- A
Data Link layer; frames
Why wrong: The Data Link layer (Layer 2) encapsulates packets into frames, not raw bits. The captured data is still in bit form, not organized into frames.
- B
Physical layer; bits
At the Physical layer (Layer 1), data is transmitted as a stream of bits. The PDU is called 'bits'. This matches the capture of '01010101...'.
- C
Network layer; packets
Why wrong: The Network layer (Layer 3) uses packets (or datagrams) for routing, not raw bits. Bits are the physical representation.
- D
Transport layer; segments
Why wrong: The Transport layer (Layer 4) uses segments (TCP) or datagrams (UDP) for end-to-end communication, not raw bits. Bits are at Layer 1.
Question 1,304 of 1,819
OSI Physical Layer PDU: Identifying Bits
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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 network technician is troubleshooting a connectivity issue between two hosts. Host A sends a web request to Host B. The technician captures packets on the link between the two hosts and sees the data as '01010101...'. At which layer of the OSI model is this data being transmitted, and what is the correct PDU name for this layer?
Quick Answer
The answer is the Physical layer, and the correct PDU name is bits. This is because the data shown as '01010101...' represents raw binary bits being transmitted over the physical medium, where the Physical layer (Layer 1) is responsible for converting these bits into electrical signals, light pulses, or radio waves for transport across the wire or air. On the CCNA 200-301 v2 exam, this question tests your ability to map the OSI model layers to their PDUs and recognize that any stream of 1s and 0s captured on a link is always a Layer 1 PDU. A common trap is confusing bits with frames or packets, but remember that frames (Layer 2) and packets (Layer 3) include headers and trailers, not just raw binary. For a quick memory tip: think of the Physical layer as the "bit bucket"—it only deals with bits, nothing more.
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
✓
Physical layer; bits
The data shown as '01010101...' represents raw binary bits being transmitted over the physical medium. At the Physical layer (Layer 1), data is encoded as electrical signals, light pulses, or radio waves, and the PDU is called bits. This matches the description of the captured data.
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.
- ✗
Data Link layer; frames
Why it's wrong here
The Data Link layer (Layer 2) encapsulates packets into frames, not raw bits. The captured data is still in bit form, not organized into frames.
- ✓
Physical layer; bits
Why this is correct
At the Physical layer (Layer 1), data is transmitted as a stream of bits. The PDU is called 'bits'. This matches the capture of '01010101...'.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Network layer; packets
Why it's wrong here
The Network layer (Layer 3) uses packets (or datagrams) for routing, not raw bits. Bits are the physical representation.
- ✗
Transport layer; segments
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Physical layer; bitsCorrect answer▾
Why this is correct
At the Physical layer (Layer 1), data is transmitted as a stream of bits. The PDU is called 'bits'. This matches the capture of '01010101...'.
✗Data Link layer; framesWrong answer — click to see why▾
Why this is wrong here
The technician sees raw bits before framing, so this is not the Data Link layer.
✗Network layer; packetsWrong answer — click to see why▾
Why this is wrong here
The Network layer deals with logical addressing and routing, not the physical transmission of bits.
✗Transport layer; segmentsWrong answer — click to see why▾
Why this is wrong here
The Transport layer is above the Physical layer and does not deal with bit-level transmission.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the Physical layer's raw bits and the Data Link layer's frames, expecting candidates to recognize that binary sequences without structure belong to Layer 1, not Layer 2.
Detailed technical explanation
How to think about this question
The Physical layer defines the electrical, mechanical, and procedural specifications for transmitting raw bits over a medium, such as voltage levels (e.g., 0V for 0, +5V for 1 in some Ethernet standards) or modulation schemes (e.g., NRZ, Manchester encoding). In real-world troubleshooting, seeing '01010101...' in a packet capture often indicates a layer 1 issue like faulty cabling, incorrect pinouts, or signal attenuation, as higher-layer PDUs would show structured headers.
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 200-301 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.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Physical layer; bits — The data shown as '01010101...' represents raw binary bits being transmitted over the physical medium. At the Physical layer (Layer 1), data is encoded as electrical signals, light pulses, or radio waves, and the PDU is called bits. This matches the description of the captured data.
What should I do if I get this 200-301 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: Jun 11, 2026
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