The invention relates to a method of transmitting packet data from a first subsystem to a wireless terminal device via the network node of a second subsystem. In the method, at least one filter is formed to guide mapping of at least one data flow of the first subsystem to at least one data flow of the second subsystem. At least one filter is attached to at least one data flow of the second subsystem. The filter comprises at least part of the interface identifier of the IP address, which at least part of the interface identifier is allocated in the wireless terminal device. Hereby, mapping of the data flow of the first subsystem to the data flow of the second subsystem is performed on the basis of the interface identifiers of the destination IP addresses in the received packets.