We questioned your knowledge of cabling testers last month, so now let’s see how well you know how to troubleshoot problems with those testers. Correct answers and explanations are in red.

1. If a copper certification tester fails an unshielded twisted-pair (UTP) cable, what is the most likely problem?
A. Cable is too long
B. Too much attenuation
C. Alien crosstalk
D. Untwisted pairs at a termination

When installing UTP cables, the most common problem is bad connections caused by untwisting the pairs at a punchdown block or jack.

2. The copper tester says you have crossed or transposed pairs two and three on a four-pair UTP cable. What’s the likely cause?
A. One end is terminated T568A and the other end T568B.
B. Wires are punched down on a jack using the color codes for punchdown blocks.
C. The solid and striped wires are reversed.
D. One of the wires is not properly connecting to a punchdown.

With two different color codes commonly used in UTP terminations and jacks having both shown on the punchdowns, it's easy to use the wrong termination at one end of a cable.

3. A Category 6a cable plant will still pass certification if it has a Cat 5 patchcord at an intermediate connection.

UTP cabling will generally degrade to the performance level of the lowest grade component used in the system, so a Cat 6A cable plant with just one Cat 5 patchcord will probably not meet Cat 6A certification requirements.

4. A time-domain reflectometer shows a reflected pulse of the opposite polarity of the test pulse. This indicates that the cable is/has _______.
A. Open
B. Shorted
C. Low impedance
D. High crosstalk

A TDR will show a pulse of the same polarity if the wires are open at the far end and opposite polarity if the wires are shorted.

5. Several recently installed UTP cables fail testing for alien crosstalk. What is the most likely cause?
A. Bad punchdowns
B. Poor-quality connectors
C. Cable bundled too neatly
D. Too much cable jacket stripped for termination

Alien crosstalk is coupling from one pair to the same pair (e.g. pair 2 to pair 2) in another cable. This problem is often associated to cables being neatly bundled together so a longer length of cable is exposed to crosstalk.

6. In a situation where one fiber optic cable test indicates high loss, what should the test tech do first?
A. Inspect the connectors for dirt or contamination
B. Test the reference test cables
C. Check the 0 dB reference on the test set
D. Test with an optical time-domain reflectometer (OTDR)

Whenever a fiber optic test shows high loss, the first step in troubleshooting is to clean connectors carefully and retest, as dirt is the most common problem.

7. In a situation where every fiber optic cable test indicates high loss, what should the test tech do first?
A. Inspect the connectors for dirt or contamination
B. Test the reference test cables
C. Check the 0 dB reference on the test set
D. Test each fiber with an OTDR

If every fiber in a cable shows high loss, the correct troubleshooting sequence is to look for bad patchcords by inspecting and testing them, then check the instrument 0 dB calibration and battery condition. If both those are OK, the problem may be a cable problem caused by installation, so it's time to try an OTDR if the cable is long enough.

8. When testing fiber optic cables, every fiber in the cable fails because of high loss. What is most likely to be the problem?
A. Bad connectors
B. Bad splices
C. Cable damage during installation
D. Any of the above

Optical fiber has higher loss caused by stress on the fibers at higher wavelengths, so if multimode fiber shows higher loss than expected at 1300 nm but not so much at 850 nm or singlemode shows higher loss than expected at 1550 nm compared to 1310 nm, the problem is likely caused by stress on the fibers.

9. What can a tech with only an optical loss test set do to determine if a cable has loss caused by stress or kinking during installation?
A. Test both directions and compare results
B. Test without reference cables
C. Test with different source power levels
D. Test at two wavelengths and compare the results

While it is possible that every connector and/or every splice has been installed improperly, it's more likely that the cable has been damaged during installation if every fiber shows problems.

10. If a cable has a kink that is causing high loss, OTDR traces should show a nonreflective loss event similar to a fusion splice at the location of the kink.

High loss caused by stress on the fiber creates a non-reflective loss event on an OTDR trace. If the fiber is broken, the trace will drop to the noise floor of the OTDR at the end of the trace but generally still be non-reflective due to the irregular broken ends of the fibers.
