The evolution of computers and networks leaves no doubt about the important role of interactive multimedia services. The quality of these new services will be a key issue for their wide deployment, and this quality is determined by the opinion of the users. The best Quality of Service (QoS) is not the highest, but the most suitable to the different users' needs. In order to provide a suitable level of QoS, an application needs to know which relevant network parameters have impact on the quality as it is perceived by the users. This paper presents a top-down approach to study the influence of network performance parameters on the user perceived quality, taking conversational speech as an example of a simple, essential and demanding application. Three types of network are considered: IP, ATM and IP over ATM. For each of these cases, the influence of the corresponding network performance parameters is studied. 1

Design patterns capture software solutions to specific problems that have evolved over time and reflect many iterations of work. Documenting such patterns promotes proven design and software reuse. There has been a growing amount of work documenting design patterns for security, however, little work specific to VoIP security. In 2005 NIST released a report on recommendations and best practices for securing VoIP, however it lacks the structure, terminology, and ease-ofunderstanding needed for both technical and non-technical audiences that is an inherent feature of design patterns.

In the Internet, transmission systems must deal with congestion in order to keep the stability of the network. However, the model used for congestion control determines some important properties of the traffic. The most important algorithm currently used, found in the TCP protocol, has characteristics that make it unsuitable for videoconferencing systems. The aim of this dissertation is to provide an insight into the field of congestion control for such systems. In particular, this work examines one of the most promising alternatives available, TCP-Friendly Rate Control (TFRC), to answer the question “is TFRC suitable for interactive videoconferencing applications?” This dissertation presents the results obtained with TFRC, focusing on some practical aspects and providing recommendations for the implementation of such a rate-based congestion control system. This work examines the scenarios where TFRC is an adequate solution, exposing the behavior that can be expected and suggesting future improvements. The thesis also presents the experiences of integrating TFRC in the UltraGrid videoconferencing application. It shows the difficulties found, demonstrating that this integration