In the case where information must be sent continuously into a SCADA network, the deployment of inbound/outbound Unidirectional Gateways is used. An outbound Unidirectional Gateway replicates servers from the plant to the external network, and a second, independent inbound gateway replicates servers from the external network into the industrial plant.
The two inbound/outbound gateway systems are completely independent. In practice, they are most often set up on different network segments, so that software elements of the two gateway solutions cannot exchange messages with each other. In an inbound/outbound gateway configuration, the outbound gateway hardware is unable to send any attack, or any information at all, back into the protected site.