Abstract: Communication system for exchanging data, via a communication network at least partly accessible to the public, between a first data processing system and a second data processing system, includes a first data processing system adapted to communicate according to a communication protocol, wherein the first data processing system is linked via a first coupling to the communication network at least partly accessible to the public, and a second data processing system, includes at least one data processing system, adapted to communicate according to the communication protocol, wherein the second data processing system is linked via a second coupling to the communication network at least partly accessible to the public, characterized in that at least one of the couplings comprises a data connection inaccessible to data traffic according to the communication protocol.