One network protocol (RTP) each, having data packets (dp) comprising an expandable header (KE) is provided for a data stream (ds1 . . . n) encoded in a manner individual to said data stream, and the key information (si1 . . . n) formed in a data stream manner individual to said data stream is inserted into an expandable header (RTPH) of a data packet (dp) of the respective data stream (ds1 . . . n) and transmitted. The key information (sp1 . . . n) is selected in a manner individual to said data stream from the expanded headers (KE) of received data packets (dp) of the respective data stream (ds1 . . . n), and the associated encoded data stream (ds1 . . . n) is decoded by means of at least one piece of selected data stream individual key information (si1 . . . n). The forming and inserting of key information (si1 . . . n) into standard expanded headers (KE) can be performed with little additional expense, thus significantly reducing the expenditure for the analysis or diagnosis of the simultaneously transmitted encoded data streams (ds1 . . . n). Advantageously, the insertion of key information (si1 . . . n) can be activated or initiated only if the diagnosis or analysis and/or the recording of the data streams is currently being performed.