VDV Testing & Troubleshooting 3 — Questions and Answers
Question 1: A technician uses a digital multimeter to test a Cat6 cable and all pairs show continuity but the cable fails the certifier's test. What is the most likely explanation?
- The multimeter test is more accurate than the certifier
- A multimeter only checks DC continuity and cannot detect high-frequency performance issues like NEXT or return loss (Correct answer)
- The certifier is miscalibrated
- The cable standard does not require certifier testing
Correct answer: A multimeter only checks DC continuity and cannot detect high-frequency performance issues like NEXT or return loss
A multimeter verifies DC continuity only; it cannot measure high-frequency parameters such as NEXT, attenuation, or return loss that a cable certifier tests.
Question 2: What is the purpose of a 'loopback' test in fiber optic troubleshooting?
- To measure the total fiber length
- To verify that a transceiver can transmit and receive by looping the output back to the input (Correct answer)
- To test the splice loss at each fusion splice
- To measure chromatic dispersion
Correct answer: To verify that a transceiver can transmit and receive by looping the output back to the input
A loopback test connects the transmit output back to the receive input to confirm the transceiver's Tx and Rx functions are operational.
Question 3: A cable TV technician finds that picture quality degrades only during rain. Which component is MOST likely at fault?
- The television set
- A loose or corroded F-connector allowing moisture ingress (Correct answer)
- The cable amplifier's AGC circuit
- The set-top box firmware
Correct answer: A loose or corroded F-connector allowing moisture ingress
Rain-related intermittent signal degradation is a classic symptom of moisture getting into a poorly sealed or corroded coaxial connector.
Question 4: When testing a data network with a cable tester, a 'split pair' fault is detected. What wiring error causes this condition?
- Both conductors of a pair are swapped end-to-end
- Conductors from two different pairs are used together, destroying the twist benefit (Correct answer)
- The cable shield is not connected
- One conductor in a pair is broken
Correct answer: Conductors from two different pairs are used together, destroying the twist benefit
A split pair occurs when one wire from each of two different pairs is used together, which eliminates the crosstalk cancellation provided by the twist.
Question 5: What does an optical power meter reading of -35 dBm on a single-mode fiber link (expected: -20 dBm) most likely indicate?
- The link is performing above specification
- Excessive loss due to a dirty connector, bend, or break in the fiber (Correct answer)
- The transmitter power is too high
- Normal variation within system tolerance
Correct answer: Excessive loss due to a dirty connector, bend, or break in the fiber
A received power 15 dB below the expected level indicates a significant loss fault such as a contaminated connector face, tight bend, or fiber break.
Question 6: A technician is troubleshooting a structured cabling installation where some outlets pass certification but users report slow network speeds on those drops. What should be checked next?
- Re-run the certifier test at a higher frequency range
- Inspect patch cords at both the outlet and patch panel for category compliance and damage (Correct answer)
- Replace the network switch
- Check the building's electrical grounding
Correct answer: Inspect patch cords at both the outlet and patch panel for category compliance and damage
Non-compliant or damaged patch cords are a common cause of performance issues that cable-run certification does not catch, since certifiers typically test the permanent link without the patch cords.
Question 7: What is the purpose of measuring 'propagation delay skew' in a twisted pair cable?
- To determine the cable's maximum pull tension
- To ensure that signals on all pairs arrive within an acceptable time window of each other, preventing data errors (Correct answer)
- To measure the cable jacket temperature coefficient
- To verify the cable shield continuity
Correct answer: To ensure that signals on all pairs arrive within an acceptable time window of each other, preventing data errors
Propagation delay skew is the difference in arrival time between the fastest and slowest pair; excessive skew can cause data corruption in high-speed protocols that use multiple pairs simultaneously.
VDV Testing & Troubleshooting 3
A technician uses a digital multimeter to test a Cat6 cable and all pairs show continuity but the cable fails the certifier's test.
What is the most likely explanation?