| 摘要 |
PURPOSE:To attain effective utilization of a frequency by using a standby line in common in providing an analog line and a digital line together. CONSTITUTION:If a fault takes place in, e.g., a digital active line 1, since the changeover request of the line is fed to a line changeover control section 4 at the transmission side via a control line from a line switching control section 3 at the reception end, a switching instruction is sent to switches 121, 513, 541 and 542 in relay stations at the sending end and the intermediate station, for example, from the section 4. A reception section 53 of the intermediate relay station converts a multi-value QAM modulation ave to be sent into a digital modulation wave of an intermediate frequency band and a digital signal is extracted by a multi-QAM demodulator 543 via the switch 541. The digital signal is converted into a multi-value QAM modulation wave of 6GHz band throghha multi-value QAM modulator 544, the switch 542 and a transmission section 55 and sent externally. The digital signal is extracted at the receiving end via a reception section 56, the switch 571, multi-value QAM modulator 573 and the switch 130 and sent to a carrier terminal station. Then the intermediate relay station relays the modulation wave of the intermediate frequency band at the changeover between the active analog line and a standby line.
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