The invention enables a distinction to be made between at least two types of time slots in serial digital bit streams that are transmitted over a link (2) between a transmitter (4) and a receiver (6) and that derive from the streams of parallel data bits which are delivered to the transmitter via an interface and subjected to parallel-series conversion in the transmitter. Subsequent to analyzing the parallel data bits in a time slot, the type of category to which this time slot belongs is decided and a plurality of marker bits are then inserted into the serial time slots, so as to obtain good transmission properties and also to restore the time slots to a parallel form. More specifically, the time slot categories are mutually separated with the aid of a different number of marker bits in the time slot, all of these marker bits having been assigned the inverted value of a bit other than the marker bits.