Optical transmission system wherein protection terminal equipment converts alarm indication signal before relaying same to working terminal equipment

An optical transmission system for communicating between first and second terminal stations, having a working system providing a first connection between the first and second terminal stations and a protection system used as a stand-by system of the working system and providing a second connection between the first and second terminal stations, said optical transmission system comprising:

• first and second switch control circuits located in the respective first and second terminal stations;
• first and second working optical transmission lines, each having first and second ends respectively disposed at and coupled to the first and second terminal stations, transmitting respective first and second optical signals from the respective first and second terminal stations to the other terminal station;
• first and second protection optical transmission lines, each having first and second ends respectively disposed at and coupled to the first and second terminal stations, transmitting respective third and fourth optical signals from the respective first and second terminal stations to the other terminal station;
• first and second optical working system terminal units located in the respective first and second terminal stations and terminating the respective first and second ends of said first and second working optical transmission lines;
• first and second optical protection system terminal units located in the respective first and second terminal stations and terminating the respective first and second ends of said first and second protection optical transmission lines;
• a first bidirectional optical transmission line connecting said first optical working system terminal unit with said first optical protection system terminal unit;
• a second bidirectional optical transmission line connecting said second optical working system terminal unit with said second optical protection system terminal unit;
• each of said first working transmission line, said second working transmission line, said first protection transmission line and said second protection transmission line further comprising first alarm indication signal generating means for detecting an abnormality in a respective one of said optical signals and, in response thereto, generating a first alarm indication signal and for transmitting said first alarm indication signal through said respective transmission line, toward a downstream side of said respective transmission line;
• and
• each of said first and second optical protection system terminal units further comprising means for transmitting a second alarm indication signal which does not activate said switch control circuits, through one of said first and second bidirectional optical transmission lines connected to said one of said first and second optical protection system terminal units receiving said first alarm indication signal, in response to said first alarm indication signal, and preventing transmission of said first alarm indication signal.

A transmission system for communicating between first and second terminal stations, having a working system providing a first connection between the first and second terminal stations and a protection system used as a stand-by system of the working system and providing a second connection between the first and second terminal stations, said transmission system comprising:

• first and second switch control circuits located in the respective first and second terminal stations;
• first and second working transmission lines, each having first and second ends respectively disposed at and coupled to the first and second terminal stations, the first working transmission line transmitting a first signal from the first terminal station to the second terminal station, and the second working terminal transmission line transmitting a second signal from the second terminal station to the first terminal station;
• first and second protection transmission lines, each having first and second ends respectively disposed at and coupled to the first and second terminal stations, the first protection transmission line transmitting a third signal from the first terminal station to the second terminal station, and the second protection transmission line transmitting a fourth signal from the second terminal station to the first terminal station;
• first and second working system terminal units located in the respective first and second terminal stations and terminating the respective first and second ends of each of said first and second working transmission lines;
• first and second protection system terminal units located in the respective first and second terminal stations and terminating the respective first and second ends of each of said first and second protection transmission lines;
• a first bidirectional transmission line connecting said first working system terminal unit with said first protection system terminal unit;
• a second bidirectional transmission line connecting said second working system terminal unit with said second protection system terminal unit;
• four signal repeaters respectively inserted between said respective first and second ends of said first working transmission line, said second working transmission line, said first protection transmission line and said second protection transmission line, each of said signal repeaters comprising first alarm indication signal generating means for detecting an abnormality in the respective first through fourth signals and, in response thereto, generating a first alarm indication signal and for transmitting said first alarm indication signal through said respective transmission line, toward a downstream side of said respective transmission line;
• and
• each of said first and second protection system terminal units, in response to receiving said first alarm indication signal, transmits a second alarm indication signal which does not activate said switch control circuits, through one of said first and second bidirectional transmission lines connected to said one of said first and second protection system terminal units receiving said first alarm indication signal and does not transmit said first alarm indication signal.