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New Member

Help with T1 Loopback Testing

I would like to better understand the different loopback commands on the controller of VWIC cards. I am troubleshooting line code violations on a T1 data circuit(with no slips) and worked with a CO tech to do initial troubleshooting.

They used their T-Bird to do a BERT test towards my VWIC card. The correct command on the controller was 'loopback diag' as lopback local showed errors on the test set.

1) I cannot find any good documentation explaining the differences between loopback local line/payload and loopback diag.

2) I would also like to understand the 'detect v54' statement. Is this command required for the CSU/DSU to respond to remote loopback requests for testing?

3) With loopback diag defined on the controller, it still showed "loopback not set" when I showed the serial interface. When will the serial interface show "loopback set"

Thanks for any help!

New Member

Re: Help with T1 Loopback Testing


Basically, loopback local is a loop back towards the router itself (DTE). The line loopback is a loop back towards the network (telco). Additional information regarding loopbacks:

If you're putting a loop on the controller (CSU/DSU) you will not see loop set. However, if you have a good local loop from the controller itself or from the provider - you should see the interface go up, up (looped). There is another loop command available at the interface level (with this set - you should see interface set).

The diag - is for a diagnostic enabled loopback.

Hope this helps.



New Member

Re: Help with T1 Loopback Testing

I am still confused on the operation of the loopback line command. First, I put a loopback plug in the T1 jack and had the CO run a test to make sure there were no problems with the wiring to my equipment.

Next, I plugged my equipment back in and used the loopback line command on the controller and had them run another test. We saw errors on the test set immediately. They also saw errors when the router responded to remote loopback requests as shown in the logs as "IBOC loopup code detected, activating line loopback".

When I use loopback diag the test set shows no errors.