Abstract: Disclosed is a method of switching between redundant routes of a communication system in which terminal stations are connected by an uplink transmission line and a downlink transmission line that are devoid of redundancy. The method includes adopting working/protection redundancy for first and second apparatuses constituting each terminal station, detecting line failure, which has occurred on the side of the first terminal station, by the second terminal station, multiplexing additional data, which includes a line failure alarm, onto main signal sent from the second terminal station to the first terminal station (or inserting the line failure alarm in the form of overhead bytes), and causing the first terminal station to perform line switching upon detecting the alarm included in the additional data or overhead bytes.

Abstract: Disclosed is a method and apparatus for preventing the occurrence of surge light that may be caused by an off-to-on transition of input signal light in an optical amplifier using an erbium-doped optical fiber. An input electrical signal, optical modulator output, leakage light, or light output is monitored to detect the off-to-on transition of the signal light, and the energy level of the input signal light or pumping light is gradually increased from a low level to an operating level.