A communication device and method for operating a communication system for transmitting time-critical data, wherein a respective individual time window within predefined time intervals is specified for data flows assigned to selected control applications running on terminals, where time windows each have an individual cycle time that is a multiple of a general cycle time or corresponds to the general cycle time, first and second communication devices each check, for the selected control applications, whether a specified time window is available for data transmission, where information about a beginning of the time window within the predefined time intervals is in each case transmitted to the terminal upon which the respective selected control application is executing in the event of an available time window, and where data flows that are assigned to selected control applications are each transmitted according to the information about the beginning of the individual time window.