In a system, the functionality is decentralized in a data transmission network so that the need of communications produced by and addressed to data terminals to pass by way of a single point in the data transmission network is diminished. A first data transmission installation is adapted to signal second data transmission installations, each of which is part of a ring-shaped data transmission topology, to be capable of building logical data transmission tunnels directed to the data terminals. Thereby, all the logical data transmission tunnels need not commence or, depending on the direction, terminate at the same point of a data transmission network. In addition, the data transmission installations present in a ring-shaped data transmission topology need not necessarily be taught routing information in order to enable each of these data transmission installations to send data transmission frames to be transmitted to another one of these data transmission installations.