An optical time domain reflectometry system , abbreviated OTDR system, adapted for diagnosing a passive optical network PON comprising an optical line termination , abbreviated OLT, an optical distribution network , abbreviated ODN and a plurality of optical network units (131, 132, 133, ... 13n), abbreviated ONUs, said OTDR system comprising: - a transmitter adapted to transmit in the downstream direction of said ODN a transmitted optical signal having a wavelength substantially corresponding to the Raman wavelength of an ONU transmitter in one ONU of said ONUs (131-13n); - a receiver adapted to receive a backscattered optical signal, resulting from reflections of said transmitted optical signal in said ODN ; and - an OTDR manager configured to: - control said transmitter to transmit said transmitted signal in a first silent time window wherein none of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a first backscattered signal resulting from transmission in said first silent time window; - control said transmitter to transmit said transmitted signal in a second time window wherein said one ONU is controlled to upstream transmit and no other ONU of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a second backscattered signal resulting from transmission in said second time window; and - subtract said first backscattered signal from said second backscattered signal to thereby obtain a reflectometry measurement for the drop section of said ODN towards said one ONU . - a transmitter adapted to transmit in the downstream direction of said ODN a transmitted optical signal having a wavelength substantially corresponding to the Raman wavelength of an ONU transmitter in one ONU of said ONUs (131-13n); - a receiver adapted to receive a backscattered optical signal, resulting from reflections of said transmitted optical signal in said ODN ; and - an OTDR manager configured to: - control said transmitter to transmit said transmitted signal in a first silent time window wherein none of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a first backscattered signal resulting from transmission in said first silent time window; - control said transmitter to transmit said transmitted signal in a second time window wherein said one ONU is controlled to upstream transmit and no other ONU of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a second backscattered signal resulting from transmission in said second time window; and - subtract said first backscattered signal from said second backscattered signal to thereby obtain a reflectometry measurement for the drop section of said ODN towards said one ONU . - control said transmitter to transmit said transmitted signal in a first silent time window wherein none of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a first backscattered signal resulting from transmission in said first silent time window; - control said transmitter to transmit said transmitted signal in a second time window wherein said one ONU is controlled to upstream transmit and no other ONU of said ONUs (131-13n) is allowed to upstream transmit; - control said receiver to receive a second backscattered signal resulting from transmission in said second time window; and - subtract said first backscattered signal from said second backscattered signal to thereby obtain a reflectometry measurement for the drop section of said ODN towards said one ONU . <EOS>