Describes the Operation table for the ISDN subscriber circuit test: It realizes the function of the circuit test for ISDN subscribers in the system. In the circuit test, the status (activated, deactivated) of the LT U port, LT power supply voltage, LT power supply overflow auto protection, manual switch-off line feeding voltage, and manual switch-on line feeding voltage are measured. The test result shows whether the ISDN subscriber circuit works in the normal state. The index of this entry is ifIndex.