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