The invention at hand concerns a method and a system for transmitting data in an optical transmission system. A measuring signal is generated having a wavelength which differs from the wavelengths of a data signal that includes the data to be transmitted. The measuring signal is coupled in the optical transmission system, reflected after passing through the transmission path and decoupled again. The coupled measuring signal is compared with the decoupled reflected measuring signal. By taking into account the comparison results, a compensation of the change of the data signal resulting from the dispersion in the fiber is performed in such a way that the data included in the data signal can be used.