Step 4: Looping to your T-1 jack

It's unlikely that your CSU is connected directly into your NIU. These two pieces of hardware might be in different sections of your building, connected by the inside wiring that ends next to your phone system in a small T-1 phone jack (called an RJ-45 jack).

In order to test to the RJ-45 jack for continuity and errors, you have to plug into it with a test set, or use a loopback plug, which is a much cheaper alternative. Loopback plugs are available in both male and female versions so that you can test your cables and the T-1 jack on the wall.

When you place the loopback plug in the jack, you allow your carrier to continue to send signals through the NIU to the phone jack. If the carrier can see the loopback plug and run test patterns without losing any data or receiving errors, you know that the section of the cabling to that point is solid. If your carrier can't see the loopback plug, or is taking errors to it, but can loop the NIU without a problem, your trouble lives in the wiring between the NIU and that T-1 jack.

Tip You can extend the test to the end of the cable that feeds into your CSU if you have a female loopback plug to receive the T-1 cable that was plugged into your CSU.

If you can loop the T-1 jack

Successfully looping your T-1 jack or the end of the cabling that leads to your CSU indicates that you have good physical continuity on your circuit to that point. If you are troubleshooting an issue where your carrier can't reach your CSU, you probably have a defective CSU. If you had quality issues and the loopback tests came back clean to the hard loop on the RJ-45 jack, you again know that any static or frame slips you are experiencing aren't being generated by the wiring. Your next phase of testing is asking your hardware vendor to validate or replace your CSU, because that is the last possible variable causing your issue.