| 摘要 |
A first terminal station (11) comprises a loopback signal producing circuit (L1 DIFFERENCE L5) for producing a loopback command signal having a predetermined number of command bits in each frame of a first PCM signal, and a signal multiplexing circuit (42) for multiplexing input signals and overhead bits into successive frames of the PCM signal with the command bits used as predetermined ones of the overhead bits. The command bits indicate a repeater station (21) which should loop back the PCM signal and a second terminal station (12) which should not loop back the PCM signal. The repeater station comprises a signal detector (45) for detecting the command bits indicative of loopback of the PCM signal at the repeater station to produce a loopback control signal, and a loopback carrying out device (63) responsive to the loopback control signal for looping back the PCM signal. The first terminal station further comprises a signal detector (65) for detecting like command bits indicative of loopback of a second PCM signal at the first terminal station to produce a terminal loopback control signal, and a loopback carrying out device (43) responsive to the terminal loopback control signal for looping back the second PCM signal as at least a part of the first PCM signal. |
