For determining OSNRreal of a real optical signal carried in an optical network link, the following has been proposed: tapping a portion of the real optical signal, altering the tapped signal portion by adding to it in-band artificial noise signal, thus obtaining a combined signal, scattering the combined signal by stimulated Brillouin or Raman scattering (SBS or SRS) in an optical element, extracting a signal back reflected by SBS/SRS from the optical element, determining OSNRcomb of the back reflected signal and deriving the OSNRreal from the OSNRcomb knowing absolute and/or relative power of the added artificial noise.