In a data packet, e.g. a PDU of the UMTS, a length indicator field of variable granularity is created, depending on the exact length of the payload to be transmitted. An additional field is provided, a single bit in length, to indicate the granularity of the length indicator field. This increases the resolution of the header, whilst keeping low an overhead of extra field bits. The method comprises: assessing the length of the data to determine the appropriate units, from a plurality of possible units, in which the length should be expressed; setting the granularity field to define the appropriate units in which the length of the data is to be indicated; and setting the length indicator field to indicate the data length.