Method for monitoring a digital transmission line

A method for monitoring an optical digital transmission line including a plurality of repeaters disposed at predetermined intervals along a pair of optical fiber cables connected between two end stations, comprising transferring controlling information through said optical digital transmission line from one of said end stations, said controlling information comprising a first signal .alpha. repeated at a predetermined period 1/f c for selectively identifying each of said repeaters, said first signal including a first alternating code S.sub.11 with a mark density of 1/2 and a second alternating code S.sub.12 having a polarity inverted with respect to that of said first alternating code S.sub.11, and a second signal .beta. constituting a continuation of said first alternating code;

• controlling each said repeater according to said controlling information by detecting and decoding the controlling information received thereby by extracting a frequency f.sub.c with a first exclusive OR gate and a band pass filter coupled thereto in the respective repeater;
• returning said first signal .alpha. from the respective repeater to said one end station at a repetition frequency f.sub.m that is a function of said respective repeater;
• and
• monitoring said repeaters and said optical digital transmission line at said one end station by extracting the frequency f.sub.m from said returned first signal .alpha. with a second exlusive OR gate and a low pass filter coupled thereto in said end station.