VDV Testing & Troubleshooting 5 — Questions and Answers
Question 1: A technician is certifying a Cat6A cable channel and the test fails for 'alien crosstalk' (AXT). What is the most effective corrective action?
- Replace all cable with Cat5e
- Increase physical separation between parallel cable runs or use F/UTP shielded cable (Correct answer)
- Reduce the cable length
- Replace the keystone jacks with Cat6 jacks
Correct answer: Increase physical separation between parallel cable runs or use F/UTP shielded cable
Alien crosstalk occurs between adjacent cables; increasing separation, using bundling best practices, or switching to shielded F/UTP cable are the recognized remedies.
Question 2: A technician uses a visual fault locator (VFL) on a fiber cable and sees the red laser light glowing through the jacket 2 meters from the end. What does this indicate?
- Normal evanescent field leakage
- A macrobend or physical damage causing light to escape the fiber at that location (Correct answer)
- The fiber is correctly transmitting light
- The jacket material is transparent to VFL wavelengths
Correct answer: A macrobend or physical damage causing light to escape the fiber at that location
A visible red glow through the fiber jacket indicates that light is escaping due to a tight bend, kink, or physical damage that disrupts total internal reflection.
Question 3: During a cable TV service call, a customer complains of a pixelating picture on all channels. The technician measures the signal level and finds it is +15 dBmV. What is the most likely problem?
- The signal level is too low
- The signal level is too high, causing receiver overload and distortion (Correct answer)
- The cable modem is interfering
- The TV tuner card is faulty
Correct answer: The signal level is too high, causing receiver overload and distortion
Signal levels above +8 dBmV can overload cable TV receivers and cause digital pixelation or analog distortion, even though the power is high.
Question 4: A network switch port shows a link light but a laptop connected to that port cannot get an IP address. Cable certification passes. What should the technician check next?
- Replace the Ethernet cable
- Verify DHCP server availability and VLAN assignment on the switch port (Correct answer)
- Re-certify the cable with a different tester
- Replace the laptop NIC
Correct answer: Verify DHCP server availability and VLAN assignment on the switch port
A passing cable certification rules out physical layer issues; the inability to get an IP address with an active link points to a Layer 2/3 issue such as an incorrect VLAN, DHCP scope exhaustion, or server availability.
Question 5: A technician suspects a ground loop is causing hum bars on a video display. Which test confirms a ground potential difference between equipment?
- Measure the coaxial cable attenuation with a TDR
- Measure the AC voltage between the chassis grounds of the two connected devices with a multimeter (Correct answer)
- Check the cable TV signal level with a spectrum analyzer
- Test the HDMI cable with a cable certifier
Correct answer: Measure the AC voltage between the chassis grounds of the two connected devices with a multimeter
Measuring AC voltage between the chassis grounds of two devices reveals a ground potential difference, which is the root cause of hum bars induced by ground loop current.
Question 6: A fiber link passes insertion loss testing but fails to support 10GBase-SR at the specified distance. Which test should the technician perform next to diagnose the limitation?
- Re-test insertion loss with a different light source
- Perform an OTDR trace to check for excessive connector losses or bandwidth-limiting events, and verify fiber modal bandwidth (Correct answer)
- Replace all connectors with LC-type
- Test the cable with a Cat6A certifier
Correct answer: Perform an OTDR trace to check for excessive connector losses or bandwidth-limiting events, and verify fiber modal bandwidth
Modal bandwidth of the fiber and discrete loss events revealed by OTDR can limit 10G reach even when the total insertion loss budget appears acceptable.
Question 7: A technician terminates a 25-pair telephone cable and some pairs have dial tone while others are silent. After using a butt set to confirm the silent pairs have no signal from the demarc, what is the correct next step?
- Re-terminate all 25 pairs
- Notify the carrier/provider that the affected pairs are not active or have a fault on the service provider side (Correct answer)
- Replace the entire 25-pair cable
- Move the silent pairs to a different punchdown block
Correct answer: Notify the carrier/provider that the affected pairs are not active or have a fault on the service provider side
If the pairs have no signal at the demarcation point (demarc), the fault is on the carrier's side and must be reported to the service provider for resolution.
VDV Testing & Troubleshooting 5
A technician is certifying a Cat6A cable channel and the test fails for 'alien crosstalk' (AXT).
What is the most effective corrective action?