DCI Testing & Troubleshooting 3 — Questions and Answers
Question 1: ACR-N is calculated by subtracting which two measurements?
- NEXT minus attenuation (Correct answer)
- Attenuation minus return loss
- FEXT minus NEXT
- Return loss minus NEXT
Correct answer: NEXT minus attenuation
ACR-N (Attenuation-to-Crosstalk Ratio at Near End) equals NEXT minus attenuation, indicating the usable signal margin at the near end.
Question 2: A Category 6 channel certification test fails for PS NEXT. What does PS NEXT measure?
- The combined crosstalk from all disturbing pairs onto one pair simultaneously (Correct answer)
- The pair-to-shield crosstalk in shielded cable
- The power sum of signal attenuation over length
- The near-end crosstalk between only two pairs
Correct answer: The combined crosstalk from all disturbing pairs onto one pair simultaneously
PS NEXT (Power Sum NEXT) measures the cumulative crosstalk effect of all disturbing pairs on a single victim pair at the near end.
Question 3: What condition does 'return loss' measure in a copper cable?
- The portion of transmitted signal reflected back toward the source due to impedance mismatches (Correct answer)
- The total signal loss over the cable length
- The crosstalk at the far end of the cable
- The resistance of the cable conductor
Correct answer: The portion of transmitted signal reflected back toward the source due to impedance mismatches
Return loss measures impedance discontinuities that cause signal reflections back toward the transmitter, which can disrupt data transmission.
Question 4: An OTDR trace shows a large reflective event followed by no backscatter. What does this indicate?
- A break or cleaved end of the fiber at that distance (Correct answer)
- A high-loss connector somewhere in the link
- A tight bend in the fiber
- A fusion splice with high loss
Correct answer: A break or cleaved end of the fiber at that distance
A large Fresnel reflection followed by the absence of backscatter indicates the fiber has a break or open end at that location.
Question 5: A cable tester shows 'FAIL – Length' even though the physical cable appears short enough. What is the most likely cause?
- The nominal velocity of propagation (NVP) is set incorrectly (Correct answer)
- The tester battery is low
- The cable has a split pair fault
- The cable is shielded and not grounded
Correct answer: The nominal velocity of propagation (NVP) is set incorrectly
NVP (Nominal Velocity of Propagation) calibrates the tester's time-to-distance calculation; an incorrect NVP produces inaccurate length readings.
Question 6: Which fiber test method uses two OTDRs measuring in opposite directions and averages the results?
- Bidirectional OTDR testing (Correct answer)
- Two-cord reference method
- OLTS single-mode measurement
- Encircled flux testing
Correct answer: Bidirectional OTDR testing
Bidirectional OTDR testing averages measurements from each end to eliminate directional asymmetry in splice and connector loss readings.
Question 7: During copper certification, a technician notices the propagation delay skew exceeds the limit. What does excessive delay skew indicate?
- The pairs in the cable have significantly different electrical lengths (Correct answer)
- The cable is too long for the category
- The attenuation exceeds the allowable limit
- A crossed pair exists in the cable
Correct answer: The pairs in the cable have significantly different electrical lengths
Delay skew is the difference in propagation delay between the fastest and slowest pair; large skew can prevent proper reassembly of parallel-transmitted data.
DCI Testing & Troubleshooting 3
ACR-N is calculated by subtracting which two measurements?