A system and a method of transmitting data via data networks. A bi-directional data connection is created even behind firewalls, including from a client not visible to a server in the Internet. In the method and system, a first connection request for establishing a first transmission channel is sent to a server of an automation system from a first data processing device of a client via a data connection. This request is answered by the server via a second transmission channel. The data connection is kept permanently open, thereby allowing mutually time-independent bi-directional sending and receiving of data between the client and the server, via at least one data network. A COM automatic marshaller which incorporates itself into the communications chain between the client and the server is provided.