- A
Multimeter
Why wrong: A multimeter can measure resistance but not cable length accurately over long distances.
- B
Cable tester
Why wrong: Basic cable testers check continuity but do not measure length; advanced certifiers may include TDR functionality, but a standard tester does not.
- C
TDR (Time-Domain Reflectometer)
A TDR is specifically designed to measure cable length by analyzing signal reflections, making it ideal for this task.
- D
Tone generator
Why wrong: A tone generator is used for tracing cables, not for measuring length.
Question 796 of 1,020
How to Use a Time-Domain Reflectometer (TDR) for Cable Length Measurement
This 220-1201 practice question tests your understanding of networking tools. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A technician is deploying a new network in a warehouse and needs to measure the length of a spool of Cat6a cable before installation to ensure it does not exceed the 100-meter segment limit. Which tool should be used?
Quick Answer
The answer is a Time-Domain Reflectometer (TDR). A TDR works by sending a high-frequency electrical pulse down the cable; when that pulse hits the end of the cable or an impedance change (like a break), it reflects back to the tool. By precisely measuring the round-trip time of that pulse and knowing the signal’s velocity of propagation (VoP) for Cat6a, the TDR calculates the exact cable length. On the CompTIA A+ Core 1 220-1201 exam, this question tests your ability to select the correct pre-installation tool for verifying the 100-meter segment limit, a common scenario in network deployment. A frequent trap is confusing a TDR with a cable tester or toner, which only check continuity or identify wires, not measure length. Remember the memory tip: TDR stands for “Time, Distance, Reflect”—it times the signal’s trip to calculate distance from the reflection.
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
✓
TDR (Time-Domain Reflectometer)
A Time-Domain Reflectometer (TDR) sends a pulse down the cable and measures the time it takes for the reflection to return from the far end or from a fault. By calculating the propagation velocity of the signal, the TDR can accurately determine the cable length, making it the correct tool to verify that a spool of Cat6a cable does not exceed the 100-meter segment limit before installation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Multimeter
Why it's wrong here
A multimeter can measure resistance but not cable length accurately over long distances.
- ✗
Cable tester
Why it's wrong here
Basic cable testers check continuity but do not measure length; advanced certifiers may include TDR functionality, but a standard tester does not.
- ✓
TDR (Time-Domain Reflectometer)
Why this is correct
A TDR is specifically designed to measure cable length by analyzing signal reflections, making it ideal for this task.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Tone generator
Why it's wrong here
A tone generator is used for tracing cables, not for measuring length.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse a basic cable tester (which only checks continuity and wiring) with a TDR, assuming any 'cable tester' can measure length, but only a TDR (or a high-end certifier with TDR capability) can provide accurate length measurements.
Detailed technical explanation
How to think about this question
A TDR works by analyzing the reflected signal's time of flight and applying the cable's nominal velocity of propagation (NVP), which is typically around 0.7c for Cat6a UTP. The TDR can also detect impedance mismatches, opens, and shorts along the cable, which is useful for identifying faults before installation. In real-world scenarios, a TDR is essential for verifying that long cable runs, such as those in a warehouse, stay within the 100-meter Ethernet segment limit to avoid signal attenuation and data errors.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Networking Tools — This question tests Networking Tools — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: TDR (Time-Domain Reflectometer) — A Time-Domain Reflectometer (TDR) sends a pulse down the cable and measures the time it takes for the reflection to return from the far end or from a fault. By calculating the propagation velocity of the signal, the TDR can accurately determine the cable length, making it the correct tool to verify that a spool of Cat6a cable does not exceed the 100-meter segment limit before installation.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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