Abstract: A method, device, and system for controlling the wavelength of an optical signal. For example, the system includes an E/O converter for converting an input electrical signal into an optical signal, an optical band-pass filter to which the optical signal is supplied, an O/E converter for converting the optical signal passed through the filter into an output electrical signal, a detecting unit for detecting a bit error of the output electrical signal, and a control unit for controlling a parameter (e.g., temperature of a laser diode) on which the wavelength of the optical signal depends so that the bit error detected decreases. In this system, the wavelength of the optical signal is maintained constant, so that a degradation in transmission quality due to wavelength changes is prevented.

Abstract: A transmission system provides a stand-by line switching facility. The transmission system includes a plurality of working line side processing equipments and a stand-by line side processing equipment to replace one of the working line side processing equipments if a fault occurs therein. The transmission system has a first unit and second unit disposed for each working line side processing equipment. The first unit issues, upon receipt of a line switching command, a single line switching control signal. The second units commonly receive the single line switching control signal and respective line switching is performed between the faulty working line and the stand-by line simultaneously.

Abstract: In an optical communication system formed by terminal stations, optical repeaters provided between terminal stations, and optical fiber cables connecting between terminal stations through optical repeaters, the system includes: an optical wavelength multiplexer provided in a terminal station to multiplex a first optical signal having a wavelength .lambda.1 with a second optical signal having a wavelength .lambda.2; an optical wavelength demultiplexer provided in the terminal station to demultiplex the first optical signal from the second optical signal; plural optical repeaters, each having a unit for amplifying the first and second optical signals in accordance with a level of the second optical signal separated from the first optical signal.

Abstract: A phase synchronization pull-in system used in a bit error detecting apparatus includes a monitored circuit for inputting an input signal and processing the input signal when an operation of the monitored circuit is to be monitored, a standard circuit for processing an output of the monitored circuit inversely to the processing in the monitored circuit, a phase synchronization circuit for inputting the input signal of the monitored circuit and adjusting a first delay time of the input signal so that the first delay time coincides with a second delay time of the output of the standard circuit corresponding to the input signal, a bit error detecting circuit for detecting a difference between the input signal, after being delayed through a variable delay circuit, and the output of the standard circuit, bit by bit, and a controller for controlling the adjusting of the first delay time, a monitoring operation, and a phase synchronization pull-in operation.