In a communication system a transmission-end input data flow with serially successive data is transmitted via a time-variant transmission channel. The transmission-end input data flow is divided into individual words. An a-priori reliability value is determined for each position of a transmittal data block by transmission-end modulation methods and coding methods, and the words of the input data flow are associated with corresponding positions of the transmittal data block according to the a-priori reliability values, and transmitted. An a-posteriori reliability value is formed on the receiving end for each word of the transmittal data block. The words having an a-posteriori reliability value falling below a pre-determined minimum value are re-requested and re-transmitted by acknowledgement of the corresponding positions.