Wavelength-division-multiplexing optical transmission system and control method thereof

A control method for a wavelength-division-multiplexing optical transmission system in which optical-node apparatuses that perform relay transmission of wavelength-division-multiplexed light are located at specified nodes, respectively, in a main optical transmission path, the method comprising, for a respective specified node:

• a step of monitoring a number of multiplexed wavelengths at said respective specified node;
• a step of transmitting the number of multiplexed wavelengths in the downstream direction;
• a step of switching level control to constant-gain control by an optical amplifier that is located at a stage before the respective optical node apparatus located at said respective specified node when there is notification from upstream of a change in the number of multiplexed wavelengths, and then after a specified amount of time, restarting level control so that the level becomes a target level that corresponds to the actual number of wavelengths;
• a step of controlling the optical transmission system by an optical transmission system control unit so that the following equation
• T<Y/(N·v)
• is satisfied, wherein T (sec) denotes a period from the time when a change in the number of multiplexed wavelengths occurs until said constant-gain control starts, v (dB/sec) denotes output-level-recovery speed of said optical amplifier, Y (dB) denotes fluctuation allowance of a receiving unit after wavelength-division-multiplexing transmission, and N denotes a maximum number of spans.

An apparatus comprising, for a respective node of a wavelength-division-multiplexed optical transmission system:

• an optical add/drop multiplexer (OADM) receiving a wavelength division multiplexed (WDM) light and dropping wavelengths from the received WDM light, so that the dropped wavelengths are provided to a receiving unit;
• an optical amplifier optically amplifying the WDM light before being received by the OADM, the optical amplifier
• performing level control when a number of multiplexed wavelengths in the WDM signal is constant,
• when receiving a notification from upstream of the respective node of a change in the number of multiplexed wavelengths in the WDM signal, switching from the level control to a constant-gain control, and
• after a specified amount of time of the constant-gain control, switches back to the level control so that a level of the amplified WDM light becomes a target level corresponding to an actual number of multiplexed wavelengths in the WDM light;
• and
• an optical-transmission-system-control unit that performs control so that the following equation is satisfied:
• T<Y/(N·v)
• wherein T (sec) denotes a period from the time when a change in the number of multiplexed wavelengths in the WDM signal occurs until the constant-gain control is started by the optical amplifier, v (dB/sec) denotes output-level-recovery speed of the optical amplifier, Y (dB) denotes fluctuation allowance of the receiving unit, and N denotes a maximum number of spans.