In a mobile communication network, the data traffic is mapped to a number of bearers (52, 54) between a terminal (10) and a gateway (26). Upon a triggering event, data packets of a specific type, which are to be monitored, are assigned to a monitoring bearer (54), which is dedicated for performance monitoring purposes. The monitoring bearer (54) and a transport bearer for carrying data packets of said specific type between a non-monitored terminal and the gateway (26) have the same traffic forwarding policy. Data packets may then be filtered from the monitoring bearer (54) at a desired node between the terminal (10) and the gateway (26), and a performance parameter may be evaluated on the basis of the filtered data packets.