In a mobile communication system (T1) a time interval selection means (TISM) in a network control means (RNC) determines a time interval and sends an indication about this time interval to a subscriber station (MS) in a time interval indication signal. A time interval signal determining means (TISDM) in the subscriber station (MS) detects the time interval and an IF measurement means (IFMM) carries out inter-frequency/inter-system measurements in the detected time interval specified by the network control means (RNC). In this time interval the temporary reduction of the quality of service QoS on the communication connection (CC) is planed by the network control means (RNC). However, independent as to whether a delay-sensitive or loss-sensitive data transmission is carried out, the network control means (RNC) can make provisions in order to compensate the temporary reduction of the quality of service. Such procedure is superior to performing IF measurements in an idle time interval of a compressed time slot in which a temporary degradation of the quality of service must always be accepted due to a compressed mode of operation.