A method and modem are provided for obtaining an optimized efficiency of an Orthogonal Frequency Division Multiplex (OFDM) data transmission, such as for power line communication, for example. The length or duration of a guard interval or cyclic prefix in an OFDM symbol is newly selected at each start-up of a modem while initializing or preparing the OFDM data transmission. The length of the guard interval can be given by the number of samples in a time-discrete representation, and the value of such number of samples that is retained for the subsequent data transmission can be selected from a plurality of pre-determined possibilities based on an evaluation of a channel quality of a communication channel including a physical line to which the modem is connected. Hence, the selected value depends on actual transmission conditions, and the optimization potential offered by a more flexible handling of system parameters is exploited to meet changing conditions on the physical line.