Protocol Data Units at each layer in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

📋 Factual

intermediate

Identify the PDU at the Network Layer

Which of the following is the correct Protocol Data Unit (PDU) name used at the Network layer in the OSI model?

A Segment

B Packet

C Frame

D Bit

💡 Hint

Think about the layer responsible for routing data between devices.

The Network layer uses packets as its PDU. Packets contain logical addressing information to route data across networks.

🧠 Conceptual

intermediate

Match the PDU to the OSI Layer

Match the following PDUs to their correct OSI model layers:
1. Frame
2. Segment
3. Packet
4. Bit

A 1 - Data Link, 2 - Transport, 3 - Network, 4 - Physical

B 1 - Network, 2 - Data Link, 3 - Transport, 4 - Physical

C 1 - Transport, 2 - Network, 3 - Data Link, 4 - Physical

D 1 - Physical, 2 - Network, 3 - Transport, 4 - Data Link

💡 Hint

Recall the order of OSI layers from bottom to top.

The Data Link layer uses frames, Transport uses segments, Network uses packets, and Physical uses bits.

🔍 Analysis

advanced

Analyze PDU Transformation Across Layers

When data is sent from an application on one device to another, which sequence correctly shows how the Protocol Data Unit (PDU) changes as it moves down the OSI layers?

A Data → Segment → Packet → Frame → Bit

B Bit → Frame → Packet → Segment → Data

C Frame → Packet → Segment → Data → Bit

D Data → Packet → Segment → Frame → Bit

💡 Hint

Consider the encapsulation process from top to bottom layers.

Data starts at the Application layer as data, then becomes a segment at Transport, a packet at Network, a frame at Data Link, and finally bits at Physical.

❓ Comparison

advanced

Compare PDUs of Transport and Data Link Layers

Which statement correctly compares the Protocol Data Units (PDUs) of the Transport and Data Link layers?

A Transport layer uses bits; Data Link layer uses packets.

B Transport layer uses packets; Data Link layer uses bits.

C Transport layer uses frames; Data Link layer uses segments.

D Transport layer uses segments; Data Link layer uses frames.

💡 Hint

Think about which layer handles error checking and flow control.

The Transport layer's PDU is called a segment, which manages data flow and reliability. The Data Link layer uses frames to handle physical addressing and error detection.

❓ Reasoning

expert

Determine the PDU Size Impact on Network Performance

If the Protocol Data Unit (PDU) size at the Data Link layer is increased significantly, what is the most likely impact on network performance?

A Increased PDU size always decreases throughput due to longer transmission times.

B Increased PDU size has no effect on network performance because only bits matter at the Physical layer.

C Increased PDU size reduces overhead, improving throughput but may increase error retransmission if errors occur.

D Increased PDU size reduces error rates because larger frames are easier to detect.

💡 Hint

Consider the trade-off between overhead and error impact.

Larger PDUs reduce the relative overhead of headers, improving throughput. However, if an error occurs, more data must be retransmitted, potentially reducing efficiency.

Practice

1. Which Protocol Data Unit (PDU) name is used at the Network layer in the OSI model?

easy

A. Segment

B. Frame

C. Packet

D. Bit

Solution

1. Step 1: Recall OSI model layers and their PDUs

   The OSI model has seven layers. Each layer uses a specific PDU name. The Network layer is the third layer.

2. Step 2: Identify the PDU for the Network layer

   The Network layer uses the term 'Packet' for its PDU, which contains logical addressing information.

3. Final Answer:

   Packet -> Option C

4. Quick Check:

   Network layer PDU = Packet [OK]

Hint: Network layer always uses 'Packet' as its PDU name [OK]

Common Mistakes:

• Confusing Frame (Data Link layer) with Packet
• Mixing Segment (Transport layer) with Packet
• Thinking Bit is a PDU name at any layer

2. Which of the following correctly matches the PDU name with the Transport layer?

easy

A. Frame

B. Bit

C. Packet

D. Segment

Solution

1. Step 1: Identify the Transport layer in the OSI model

   The Transport layer is the fourth layer and is responsible for end-to-end communication.

2. Step 2: Recall the PDU name for the Transport layer

   The Transport layer uses 'Segment' as its PDU name, which contains data with transport headers.

3. Final Answer:

   Segment -> Option D

4. Quick Check:

   Transport layer PDU = Segment [OK]

Hint: Transport layer PDU is always called Segment [OK]

Common Mistakes:

• Choosing Frame which belongs to Data Link layer
• Selecting Packet which belongs to Network layer
• Confusing Bit with a PDU name

3. Consider this simplified data flow: Application data is passed down to the Transport layer, then to the Network layer, and finally to the Data Link layer. What is the correct sequence of PDUs at each layer?

medium

A. Packet -> Segment -> Frame

B. Segment -> Packet -> Frame

C. Frame -> Packet -> Segment

D. Bit -> Segment -> Packet

Solution

1. Step 1: Identify PDUs for each layer in order

   Transport layer uses Segment, Network layer uses Packet, Data Link layer uses Frame.

2. Step 2: Arrange PDUs in the order data passes down

   Data flows from Transport (Segment) -> Network (Packet) -> Data Link (Frame).

3. Final Answer:

   Segment -> Packet -> Frame -> Option B

4. Quick Check:

   Transport=Segment, Network=Packet, Data Link=Frame [OK]

Hint: Remember data flows: Segment, Packet, then Frame [OK]

Common Mistakes:

• Mixing order of PDUs
• Confusing Packet and Segment
• Including Bit as a PDU in this sequence

4. A student wrote this statement: "The Data Link layer PDU is called a Packet." What is wrong with this statement?

medium

A. Packet is the PDU for the Network layer, not Data Link layer.

B. Packet is the PDU for the Transport layer, not Data Link layer.

C. Packet is the PDU for the Application layer, not Data Link layer.

D. Packet is the PDU for the Physical layer, not Data Link layer.

Solution

1. Step 1: Identify the correct PDU for Data Link layer

   The Data Link layer uses 'Frame' as its PDU, not 'Packet'.

2. Step 2: Identify which layer uses 'Packet'

   The Network layer uses 'Packet' as its PDU.

3. Final Answer:

   Packet is the PDU for the Network layer, not Data Link layer. -> Option A

4. Quick Check:

   Data Link = Frame, Network = Packet [OK]

Hint: Data Link layer PDU is Frame, not Packet [OK]

Common Mistakes:

• Confusing Packet with Frame
• Mixing up Network and Data Link layers
• Assuming Packet is a general PDU name

5. In a network communication, data is encapsulated at each layer with its PDU. If a message is 1000 bytes at the Application layer, and each layer adds a header of 20 bytes (Transport), 30 bytes (Network), and 14 bytes (Data Link), what is the total size of the Frame sent over the Physical layer?

hard

A. 1064 bytes

B. 1066 bytes

C. 1064 bits

D. 1064 kilobytes

Solution

1. Step 1: Calculate total header size added by layers

   Transport header = 20 bytes, Network header = 30 bytes, Data Link header = 14 bytes. Total headers = 20 + 30 + 14 = 64 bytes.

2. Step 2: Add headers to original Application data size

   Application data = 1000 bytes. Total Frame size = 1000 + 64 = 1064 bytes.

3. Final Answer:

   1064 bytes -> Option A

4. Quick Check:

   1000 + 20 + 30 + 14 = 1064 bytes [OK]

Hint: Add all headers in bytes to original data size [OK]

Common Mistakes:

• Confusing bytes with bits
• Adding headers incorrectly
• Misreading units as kilobytes