For switching or transmitting data packets, one can provide communication systems which consist of several modules —operating in parallel on segments of a packet —to increase speed and handling capacity. One module acts as master, others are slave modules controlled by control signals derived by the master module. It is important to correctly synchronize in each module the data segment and the respective control signal of each packet, because in large systems the data paths carrying packet segments and the control signal paths may have substantially different delays. The invention provides for measurement of the propagation delay differences and for introducing a controlled delay in each slave module, so that data segments and control signals can be correctly correlated by delaying either the one or the other. Synchronization packets are transmitted besides normal data packets, for obtaining time stamps which are used to determine the delay difference.