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Difficulty: HardTroubleshooting Physical Cabling and Connectors

A network engineer is troubleshooting a newly deployed 10GBASE-LR single-mode optical fiber link between two distribution switches spaced 2 kilometers apart. An optical power meter registers an excessive loss of 14 dB14\text{ dB} across the link, significantly exceeding the maximum insertion loss budget of 2.5 dB2.5\text{ dB}, despite both transceiver end-face connectors passing visual inspection with a fiber microscope. Which diagnostic tool should the engineer utilize to measure reflections and pinpoint the precise location and cause of the attenuation fault along the fiber run?

  1. Optical Time-Domain Reflectometer (OTDR)Answer
  2. B
    Visual Fault Locator (VFL)
  3. C
    Optical Power Meter (OPM) with a stabilized light source
  4. D
    Tone generator with an inductive probe

Answer

The Optical Time-Domain Reflectometer (OTDR) is the correct tool because it analyzes Rayleigh backscattering and Fresnel reflections over distance to pinpoint specific attenuation faults.
The Optical Time-Domain Reflectometer (OTDR) is designed specifically to trace optical fiber cables by sending light pulses down the fiber and analyzing backscattered light. It produces a graph displaying loss against distance, allowing engineers to identify the exact distance to high-attenuation events such as macrobends, splices, or physical breaks.

Step-by-Step Solution

1
Evaluate the symptom and requirements
The optical fiber link experiences excessive signal loss (14 dB14\text{ dB}) somewhere along a 2 km span, but connectors are clean.
End-to-end power testing confirmed an issue exists, but does not provide distance or location details.
2
Assess diagnostic tool capabilities for fiber optics
An OTDR transmits pulsed light signals down the fiber strand and records the amplitude and timing of reflected light signals.
This signature trace identifies event locations (e.g., sharp bends, bad splices, or physical stress) relative to distance from the transmitter.
3
Select the correct physical layer diagnostic tool
Select the Optical Time-Domain Reflectometer (OTDR).
It is the only instrument capable of pinpointing fault distances along an extended optical cable pathway.

Key Concept

Pinpointing Fiber Optic Cable Faults with OTDR
Estimated Time:2m 0s
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Topics

Troubleshooting Physical Cabling and Connectors

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