CCNA Network Infrastructure and Connectivity Practice Question
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?
⚠ Common exam trap
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.
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.
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) takes packets from the Network layer and encapsulates them into structured frames, complete with MAC addresses, a preamble, and a frame check sequence. A captured stream of '01010101...' shows no such structure or header information; it is simply the raw physical symbol stream as it appears before any Layer 2 framing boundaries are applied. Therefore, while frames are a Layer 2 PDU, the evidence given is clearly a bit-level representation, not an organized frame.
- ✓
Physical layer; bits
Why this is correct
The Physical layer (Layer 1) is responsible for the transmission and reception of unstructured raw bit streams over a physical medium, such as copper wire, fiber, or radio waves. At this layer, the PDU is simply the bit, and there is no recognition of packets, frames, or segments—only signaling states that represent 0s and 1s. The captured data '01010101...' exactly matches this definition, making 'Physical layer; bits' the correct answer.
- ✗
Network layer; packets
Why it's wrong here
The Network layer (Layer 3) operates with packets (or datagrams) that carry logical addressing, such as IPv4 or IPv6 addresses, and are used to route data across internetworks. A raw bit sequence like '01010101...' lacks any IP header fields, addressing, or other packet structure; the Network layer is one of the higher layers that must interpret bits that were physically sent. Thus, selecting 'Network layer; packets' misidentifies the abstraction level at which this capture occurs.
- ✗
Transport layer; segments
Why it's wrong here
The Transport layer (Layer 4) produces segments (TCP) or datagrams (UDP) that include port numbers, sequence and acknowledgment numbers, and checksums to support end-to-end communication between applications. These are logical data units that require the bits to be assembled and interpreted according to a header format, which is not visible in a raw '01010101...' capture. Because the captured data is just the electrical or optical representation of bits, it cannot be a Transport layer segment.
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
The Physical layer (Layer 1) is responsible for the transmission and reception of unstructured raw bit streams over a physical medium, such as copper wire, fiber, or radio waves. At this layer, the PDU is simply the bit, and there is no recognition of packets, frames, or segments—only signaling states that represent 0s and 1s. The captured data '01010101...' exactly matches this definition, making 'Physical layer; bits' the correct answer.
✗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?”
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 |
Go deeper
Related to this question
Learn chapter
The OSI Model
Key term
PDU
A PDU (Protocol Data Unit) is the unit of data at a specific layer of the OSI or TCP/IP model, containing both header and payload information as it travels across a network.
Key term
Open Systems Interconnection
The Open Systems Interconnection (OSI) model is a conceptual framework used to understand how data travels from one computer to another across a network.
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Courseiva writes every 200-301 question from scratch — 1,389 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.