oriented networks

This paper describes a research project on specification and implementation of a fast, reliable and context sensitive transfer system. The design considers the behavior of various connection oriented networks and the requirements of request/response based communication schemes. First we describe the current situation. This is based on recent measurements and experiences with remote procedure call (RPC) over ATM and GSM networks. The major basis is the RPC of the OSF Distributed Computing Environment. Furthermore, we discuss different approaches and existing solutions including VMTP, IIOP, RMI, IPv6 and XTP.

QoS support for advanced RPC interactions

QoS support is necessary for nearly every communication between computers. We present an approach for supporting request response communication behavior with particular respect to an ATM LAN environment. The goal is to extend the RPC protocol with mechanisms for QoS-specification mapping and -monitoring for the effective transmission of data in such an ATM environment. As a first basis for validating our conceptual work, we present a facility to transmit multimedia data by extending the RPC pipe mechanism.

Infrastructure support for cooperative mobile environments

With the widespread use of distributed systems on one hand and the rapid deployment of mobile computing and communication infrastructure on the other, it becomes important to link both technologies together. This paper first outlines new problems arising from distributed mobile computing and then presents a software support architecture and system for mobile applications. We then discuss a system model for structuring mobile applications, a station software infrastructure for managing resource access in dynamic mobile environments, and a description technique for specifying behavioural aspects of mobile applications. The implementation is based on the remote procedure call of the OSF Distributed Computing Environment and on Microsoft RPC. First experiences with our prototype are reported and directions for future development are outlined.

RPC over advanced network technologies: evaluation and experiences

This paper describes measurements and experiences with remote procedure call (RPC) over different network technologies, and with different levels of functionality. The major basis is the RPC of the OSF Distributed Computing Environment. We first discuss performance based on local area networks; also including secure RPC features. We then present performance results over WAN connections via the Internet and compare them with our findings in LAN settings. Another major focus area is the investigation of the RPC behavior using an ATM network as a basis for transmission. Major limitations and further requirements of client/server interactions in high performance networks are illustrated. The RPC is also evaluated in mobile environments using a cellular, GSM-based WAN. In this domain, various additional requirements have to be considered as discussed in the paper. Finally, steps towards a new integrated and enhanced request-response protocol are pointed out as a conclusion.

An adaptive data distribution system for mobile environments

Common transport systems lack the appropriate mechanisms to deal with the problems of mobile computing systems (e.g. temporary inaccessibility, transient network addresses of mobile hosts, along with varying quality of service parameters of physical network connections). Therefore, a need for new adaptive data distribution mechanisms.

Distributed Compression of Live Video - An Application for Active Networks

A rising number of video conferencing applications uses software codecs for video compression. This allows to support several compression schemes without the necessity for expensive dedicated hardware for every new standard. Using inexpensive framegrabber boards, it is possible to support several different compression schemes as H.261 or H.263. But the computing power of todays desktop system does usually not allow for the simultaneous compression and decompression of high quality video. One solution is to equip these systems with dedicated hardware, trading off the above mentioned flexibility and introducing considerable costs per machine. This paper presents an approach for the distributed compression of live video, exploiting cheap LAN capacity and thus dividing the compression process into a short preprocessing on the desktop and a second stage performing the more complex operations on a dedicated compression server, that may be shared by a workgroup or even more people.

A Framework for High Quality/Low Cost Conferencing Systems

This paper presents a framework for the development of advanced video conferencing systems with very high quality. The design and performance of the developed components like session management, real-time scheduler and a specific transport system are outlined. The core of the framework consists of a toolkit for the processing of audio and video data. It integrates capturing, compression, transmission and presentation of media streams into an object oriented class library. The specifics of audio/video hardware, preferred operation modes, compression algorithms and transport protocol properties are encapsulated. Due to the achieved abstraction level complex application software can easily be developed resulting in very short design cycles. Contrary to existing systems the developed application is regarded in its entirety. The focus on highest possible performance throughout all components is unique to our approach. Due to a number of optimisations, specific compression hardware is not required which leads to low overall system costs.

Off-Line Verteilung multimedialer Daten in mobilen Systemen

Der vorliegende Beitrag beschaftigt sich mit der Off-Line Verteilung multimedialer Daten in mobilen Systemen. Es werden die Problematiken der Themenfelder multimedialer Datenaustausch, Verteilung von Informationen und Anforderungen durch Nutzermobilitat unter diesem Gesichtspunkt analysiert. Daran anschliesend werden aus den spezifizierten Problemen die Anforderungen an ein System zur multimedialen Informationsverteilung herausgearbeitet. Es wurde ein adaptiver Transportdienst zur Verteilung von multimedialen Informationen in mobilen Umgebungen konzipiert. Die Implementierung des Systems ermoglichte praktische Untersuchungen, die zur Validierung herangezogen wurden.