In order to transmit data packets in multi-RAT networks, a method and system of network controllers are provided including simultaneous establishment of radio connections over multiple different Radio Access Technologies or RATs, with User Equipment or UE 13, being one radio connection firstly established over one primary RAT under a single PDP context and one or more radio connections established over at least one secondary RAT, different from the primary RAT, and under the same single PDP context; simultaneous transmission of data packets over the different RATs towards the UE 13 and combination of the transmitted data by higher layer protocols at the UE 13. In the present method/system, a connection is established for user plane transmission on the same PDP context between a primary network controller 11 of the primary RAT and the secondary network controller 12 of a secondary RAT. More than one secondary RAT may be involved.