| 摘要 |
PURPOSE:To make the place of occurrence of the cause of abnormality clear by adding a return code in controlling signals sent out to a digital subscriber's line and returning the controlling signal in the digital subscriber's line when the return code is detected by a returning means in the digital subscriber circuit. CONSTITUTION:A returning means 300 is provided in a digital subscriber's circuit 3. The returning means 300 watches presence of a return code in controlling signals transmitted from a line processor 1 to the control channel of a digital subscriber's line 4 through a digital subscriber interface circuit 2, and when the return code is detected, folds the controlling signals in the digital subscriber circuit 3, and returns to the digital subscriber interface circuit 2. By collating transmitted controlling signals and returned controlling signals, controlling signal communicating function between the digital subscriber interface circuit 2 and digital subscriber circuit 3 can be tested separately from controlling signal communicating function between the digital subscriber circuit 3 and a circuit terminal device 5, and accordingly, the place of occurrence of the cause of abnormality is made clear.
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