- A
The phone handset draws too much power from the line
Why wrong: Power draw is not the issue; it's signal interference.
- B
The missing filter allows voice frequencies to interfere with DSL frequencies
DSL and voice share the same line; filters separate them to prevent interference.
- C
The filter is needed to amplify the DSL signal
Why wrong: Filters block interference, they do not amplify signals.
- D
The phone handset creates a short circuit without the filter
Why wrong: A missing filter does not cause a short circuit.
Question 35 of 1,020
DSL Missing Filter Causes Disconnects
This 220-1201 practice question tests your understanding of internet connection types. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 troubleshooting a DSL connection that keeps disconnecting. The modem shows the DSL light blinking. After checking the phone line, they find a filter is missing on a nearby phone handset. How does this cause the issue?
Quick Answer
The missing filter allows voice frequencies to interfere with DSL frequencies, which is why the connection keeps disconnecting. DSL operates at a higher frequency range than standard voice calls, and a microfilter acts as a low-pass filter, blocking those voice-frequency signals from bleeding into the DSL line. Without a filter on a nearby phone handset, the handset’s electrical noise and voice signals create crosstalk that corrupts the DSL signal, causing the modem’s DSL light to blink and the link to drop intermittently. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding of DSL installation and troubleshooting—a common trap is assuming any phone device can share the line without a filter, or confusing DSL filters with surge protectors. Remember the memory tip: “No filter, no signal—voice noise kills the DSL.”
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
✓
The missing filter allows voice frequencies to interfere with DSL frequencies
DSL operates at frequencies above the voice band (typically 25 kHz to 1.1 MHz for ADSL), while analog phone handsets use the 0–4 kHz range. A missing filter (microfilter) allows the phone handset's impedance changes and noise from voice-band signals to reflect onto the DSL line, causing crosstalk and signal-to-noise ratio degradation. This interference triggers frequent retrains and disconnections, visible as a blinking DSL light.
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.
- ✗
The phone handset draws too much power from the line
Why it's wrong here
Power draw is not the issue; it's signal interference.
- ✓
The missing filter allows voice frequencies to interfere with DSL frequencies
Why this is correct
DSL and voice share the same line; filters separate them to prevent interference.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The filter is needed to amplify the DSL signal
Why it's wrong here
Filters block interference, they do not amplify signals.
- ✗
The phone handset creates a short circuit without the filter
Why it's wrong here
A missing filter does not cause a short circuit.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse a missing filter with a power or short-circuit issue, overlooking the fundamental principle that DSL and POTS share the same wire pair but require frequency separation to avoid interference.
Detailed technical explanation
How to think about this question
DSL uses discrete multitone (DMT) modulation, splitting the frequency spectrum into 256 or more subchannels (4.3125 kHz each). A missing filter allows the phone's off-hook impedance (typically 600 ohms) and nonlinearities (e.g., from the ringer or hybrid circuit) to introduce harmonics and noise into the DSL subchannels, especially the lower ones, causing bit errors and forcing the modem to reduce the SNR margin below the threshold (usually 6 dB) for a stable link. In real-world scenarios, even a single unfiltered cordless phone base can cause intermittent drops that are hard to diagnose without a spectrum analyzer.
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Internet Connection Types — This question tests Internet Connection Types — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The missing filter allows voice frequencies to interfere with DSL frequencies — DSL operates at frequencies above the voice band (typically 25 kHz to 1.1 MHz for ADSL), while analog phone handsets use the 0–4 kHz range. A missing filter (microfilter) allows the phone handset's impedance changes and noise from voice-band signals to reflect onto the DSL line, causing crosstalk and signal-to-noise ratio degradation. This interference triggers frequent retrains and disconnections, visible as a blinking DSL light.
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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