Method for operating a communication system for transmitting time-critical data, in which - selected datagrams are transmitted from first communication devices at source network nodes to second communication devices at destination network nodes within predefined periodic time intervals, - the selected datagrams are assigned to data streams and are transmitted via paths that comprise third communication devices at intermediate network nodes, - the predefined time intervals are synchronized at all network nodes, - terminals connected to the first communication devices specify, in each particular case, quality of service parameters for the data streams in order to reserve resources to be provided by the first, second and third communication devices, - terminals connected to the second communication devices specify, in each particular case, a data stream identifier for a reservation request, - each communication device along a path for a data stream takes the quality of service parameters as a basis for checking, in each particular case, in the event of a reservation request, whether there are sufficient resources available in the respective communication device for data transmission while observing the specified quality of service parameters, - if there are sufficient resources then configuration control units of the communication devices along a path ascertain, in each particular case, configuration information for a data stream and set up the respective communication device for resource provision for the data streams in accordance with the ascertained configuration information, characterized in that - an individual time window within the predefined time intervals is specified, in each particular case, for data streams assigned to selected control applications running on terminals (201, 202), the time windows having, in each particular case, an individual cycle duration that is a multiple of a general cycle duration or corresponds to the general cycle duration, - the first and/or second communication devices (101, 102) check, in each particular case, for the selected control applications whether a specified time window is available for data transmission, - if a time window is available then information about a beginning of the time window is transmitted, in each particular case, within the predefined time intervals to the terminal (201, 202) on which the respective selected control application runs, - data streams assigned to selected control applications are transmitted, in each particular case, in accordance with the information about the beginning of the individual time window. - selected datagrams are transmitted from first communication devices at source network nodes to second communication devices at destination network nodes within predefined periodic time intervals, - the selected datagrams are assigned to data streams and are transmitted via paths that comprise third communication devices at intermediate network nodes, - the predefined time intervals are synchronized at all network nodes, - terminals connected to the first communication devices specify, in each particular case, quality of service parameters for the data streams in order to reserve resources to be provided by the first, second and third communication devices, - terminals connected to the second communication devices specify, in each particular case, a data stream identifier for a reservation request, - each communication device along a path for a data stream takes the quality of service parameters as a basis for checking, in each particular case, in the event of a reservation request, whether there are sufficient resources available in the respective communication device for data transmission while observing the specified quality of service parameters, - if there are sufficient resources then configuration control units of the communication devices along a path ascertain, in each particular case, configuration information for a data stream and set up the respective communication device for resource provision for the data streams in accordance with the ascertained configuration information, - an individual time window within the predefined time intervals is specified, in each particular case, for data streams assigned to selected control applications running on terminals (201, 202), the time windows having, in each particular case, an individual cycle duration that is a multiple of a general cycle duration or corresponds to the general cycle duration, - the first and/or second communication devices (101, 102) check, in each particular case, for the selected control applications whether a specified time window is available for data transmission, - if a time window is available then information about a beginning of the time window is transmitted, in each particular case, within the predefined time intervals to the terminal (201, 202) on which the respective selected control application runs, - data streams assigned to selected control applications are transmitted, in each particular case, in accordance with the information about the beginning of the individual time window.