Method for monitoring a digital transmission line

DETAILED DESCRIPTION The present invention overcomes the drawbacks noted above in the prior art, and it seeks to provide a digital transmission line monitoring control system capable of transmission line monitoring control without use of mark density modulation and with the mark density held at 1/2 at all times.
A feature of the invention is that use is made as a monitoring control signal of an alternating code with the polarity thereof inverted at a certain frequency for out-of-service monitoring of a digital transmission line.
More specifically, according to the invention there is provided a monitoring control system for a digital transmission line including a repeater provided between two end stations, wherein an alternating code is transmitted, with the polarity thereof inverted at a frequency determined by monitoring control information, from said repeater or one of said end stations for transmission of information for the transfer of control information from repeater station to said end stations or from said end stations to said repeater station, on the information reception side an exclusive OR of said received signal and an alternating code obtained through frequency division of a clock signal extracted from said received signal is obtained, said frequency component is reproduced from said exclusive OR, and monitoring control information transmitted from said information transmission side is identified in a step of generation of the reproduced frequency component.