| 摘要 |
PROBLEM TO BE SOLVED: To provide an ISDN exchange that can check lines even when they are not open for communication. SOLUTION: When a test switch 15 sets a CPU 12 to a test mode, the CPU 12 controls a changeover switch 18 to change the connection of a terminal from a DSU 10 to a layer 1 controller 19. The CPU 10 transmits a preset setup message to the terminal and confirms the arrival of the message to the terminal in this ISDN exchange.
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