PURPOSE:To provide extending performance to an optical data transmission system by setting a single optical switch of the 2nd stage to plural optical switches of the 1st stage among those optical switches for bypass set inside a node station. CONSTITUTION:The optical switches 107 and 108 of the 2nd state are connected to an optical fiber cable 106 against the optical switches 104 of the 1st stage. In this case, the switches 104 are opened and the switches 107 and 108 are bypassed respectively. When the optical switches of the 2nd stage are bypassed like the switches 108, the optical switches of the 1st stage are opened like the switch 104 and not bypassed to transmit no optical signal. Therefore the number of optical switches that transmit light can be cut down to 1/2 conventional number of switches when the optical signal is transmitted to a certain terminal from another terminal. Thus the attenuation degree of light caused by the optical switches can be approximately reduced down to half.
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