| 摘要 |
PURPOSE:To enable the switching of a transmission path to be performed by each station or the opening/closing of the loop of a sub signal transmission line to be controlled by a master station, by providing switches on two pairs of loop-shaped sub signal transmission lines, and constituting a switch circuit so that an inserted signal can be switched and transmitted to one of the transmission lines, and it is received again and the signal can be taken out. CONSTITUTION:When a digital transmission line in an up direction is disconnected between the stations C and D, the station D changes over the switches S4-S6. And a sub data signal inserted by the station D is switched and outputted from the sub signal transmission line 21 to the sub signal transmission line 12. The station C changes over the switches S1, S2, and S6. By changing over those switches, the sub data signal in the up direction is transmitted on the sub signal transmission line 22 in a down direction to the station D via baseband switches B, A, F, and E in order. In such a way, it is possible to multiplex a sub signal on a main signal and transmit in both directions with a spare system even when only one transmission line in the up and down directions respectively is provided.
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