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Difficulty: MediumTroubleshooting Physical Cabling and Connectors

A network administrator installs a multimode fiber optic patch cable between two rack-mounted switches in a data center. Although physical link connectivity is established, the switch management interface logs a high rate of bit errors and optical power loss. An OTDR scan reveals a sudden, localized non-reflective loss at a point where the cable is tightly zip-tied around a sharp corner of the cable tray, with no glass break detected. Which of the following physical layer issues is the most likely cause of this signal degradation?

  1. Exceeding the minimum bend radius of the fiber optic cableAnswer
  2. B
    A complete conductor open caused by a severed fiber core strand
  3. C
    Transceiver Tx and Rx polarity reversal at the LC connector
  4. D
    Near-End Crosstalk (NEXT) induced by unshielded copper power cables

Answer

Exceeding the minimum bend radius of the fiber optic cable is the most likely cause of the optical signal attenuation.
Exceeding the cable's minimum bend radius causes macrobending, where light escapes from the core into the cladding due to micro-distortions in total internal reflection. This manifests as localized optical power loss and high bit error rates on an OTDR scan while still maintaining a physical link.

Step-by-Step Solution

1
Analyze the symptom and diagnostic findings reported in the scenario.
The link establishes physically, but experiences elevated optical signal loss and bit errors, with OTDR pointing to a sharp corner where the cable is tightly secured.
Identifying the localized physical anomaly helps isolate cause between physical breakage, connector fault, or geometry distortion.
2
Evaluate the optical properties of fiber under mechanical stress.
Bending fiber cable beyond its minimum bend radius alters the internal angle of incidence, causing light rays to leak out of the core into the cladding (macrobending loss).
Macrobending creates non-reflective attenuation at the bend site without causing a total optical discontinuity.
3
Select the corrective action and physical cause.
Loosening the zip-ties and re-routing the fiber cable to respect its minimum bend radius will restore full optical signal strength.
Restoring proper bend radius eliminates macrobending optical losses.

Key Concept

Fiber Optic Bend Radius and Macrobending Loss
Estimated Time:1m 15s
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Topics

Troubleshooting Physical Cabling and Connectors

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