The invention relates to transfer of packet data from a first subsystem via a network node in a second subsystem to a terminal in the second subsystem. The method includes negotiating identifiers identifying a data flow of the first subsystem via a separate signalling element during the set-up of an application-plane logical connection of the terminal. The identifiers are transmitted from the signalling element to the network node. In the method, a filter is generated at least on the basis of the identifiers for directing the mapping of at least one data flow of the first subsystem to at least one data flow of the second subsystem. The filter is bound to at least one data flow of the second subsystem, and at least one data flow of the first subsystem is mapped to at least one data flow of the second subsystem on the basis of the filter.