A process and system for controlling the use of satellite transmission capacity in terrestrial networks for the substitution of out-of-order data lines. The circuit/process is characterized in that an independent, local control apparatus, which monitors a backup terminal with the software specially written for this application, cooperates in limited manner with the data transmission device of the user and, based on the analysis of a data control signal, detects the need for alternative routing. Via communication lines and modems, the control apparatus switches on the transmission carrier of the affected satellite modem which is connected to a satellite antenna. The satellite antennas are in communication with the satellite via the transmission carrier. All other, non-affected terminals in the network also receive the transmission carrier of the affected satellite modem. The transmission capacity of the asynchronous overhead of the satellite modem is used for the transmission of destination addresses. A hub is provided, which is connected via an interface card or a modem to the terrestrial network that, in turn, is connected via lines to routers. The line is assumed to be out of order. Customer devices or terminals are connected via lines to the routers. The system-specific software-controlled circuit permits independent, decentralized administration of the satellite transmission channels by many satellite terminals without participation of a controlling central station. Thus, even when the terrestrial transmission path is out of order, a free-running alternative routing via a different medium has been achieved.