Abstract: An apparatus and a method for monitoring an optical transmission line. An optical pilot signal of a predetermined duration is transmitted along said optical transmission line and a return signal is sent back from a signal returner along the same transmission line corresponding to at least part of said optical pilot signal. The signal returner is positioned at a predetermined point along the optical transmission line. An optical detection apparatus detects said optical pilot signal and also detects said return signal. A monitoring unit receives detection signals from said optical detection apparatus and determins the time relationship between the predetermined duration of the optical pilot signal and the round-trip transit time of the optical pilot signal. A first monitoring signal is generated when the determined time relationship has a predetermined value, and at least one further monitoring signal is generated in other cases.

Abstract: To diminish the risk of interruption of traffic in a submarine optical cable system in the region of the shore where damage is most likely to occur, the landing-stage of the system has duplicated spaced-apart cables between a submerged branching unit and an onshore submarine line terminal endstation. Various changeover modes are disclosed, including those where both cables are coupled to signals at all times and changeover is accomplished by blocking amplifiers.

Abstract: An apparatus and a method for monitoring an optical transmission line. An optical pilot signal of a predetermined duration is transmitted along said optical transmission line and a return signal is sent back from a signal returner along the same transmission line corresponding to at least part of said optical pilot signal. The signal returner is positioned at a predetermined point along the optical transmission line. An optical detection apparatus detects said optical pilot signal and also detects said return signal. A monitoring unit receives detection signals from said optical detection apparatus and determins the time relationship between the predetermined duration of the optical pilot signal and the round-trip transit time of the optical pilot signal. A first monitoring signal is generated when the determined time relationship has a predetermined value, and at least one further monitoring signal is generated in other cases.