US Patent 7574133 Virtual protection channel for fiber optic ring network

A single, normally inactive, protection channel provides protection against transponder failure in a fiber optic ring network. The protection channel is established and put into operation by a series of steps that are triggered by the detection of loss of light from one transponder by the other transponder. Each node reacts to a loss of light from another node in the same way: by rerouting incoming traffic from external sources from a working input-output interface to a protection interface, to establish a path for the protection signal, and by sending a signal enabling the two nodes to resume communication.