Dr. I Doctor
Dr. I Doctor's Informational Juggernaut
March 1, 2008
T1/T3 Circuit Troubleshooting Using Loopback
Dear Doctor,
We have several T1 and T3 circuits between our main office and branch locations. These all connect to a central router, which relays traffic between the branches and to our centralized server farm. Lately we've experienced a series of circuit outages that we can't diagnose. We suspect the telco provider, but it always runs a "loop-back" test to our equipment at each location and reports that the failed circuit is operating normally, indicating a hardware failure with our gear. But after the provider conducts its tests, the circuit mysteriously comes back online. We ask for support from our hardware vendor, and the vendor says it's the telco's fault. How can we stop the finger pointing and get to the bottom of this?
Gentle User,
The failure is not just with your routers and circuits, but with the hardware vendor and telco. You are the victim of misdirection from both parties.
In its loop-back test, the telco sends a special code to its premises equipment (called the Network Interface Unit) that creates a temporary connection between the send-side and receive-side of the circuit, so that test data transmitted by the telco "loops back" to the telco where it can be verified. The telco's misdirection is that a successful loop-back test proves a circuit is working normally. It does no such thing, because the telco loop-back test doesn't exercise two critical components in a circuit: the cabling between your router and the NIU, and the "customer" side of the NIU's electronics and connectors.
The second bit of misdirection is from your hardware vendor, which rather than finger pointing should be performing its own end-to-end loop-back tests. Only when both tests are correlated can you identify the failing component in the circuit. Dr. I Doctor recommends that you get the hardware vendor and telco on a three-way call, then insist that they run the full set of tests before unhostering their fingers.
Posted by mbeckman at March 1, 2008 4:42 PM
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