A method and a GGSN support node for sending data packets to a mobile station in a mobile communications system from an external communication system. The GGSN receives data packets from the external communication system in a first plurality of data flows which it maps to a second plurality of data flows in the mobile communications system. It establishes at least one filter for controlling the mapping and associates the filter with the mobile station. It also maps at least one of the data flows on the basis of the filter and configures the filter on the basis of information which preferably originates from the mobile station.