Provided is a computer-implemented method and a transmission apparatus for transmitting data between a first network and a second network having high and low security requirements, wherein a first session is set up between the first and second networks, a first data packet is transmitted from a transmitting unit in the first network via a first one-way communication unit to a receiving unit in the second network, and a second session is set up and a second data packet is transmitted from a transmitting unit in the second network via a second one-way communication unit to a validation unit, the second data packet is validated in the validation unit on a prescribed rule, positive validation of the second data packet results in a third session being set up, and the second data packet is transmitted from the validation unit to a receiving unit in the first network.