A data signal is transmitted in a wireless communication system from a primary station to a secondary station. The data signal comprising a medium access control header (MAC-hs header) and a protocol data unit associated with the medium access control header. The medium access control header comprising a set at least one field (e.g. TSN, SEG, LI, FMT, LCid), wherein a size allocated to at least one field of the set is determined based on a length of the protocol data unit.