28.2.3 Failure monitoring method

Ring faults are monitored under the Ring Protocol by having the master node regularly send control frames called health check frames, and then monitor whether or not these health check frames were received. When a health check frame does not arrive within a fixed time, the master node determines that a ring fault has occurred, and performs fault operations. Also, when a health check frame is received again during a ring fault, the master node determines recovery from the ring fault, and performs restoration operations.