| 摘要 |
PURPOSE:To improve thunder resistance of communication by conducting maintenance communication between a node station where a multiplex/separation circuit is disconnected from an optical loop network and other node station through an optical fiber transmission line through which a main signal flows. CONSTITUTION:Each node station is provided with a modulation circuit 24 for sub-signal, a function superimposing a modulated signal onto the main signal transmitted between node stations, and a demodulator 21 separating and demodulating the modulated signal superimposed on the transmitted main signal and extracting the sub-signal. Then the input of the modulation circuit 24 is provided with a changeover switch SW4 so as to input normally the demodulated sub-signal to the modulation circuit 24 and when the own node station transmits the sub-signal, the communication by usnig the sub-signal is conducted between the node stations by changing over the changeover switch SW4 to the sub-signal position. |
