1. A method of evaluating or estimating the performance of a specific user service executed in a communication network, said user service relating to the actual content or result delivered to and perceived by users consuming said user service, comprising the following steps: identifying one or more user-independent system services required for executing said user service, each system service being technically provided by equipment and protocols used in the network and/or user terminals, measuring a limited set of technically measurable system service key performance indicators S-KPIs for each identified system service, the S-KPIs being selected and established to reflect the performance of the respective system service(s), wherein relevant data for measuring said S-KPIs is collected from sensors and counters from the following sources: a) infrastructure data, providing information on how the involved nodes handle the delivery of services, b) traffic data, containing information for each communication session when executing the user service for each user, as measured by at least one passive probe, and c) end user data, as measured by at least one active probe in the user equipment, evaluating or estimating the performance of said user service based on the measured S-KPIs; and wherein the SKPIs are measured by aggregating data collected from sensors and counters installed in the communication path, when the respective system services are executed.