Abstract: A data transmission system comprises a transmitter transmitting data messages to n receivers by means of a link. Each message is preceded by a preamble. Each preamble is such that it can either be recognized only by one of the receivers (to whom, alone, the message is directed) or it can be recognized by all receivers when the message is intended for all of them. Each preamble is recognized if at least x of its y bits are recognized. At each receiver, the received preamble is sampled at a multiple of the bit rate and the respective pulse trains produced by the same-numbered samples in each bit period are individually compared with the two predetermined preambles to be recognized by that receiver.