The present invention provides for a method of forming at least one ARQ PDU from an ARQ service data unit (SDU), the ARQ PDU comprising a header portion and a data portion, and the method including selective addition of a Length Indicator field to the said header portion responsive to the determination of the presence in the PDU of the last bit of an ARQ SDU and if a HARQ PDU is formed of a plurality of ARQ PDUs wherein the last of the ARQ PDUs is arranged not to include an LI within its header if it is found not to be carrying the last bit of the ARQ SDU and LI will be added to all other ARQ PDUs though they are not carrying last bit of ARQ SDU.