A system and a method provide a real-time-critical communication and a non-real-time-critical communication in a switched data network consisting of users and switching units, for example a distributed automation system, by a cyclic operation. In a transmission cycle, there exists for all users and switching units of the switched data network in each case at least one section for transmitting real-time-critical data and at least one section for transmitting non-real-time-critical data, as a result of which the real-time-critical communication is separated from the non-real-time-critical communication. Since all users and switching units are always synchronized to a common time base, the respective sections for transmitting data in each case take place at the same time for all users and switching units, i.e. the real-time-critical communication takes place independently in time from the non-real-time-critical communication.