| 摘要 |
PROBLEM TO BE SOLVED: To provide a communication system where loopback processing on the occurrence of a fault in a ring system is simplified and number of exchange resources in a node unit is decreased. SOLUTION: On the occurrence of a fault in an adjacent node unit E 5, a node unit D 4 supplies loopback processing of communication data that are other than monitor control data with a virtual path identifier 'VPI=0' assigned for monitoring and independent of a terminal connected to its own unit among data received from a primary ring line 10c to a secondary ring line 110c on the basis of 'VPI=1' assigned for communication.
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