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Difficulty: MediumTroubleshooting Physical Cabling and Connectors

Match each physical layer cabling symptom or test result on the left with its most likely root cause on the right.

  • A wiremap tester indicates pin-to-pin continuity on pins 1 through 8, yet a cable certifier reports severe Near-End Crosstalk (NEXT) failure.Split pair wiring mistake where conductors from different twisted pairs were interchanged during connector termination.
  • An optical power meter reveals abnormal insertion loss and high back reflection localized precisely at an LC patch panel junction.Contaminated or dirty fiber optic connector ferrule end-face.
  • A Category 6 cable run negotiates a link speed of only 100 Mbps instead of 1 Gbps, and a continuity test reveals an open circuit on pin 4.Incomplete 4-pair conductor connectivity required for Gigabit autonegotiation.
  • A Power over Ethernet (PoE+) switch port shuts down due to excessive voltage drop, and a cable analyzer reports abnormally high DC resistance.Use of non-standard Copper-Clad Aluminum (CCA) cabling instead of solid copper conductors.

Answer

1. Pin-to-pin continuity with severe NEXT matches Split pair wiring mistake. 2. Insertion loss and high back reflection at LC junction matches Contaminated fiber optic ferrule end-face. 3. 100 Mbps link fallback with open pin 4 matches Incomplete 4-pair connectivity for Gigabit. 4. High DC resistance and PoE voltage drop matches Use of Copper-Clad Aluminum (CCA) cabling.
Each physical cabling symptom maps directly to its specific physical layer failure mechanism: split pairs preserve pin continuity while causing NEXT due to untwisted pair pairing; fiber ferrule end-face debris scatters light causing insertion loss and return loss at connector junctions; pin 4 opens prevent 4-pair Gigabit autonegotiation and force a 100 Mbps fallback; and non-standard CCA cables introduce excessive DC resistance under PoE load.

Step-by-Step Solution

1
Analyze the first scenario involving pin continuity accompanied by NEXT failure.
Identify that split pair cabling errors maintain straight-through electrical continuity but break pair twists, causing crosstalk.
Differential signaling in twisted-pair copper relies on tight twists to eliminate electromagnetic induction between adjacent pairs.
2
Analyze the second scenario involving attenuation and reflection localized at a fiber LC connector.
Determine that microscopic contamination on the ferrule surface impedes light transfer and reflects light back toward the transmitter.
Clean ferrule end-faces are essential to ensure glass-to-glass contact without air gaps.
3
Analyze the third scenario where a Cat 6 connection drops from 1 Gbps to 100 Mbps due to an open conductor on pin 4.
Recognize that 1000BASE-T autonegotiation requires all 4 pairs, whereas 100BASE-TX operates over only 2 pairs (1/2 and 3/6).
A break on pins 4, 5, 7, or 8 prevents 4-pair operation, causing the switch/NIC to downgrade the link.
4
Analyze the fourth scenario where PoE causes excessive voltage drop and high DC loop resistance.
Identify Copper-Clad Aluminum (CCA) as a non-compliant conductor material with higher DC resistance than solid copper.
Aluminum has lower conductivity than copper, leading to higher I^2R power loss and voltage drops when powering remote PoE devices.

Key Concept

Troubleshooting Physical Cabling and Connectors
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Topics

Troubleshooting Physical Cabling and Connectors

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