In a circuit switched mobile telecommunications network a performance analysis is provided. In accordance with the invention a method is implemented, which can be carried out both OFF-LINE and ON-LINE, wherein transport channels carrying the circuit switched traffic of a service are found, then bit-streams of the circuit switched traffic are demultiplexed. A traffic database is built and a set of Key Performance Indicators and usage measures characterizing the service are defined and calculated. A passive traffic monitor is also disclosed, which is attached to a standardized interface of the circuit switched mobile telecommunications network consisting of mobile terminals, radio access network and core network. The passive traffic monitor comprises a trace analyzer connected to a signaling analyzer, to a user plane protocol analyzer and to a traffic database writer. The traffic database writer generating a traffic database is linked to the signaling analyzer, to the trace analyzer and to the user plane analyzer.