The routes for transmitting optical signals between the nodes (N1-N6) of a transparent network are selected according to the estimated error rate (ERe) values presented by the signals received after being transmitted along these routes. To limit the number of measurements to be made before the network is commissioned, a function (G) relating to the parameters characteristic of the network's optical links is used. This function is an interpolation function providing, for each set (QoT) of parameters associated with a given route, a corresponding estimated error rate (ERe) value. To evaluate the accuracy of the interpolation, during the network's operation, error rate measurements are performed, relating to signals effectively transmitted along given routes, and these measurements serve either to readjust the margin of uncertainty to be applied to exploit the estimated error rates (ERe), or to modify the function (G) to improve its interpolation accuracy.