A method of data segmentation in a transmitter of a mobile communications system includes initiating alternate extension bit interpretation, a last data segment of a Service Data Unit exactly ending at an end of a Protocol Data Unit, no Length Indicator in the Protocol Data Unit indicating the end of the Service Data Unit, a length of a next Service Data Unit following the Service Data Unit exactly equaling to a length of a data field of a next Protocol Data Unit following the Protocol Data Unit, and setting a first extension bit of the next Protocol Data Unit to be a specified value for indicating that the next Protocol Data Unit comprises a complete Service Data Unit, which is not segmented, concatenated or padded, and indicating that a Service Data Unit exactly ended in the Protocol Data Unit.