<span lang="en">Data network services have been developed since their begining and nowadays they are showing a great expansion. This is the case of the streaming services such as voice transmision over IP (VoIP). The technology is in an advanced state, however, the Quality of Service (QoS) measurement and evaluation are still being studied. This paper analyzes the relationship between the user's perceived quality and the impairment factors of the network, as well as, the methodology followed to measure the required parameters. The results are expected to be interesting not only for the users but also for the Internet providers, so that it gives them the oportunity to improve the service offered in an optimal way.</span>