A method in an arrangement for monitoring a transmission line to a radio device in a communication network, wherein the transmission line comprises an optical part carrying signals in an optical domain and an electrical part carrying analog information signals in an electrical domain. The arrangement selects a carrier in the electrical part for monitoring the transmission line by assigning a test signal to a subcarrier out of a number of subcarriers of an optical carrier, wherein the subcarrier in the optical domain corresponds to the selected carrier in the electrical domain. Furthermore, the arrangement monitors the transmission line by analyzing a signal, associated with the test signal, received over the assigned subcarrier or another subcarrier.