The invention relates to an efficient method for operating a packet-oriented communication system, and for classifying a packet data stream, comprising the steps of: acquiring measurement values (10) with respect to the size of the time intervals and the direction of transmission of the data packets of the packet data stream, examining the measured values (15, 20, 25, 30) and, if a transmission of data packets of similar size at regular short intervals is bidirectional intermittently only in individual separated by pauses data transmission sections, Examining the time length of the individual data transmission sections as well as the time length of the individual pauses, determining (40) if the time length of the data transmission portions and the temporal length of the pauses in accordance with predetermined respective reference distributions with characteristic parameters for voice telephony are randomly distributed, and if so with a temperature above a threshold statistical significance of the case (45), classifying the packet data stream as voice telephony (50). The invention further relates to a network node of a packet-oriented communication system.