This thesis considers aspects of synchronous group communication regarding the scalability with respect to the number of participating users is such scenarios. The goals of the thesis are twofold:

  • Design und development of a scalable conferencing service
  • Modeling groupware scenarios for evaluation

In the first part of the thesis, related work in the area of conferencing is presented, e.g. the H.323 standard of the ITU or the approaches of the IETF working groups. The outcome of this presentation is that current systems mainly suffer from bad scalability in terms of participating users or from restrictions of provided functionality.
As a consequence, an own approach for a conferencing service is proposed (Scalable Conferencing Control Service, SCCS) following the paradigm of tightly coupled environments, i.e. the system is built of interconnected providers in a tree. Apart from the provision of main conferencing services, a major effort is concentrated on the scalability of the service. For that, specific protocol mechanisms are developed to shorten the response time of the system. Furthermore, underlying services on network and transport layer are used as efficient as possible to improve the scalability for control as well as user data traffic.
The provided services, the used protocol mechanisms, and the implementation design of SCCS are presented in the thesis.

In the second part, a modeling methodology is proposed for the evaluation of SCCS protocol mechanisms, called Group Communication Description Language (GCDL). This methodology provides a framework for modeling group communication scenarios in general and therefore covers a wide spectrum of requirements for such modeling techniques. The framework defines an object model for structural description of the system under consideration. Furthermore, a process model is proposed for the instantiation of the involved entities with a specific scenario.
In general, GCDL is designed to be independent from the specific behavior description of the different system entities. With this, analytical as well as simulative evaluation of group communication systems should be enabled. In the thesis, the specification language SDL is used for behavior description, which is mapped onto an event-based simulation applying a proposed simulation framework. For an analytical evaluation of groupware systems, an approach is proposed using modules of stochastic Petri Nets for the evaluation.

The proposed modeling methodology is used for the evaluation of two SCCS protocol mechanisms, namely the resource management and the dynamic reconfiguration. On the one hand, the performance evaluation depicts the improved scalability of the used protocol mechanisms. On the other hand, the applicability of the modeling framework is demonstrated. For the evaluation, complex group communication scenarios are considered with complex social protocols describing the interaction among the participating users in the system.