VDV Testing & Troubleshooting 4 — Questions and Answers
Question 1: A technician is using a time domain reflectometer (TDR) on a coaxial cable and the display shows a positive reflection spike. What does a positive reflection indicate?
- An open circuit or high-impedance fault (Correct answer)
- A short circuit or low-impedance fault
- Normal matched termination
- Cable length exceeds instrument range
Correct answer: An open circuit or high-impedance fault
A positive reflection on a TDR trace indicates an open circuit or impedance higher than the cable's characteristic impedance, such as a missing terminator.
Question 2: Which fiber optic test is used to verify the end-to-end optical loss budget of a newly installed link before connecting active equipment?
- OTDR trace analysis
- Optical loss test using a light source and power meter (insertion loss test) (Correct answer)
- Visual fault locator inspection
- Chromatic dispersion measurement
Correct answer: Optical loss test using a light source and power meter (insertion loss test)
An insertion loss test using a calibrated light source and optical power meter directly measures the total end-to-end attenuation against the system's loss budget.
Question 3: A BNC connector on a 75-ohm coaxial cable shows 50 ohms impedance on a TDR. What is the most likely problem?
- The TDR is set to the wrong velocity of propagation
- A 50-ohm BNC connector was mistakenly installed instead of a 75-ohm connector (Correct answer)
- The cable has excessive attenuation
- The connector is properly installed
Correct answer: A 50-ohm BNC connector was mistakenly installed instead of a 75-ohm connector
50-ohm and 75-ohm BNC connectors are physically compatible but electrically mismatched; using a 50-ohm connector on 75-ohm cable creates a measurable impedance discontinuity.
Question 4: During troubleshooting, a technician discovers that a Cat5e cable run passes all wire map tests but fails the Power Sum ELFEXT (PS-ELFEXT) test. What does this failure indicate?
- The cable run is too short
- Excessive far-end crosstalk which degrades signals at the distant end of the cable (Correct answer)
- The cable has incorrect pair colors
- The cable shield is missing
Correct answer: Excessive far-end crosstalk which degrades signals at the distant end of the cable
PS-ELFEXT measures power sum equal-level far-end crosstalk; a failure indicates the cable or terminations are coupling noise onto pairs at the far end, impairing high-speed data transmission.
Question 5: A technician notices that a wireless access point connected to a PoE switch port does not power on. After confirming the AP supports PoE, what is the NEXT step?
- Replace the access point
- Test the patch cable with a cable tester and verify the switch port PoE output with a PoE tester (Correct answer)
- Update the AP firmware
- Move the AP to a different location
Correct answer: Test the patch cable with a cable tester and verify the switch port PoE output with a PoE tester
A faulty patch cable or a non-functional switch PoE port are common causes; verifying the cable and measuring the PoE output isolates the fault before replacing equipment.
Question 6: When performing a signal level measurement on a CATV drop, the technician finds the downstream level is -12 dBmV at the outlet. What is the appropriate action?
- The level is acceptable; no action needed
- Add an amplifier to boost the signal, as -12 dBmV is below the -8 dBmV minimum for good reception (Correct answer)
- Replace the cable modem
- Reduce the input signal level at the tap
Correct answer: Add an amplifier to boost the signal, as -12 dBmV is below the -8 dBmV minimum for good reception
Cable television outlet signal levels should typically be between -8 dBmV and +8 dBmV; -12 dBmV is below minimum and requires amplification or reduction of splitter/cable losses.
Question 7: An OTDR shows a flat line with no return signal after a certain distance on a multimode fiber. What does this indicate?
- A very clean connector at that distance
- A complete fiber break or macrobend causing total signal loss beyond that point (Correct answer)
- The fiber transitions to single-mode at that point
- The OTDR has reached its dynamic range limit on a normally functioning cable
Correct answer: A complete fiber break or macrobend causing total signal loss beyond that point
A flat line (noise floor) after a point on an OTDR trace indicates no backscatter is returning, which means the fiber is broken or has a severe bend causing complete signal loss.
VDV Testing & Troubleshooting 4
A technician is using a time domain reflectometer (TDR) on a coaxial cable and the display shows a positive reflection spike.
What does a positive reflection indicate?