Patent ID: 7139484

Claim:
A signal transmission method in a wavelength-division-multiplex (WDM) transmission system, comprising the steps of: transmitting a WDM signal from a first WDM terminal to a second WDM terminal regardless of whether an optical signal is added or dropped at an optical add-drop multiplexer (OADM) node; and transmitting adding data by a network element using a further optical signal of an idle wavelength of said WDM signal which has no transmission data, wherein the OADM node is adapted to transmit to the network element an optical signal of a specific wavelength of said WDM signal transmitted between said first and second WDM terminals, the OADM node is adapted to add to said WDM signal the further optical signal of an idle wavelength of said WDM signal received from said network element, said first WDM terminal stops transmission of an optical signal of an idle wavelength of said WDM signal which has no transmission data, said network element individually understands the idle wavelength of said WDM signal and transmits the adding data to said OADM node using an optical signal of the same addition wavelength as the idle wavelength individually understood by said network element and generated by the transmission stop, and said OADM node adds to said WDM signal the optical signal of the addition wavelength from said network element instead of the optical signal stopped by said first WDM terminal.