N10-009 Question 301
Single answerA technician installs a new copper Ethernet run between a switch in a manufacturing area and a workstation 85 meters away. The cable was punched down to wall jacks on both ends and passes through an area with several large electric motors. After installation, the link LED comes up at 1 Gbps, but users report intermittent packet loss and very low throughput whenever the motors are running. A cable tester shows high near-end crosstalk (NEXT) and occasional errors on multiple pairs. Which of the following is the MOST likely cause of the problem?
- A
The technician used UTP instead of STP in a high-EMI environment
- B
The technician used single-mode fiber transceivers instead of multimode fiber transceivers
- C
The technician accidentally transposed TX and RX pairs on one end of the cable
- D
The total cable length exceeds the maximum allowed for twisted-pair Ethernet
Show answer and explanation
Correct answer: A
Explanation
The best answer is that the technician used UTP instead of STP in a high-EMI environment. Large electric motors can generate significant electromagnetic interference, which can degrade copper Ethernet signals. In industrial or manufacturing spaces, proper cable selection matters: STP can help reduce interference when installed correctly, while UTP is more vulnerable in electrically noisy areas. The mention of high near-end crosstalk (NEXT) and errors across multiple pairs points to a physical-layer cabling issue rather than a logical or configuration problem.
From a Network+ perspective, candidates should distinguish among several common cable problems:
- Incorrect cable type: choosing UTP where STP is appropriate, or selecting the wrong category for required bandwidth, can produce degraded performance even if link lights appear normal.
- Signal degradation: EMI, crosstalk, and attenuation can cause intermittent errors, retransmissions, and low throughput.
- Improper termination: poor punchdowns, excessive untwisting at the jack, and split pairs can also increase NEXT and return loss.
- TX/RX transposed: more likely to cause no link or immediate negotiation issues, not EMI-correlated packet loss.
Relevant best practices align with TIA/EIA structured cabling guidance and Ethernet physical-layer standards under IEEE 802.3: maintain pair twists as close as possible to the termination point, observe cable-category limits, keep copper away from EMI sources when possible, and use shielded cabling where the environment requires it. Also note that shielding is effective only when the overall cabling system is designed and grounded properly.
- A. Correct.
Correct. The scenario specifically mentions a copper Ethernet run installed near large electric motors, which are a common source of electromagnetic interference (EMI). In that type of environment, using shielded twisted pair (STP) is often preferable to unshielded twisted pair (UTP), provided it is installed and grounded properly. The symptoms also align: the link can still come up at 1 Gbps, but interference and signal degradation can cause intermittent packet loss and poor throughput under load or when nearby equipment is active. The reported high NEXT further supports a cabling quality issue that can be worsened by poor cable choice and installation in noisy environments.
- B. Incorrect.
Incorrect. This option refers to fiber optic cabling and optics, but the scenario clearly states this is a copper Ethernet run punched down to wall jacks. Single-mode versus multimode is a valid cable-selection issue in networking, but it does not match the described installation. A candidate might choose this if focusing only on the idea of using the wrong media type, but the details rule it out.
- C. Incorrect.
Incorrect. A TX/RX transposition issue would more likely prevent link establishment entirely unless corrected by auto-MDI/MDIX, which is common on modern Ethernet equipment. Even if auto-MDI/MDIX handled the crossover, it would not explain the strong correlation between errors and the motors running, nor would it typically produce a tester result showing high NEXT on multiple pairs. This distractor targets the common misconception that any intermittent Ethernet issue is due to pair reversal.
- D. Incorrect.
Incorrect. The workstation is 85 meters away, which is within the standard 100-meter maximum channel length for twisted-pair Ethernet, assuming the full channel including patch cables remains within spec. Attenuation can become a problem when distances exceed standards or when cable quality is poor, but the distance given does not by itself indicate an overlength run. The interference pattern tied to the motors is a much stronger clue.