The present invention relates to a data packet header comprising a first bit field consisting of a first sequence of bits with identical values and a second bit field consisting of a second sequence of bits, the second sequence of bits being subject to a run-length restriction, and located immediately after the first sequence of bits in the order of transmission. The number of bits in the first sequence is equal to or exceeding the run-length restriction and the bits of the second sequence have opposite identical values to the value of the first bit of the second sequence of bits. The invention equally relates to a correlator for detecting a data stream and arranged to determine a correct clock phase for the data stream.