Abstract: A method and an arrangement for synchronizing two more individual communication networks of the time multiplex type, in order to form a composite network thereof, wherein the individual networks having nodes, in which cyclic transmission of time frames is performed, which include time slots intended for the data transmission. One node in the composite network is assigned the role as a superior master node determining the transmission speed of the individual communication networks. This is obtained by adding a fixed idle pattern to each time frame set out from the superior master node, followed by a fixed triggering pattern as a start of the next time frame. At the receipt of the triggering pattern at a master node in an individual communication network, which synchronizes the data transmission of the communication network, the master node starts sending a new time frame.