A method and apparatus of communicating data between a cell based network and a packet based network. Received segments of data units are formed into new data segments which are transmitted irrespective of whether a complete data unit has been received. The received segments may be stored, used to form cells and/or transport packets, and transmitted to the cell based network or the packet based network. Also, conditions may be detected, such as timer expiration or a maximum reception capacity achieved, before the segments are used to form the cells and/or transport packets. Forming transport packets may comprise including segments belonging to a specific data unit in a transport packet and/or adding an indication of a specific data unit to which a segment belongs. These operations may be performed by a base station, an interworking unit, or a radio network controller.