Mesure de la qualité dans les réseaux informatiques

Quality is an essential concept for computer networks today given that they are used on a larger and larger scale by an ever growing variety of people. Its study is of utmost importance for enabling judgments on the usefulness and value of computer networks and the services they provide. Quality in general is about the correspondence between the expectations of a subject and the properties of an object. Quality in computer networks has two aspects: quality of service (QoS) and user-perceived quality (UPQ). Quality of service treats the above mentioned correspondence in terms of network performance parameters: throughput, loss and delay ­ a triplet with two degrees of freedom. Perceived quality asserts a higher point of view, that of users, and concerns their expectations in terms of application performance. In this case, application specific metrics are defined, that permit assessing in the most objective possible way the degree of user satisfaction. We developed a methodology and test systems that allow assessing the QoS and UPQ for computer networks and applications running over them. On one hand we were able to evaluate the amount of degradation induced by network elements and the service differentiation introduced by current QoS mechanisms. In the latter case we showed the drawbacks that these mechanisms still have. On the other hand we established the correlation that exists between the network QoS and the application UPQ by simultaneously assessing the metrics associated to these two aspects of quality. The studies on file transfer and IP telephony allowed us to establish in an objective fashion the requirements the network must meet for applications to deliver their users a specified satisfaction level.