220-1101 Hardware and Network Troubleshooting Practice Question
A user reports that their wired desktop computer loses network connectivity intermittently. The technician notices that the link light on the network interface card (NIC) turns off when the computer's CPU is under heavy load, such as during video rendering. The technician has already updated the network driver and replaced the Ethernet cable with a known-good cable. The switch port is active and works with other devices. Which of the following is the MOST likely cause of this issue?
⚠ Common exam trap
The trap here is that candidates may focus on Layer 1 issues like cabling or switch ports, but the key clue is the link light turning off specifically under CPU load, which points to a power delivery problem rather than a connectivity fault.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Power supply unit (PSU) is failing to provide adequate power to the NIC under load
When the CPU is under heavy load, the system draws more power from the PSU. If the PSU is failing or underrated, it may not deliver stable voltage to the NIC, causing the NIC to reset or lose link. This explains why the link light turns off specifically during video rendering, not during idle or light use.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Faulty switch port
Why it's wrong here
A faulty switch port would cause link flapping or complete loss of connectivity independent of the workstation's processing load. Here, the same port is known to work with another device, and the failure only occurs when this PC is under load, so the port's physical layer circuitry is not the cause. Switch ports also do not degrade due to a client's PSU voltage fluctuations; the issue tracks the computer itself.
- ✓
Power supply unit (PSU) is failing to provide adequate power to the NIC under load
Why this is correct
A marginal PSU can provide adequate voltage at idle but sag under load, especially on the +3.3 V or +5 V rails that feed the NIC's PHY transceiver. When the CPU draws more current, rail ripple and voltage droop can exceed tolerance, causing the NIC to reset or lose its carrier link. This is precisely why the symptom is load-dependent: the link light goes off only during heavy usage, then returns when the load drops.
- ✗
Network congestion
Why it's wrong here
Network congestion saturates switch buffers or uplinks, producing packet loss, high latency, and TCP retransmissions, but it cannot extinguish the NIC's link LED. The link light indicates that a valid electrical carrier signal is being received from the switch PHY; congestion operates at layers 2-3, not at the physical PMA layer. Even with 100% buffer utilization, the physical link remains electrically active and the port stays 'up'.
- ✗
Incorrect duplex settings
Why it's wrong here
Duplex mismatch occurs when one side is forced to full duplex while the other is on half duplex, causing late collisions and CRC errors. However, both PHYs remain synchronized and the carrier signal is never interrupted, so the link LED stays lit. The system would show extreme slowness or errors, not intermittent link down events, distinguishing it from a power-related NIC reset.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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