When it is known that a problem exists in a T1 network application, a different approach is suggested. In the verification procedure above, the initial test passed data through the entire T1 circuit, including both local and far end CSUs. This was done because no troubles were expected.
With trouble known to exist, begin by establishing what is good. Start with a repeater loopback test in the local CSU.
The repeater loopback test takes the transmit data just as it is about to leave the network port of the CSU and feeds it directly into the receive circuitry of the CSU. If the CSU is transmitting bad data, it will now receive bad data and the problem will be located (the CSU). If the CSU is transmitting good data but the receive circuitry is defective, the signal received will fail and the CSU will declare an alarm.
Use the “R” command on the Diagnostics Menu to initiate a repeater loopback. A warning message appears:
Service Affecting, Are you sure ? (Y/N)
Press “y” because you are sure. During a repeater loopback test the NET LED on the CSU should be green and the EQPT LED should be amber. The STAT LED will usually turn red because of a transient bit error condition which occurs at the instant the test begins. Ignore the STAT LED when conducting loopback tests on a CSU.
If the EQPT LED blinks amber to red during a repeater loopback, errors are being detected in the DSX-1 data presented by the local equipment. Verify cabling and option compatability.
If the NET LED is not green during a repeater loopback test, contact Verilink Technical Support for assistance.
If the repeater loopback test passes, the CSU is not defective.
For troubleshooting tips related to non-CSU problems, see Table 5-6 below:
Verilink NCC 2020 User Manual