1. A method of decoding a coded data stream of a plurality of cells, the uncoded data stream comprising a plurality of segments each formed of a preamble of fixed length and a series of data bits, wherein: the coded data stream includes at least two cells and one transition for each data bit and a portion without a transition at the beginning of each preamble, the said portion having the maximum period in the coded data stream without a transition and being longer than a bit period, the method characterised by: detecting transitions in the coded data stream; determining the maximum period in the data stream without a transition; determining a first threshold corresponding to a period of time less than the maximum period and greater than the data bit period; determining whether a considered transition corresponds to a data bit or a preamble based on the time between the considered transition and a preceding transition compared with said first threshold; and if the considered transition corresponds to a data bit, determining the data bit based on the considered transition and the preceding considered transition.