The invention relates to a method of operating an optical transmission system (100a, 100b, 100c), wherein at least one optical data signal is transmitted over an optical transmission link (120), which particularly comprises at least one optical fiber (120a).The inventive method is characterized by modulating (200) said data signal with a test signal (s) having a predetermined modulation frequency fmod to obtain a modulated data signal (Pin), by receiving (210) a reflected portion (Pback) of said modulated data signal (Pin), and by determining (220) a fiber quality measure (a) depending on said received reflected portion (Pback) of said modulated signal (Pin).