CompTIA Network+ (N10-009)Network TroubleshootingMedium
A technician installed a new Cat 6 cable run that passes directly alongside fluorescent light fixtures in a drop ceiling. Users on that segment report intermittent packet loss and slow performance, especially when the lights are on. Which issue is MOST likely occurring?
- ADuplex mismatch
- BElectromagnetic interference (EMI)
- CFiber attenuation
- DDNS resolution failure
Show answer & explanationAnswer & explanation
Correct answer: B. Electromagnetic interference (EMI)
Fluorescent light fixtures and ballasts generate electromagnetic fields that can induce noise onto nearby unshielded copper cabling, causing intermittent errors and packet loss that correlate with the lights being powered on. Moving the cable away from the fixtures or using shielded cable resolves this.
Why the other options are wrong
- A. Duplex mismatch causes consistent late collisions/CRC errors regardless of lighting, not correlated with fixtures.
- C. Attenuation applies to fiber optic signal loss, not copper cable interference from lighting.
- D. DNS failures would cause name resolution issues, not physical-layer packet loss tied to light fixtures.
Electromagnetic Interference (EMI)
Noise induced onto copper cabling from nearby electrical or magnetic sources such as fluorescent lights, motors, or power lines, causing signal degradation.
- Common sources: fluorescent ballasts, motors, generators, power cables
- Symptoms: intermittent errors correlating with equipment operation
- Mitigated by shielded cable (STP), proper cable routing, or distance from source
Memory trick: Lights flicker, cables suffer — EMI sneaks in near fluorescent ballasts.