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6.2.1. Cable Problems and Signal Degradation
Incorrect Cable Issues:
- Single-mode vs. multimode fiber mismatch
- Wrong cable category for required speed (Cat5e for 10G won't work)
- STP vs. UTP in wrong environment
Signal Degradation:
| Issue | Cause | Symptom |
|---|---|---|
| Crosstalk | Signal interference between cable pairs | CRC errors, slow speeds |
| EMI | Electromagnetic interference from external sources such as fluorescent lights, motors, and power cables | Intermittent errors; fix by rerouting cable or using shielded (STP) cable |
| Attenuation | Signal loss over distance | Connection drops, errors increase with distance |
Other Cable Issues:
- Improper termination: Bad crimping, incorrect pinout
- TX/RX transposed: Transmit/receive wires crossed
- A tester showing pairs 1-2 and 3-6 swapped while the other pairs run straight has found a crossover cable: intended for linking like devices on ports without Auto-MDI-X, but a fault on a run meant to be straight-through. A rollover (console) cable reverses all eight pins (1↔8, 2↔7, and so on).
- Opens/shorts: Broken conductors or conductors touching
- Unseated or mis-patched connections: A cable knocked loose, or a patch cord moved at the patch panel so the drop no longer reaches its switch port—if the cable itself tests good, check seating at both ends and trace the cross-connect
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Written byAlvin Varughese
Founder•20 professional certifications