220-1101 Networking Practice Question
A user reports that their desktop computer loses network connectivity every few minutes, especially when the nearby overhead fluorescent lights are turned on. The technician observes that the link light on the NIC occasionally turns off during these events. The Ethernet cable is a Cat5e unshielded twisted pair (UTP) cable routed through the ceiling near the light fixtures. Which of the following is the MOST likely cause of the intermittent connectivity?
⚠ Common exam trap
Test-takers frequently assume a physical-layer issue like a faulty NIC (Option A) when the real culprit is environmental interference, which CompTIA tests by linking the symptom to a specific external trigger (fluorescent lights).
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
✓
Electromagnetic interference (EMI) from the fluorescent lights is affecting the unshielded cable
Unshielded twisted pair (UTP) cables lack shielding against electromagnetic interference (EMI). Fluorescent lights, especially older magnetic-ballast fixtures, generate strong EMI that can induce noise in nearby UTP cables, corrupting data signals and causing the NIC's link light to drop. This explains the correlation between the lights turning on and the loss of connectivity.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The network interface card (NIC) is faulty and needs replacement
Why it's wrong here
A failing NIC typically results in persistent and progressively worsening connectivity failure, packet loss, or complete LAN dropout regardless of external conditions. Intermittent loss that only occurs when fluorescent lights are active points to external signal degradation, not hardware failure. Furthermore, the link would not consistently recover when the lights are off if the card itself were faulty.
- ✗
The switch port is configured with an incorrect VLAN assignment
Why it's wrong here
An incorrect VLAN assignment on the switch port would place the workstation in the wrong broadcast domain, immediately blocking DHCP and all unicast traffic, creating a total and continuous lack of access rather than intermittent link drops. VLAN misconfigurations are static and do not change with lighting conditions or time of day. Because the symptom is tied to fluorescent light activity, the fault lies at the physical layer, not in logical network segmentation.
- ✓
Electromagnetic interference (EMI) from the fluorescent lights is affecting the unshielded cable
Why this is correct
Fluorescent light fixtures generate broadband electromagnetic interference, especially from their ballasts, which can couple into unshielded twisted pair (UTP) cabling and corrupt data signals. This EMI-induced signal degradation causes the NIC to lose carrier or resynchronize, resulting in intermittent link drops that correlate with the lights being on. Using shielded cabling such as STP/FTP or rerouting the cable away from the light source mitigates this physical-layer issue.
- ✗
The DHCP server has exhausted its IP address scope
Why it's wrong here
DHCP server address exhaustion prevents clients from obtaining an IP lease, but an existing connection with a valid lease continues to work at the physical and data-link layers; the NIC retains link and can communicate with already-assigned IPs. Moreover, scope exhaustion would affect any device attempting to join the network, not just a single workstation, and it would not produce link-level connectivity drops tied to fluorescent lighting. The observed behavior is a physical-layer phenomenon, not a DHCP server issue.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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