Testing with Loopback IF & DIG

The diagnostic tool loopback can be used to check the proper operation of an IDU or OMU up to the point where the signal leaves the IDU/OMU. This can be useful for determining if one side or another may have a hardware issue.

Setting a loopback mode other than off activates one of the loopback modes for test purposes.  This must be turned off by a command when done with testing.  The CLI is still active while the mode is on to allow monitoring of various parameters.  

During IF and digital loopback, the transmitter signal from the far end will still be present at the input to the IDU and may cause a poor result.   Therefore, the odupower OFF on the remote end of may be required to eliminate any signal from the far end which may corrupt the result.

Consider using loopback_auto which will automatically run loopback if and loopback dig on a far end radio, then restore the link.  This allows troubleshooting the far end remotely, a method for those using the ibm interface without out of band management (OBM) or those who prefer a simpler approach to the manual testing method. All data ports are disabled while the loopback is running to prevent a traffic loop at the far end switch.

Caution: All loopback modes will stop live traffic from passing across the link and will cause data traffic to loop back out the data port. It is important for this reason that data ports be disconnected or disabled prior to activating loopback.

To use the loopback function to diagnose a particular unit:

1. Establish out of band management access to the unit.
2. If this is a remote unit, consider using the reload in x command as a precaution prior to starting the test. You can also use the loopback_auto function described above.
3. Disable or disconnect the data ports. If you're using in band management (IBM) you will also be disabling your management access, so you must use the loopback_auto function discussed previously.
4. For units other than Apex, disable ODU power with the command odupower off.
5. For Apex, ensure that opmode is on. For all other products, continue to the next step.
6. Activate loopback if.
7. Run the linktest command, possibly with multiple iterations like linktest 10.
8. Check that the lock status is 1 (locked) and record the MSE. The MSE should be -36 or better, typically -36 to -40. If the loopback IF result is in the -36 to -40 range, you can skip the loopback dig testing.
9. For Apex, set opmode off. Skip this step for all other products.
10. Activate the digital loopback mode by using the command loopback dig. This causes the signal to be looped in the IDU/OMU before it goes to the IF stage.
11. Run the linktest command, possibly with multiple iterations like linktest 10.
12. Check that the lock status is 1 (locked) and record the MSE. The MSE should be -40 or better, typically -42. 
13. Set loopback off.
14. Turn the ODU on, odupower on for systems other than Apex.
15. Enable or reconnect the Ethernet data ports as needed.
16. If the reload in x command was used, be sure to cancel the reload with reload cancel.