Protocol Data Units at each layer in Computer Networks - Step-by-Step Execution

Concept Flow - Protocol Data Units at each layer

Application Layer

↓

Data: Message

↓

Transport Layer

↓

Segment

↓

Network Layer

↓

Packet

↓

Data Link Layer

↓

Frame

↓

Physical Layer

↓

Bits on the wire

Data starts as a message at the Application layer and is wrapped with headers at each layer, changing its name: segment, packet, frame, and finally bits for transmission.

Execution Sample

Computer Networks

Application Layer: Data = "Hello"
Transport Layer: Segment = add header + Data
Network Layer: Packet = add header + Segment
Data Link Layer: Frame = add header + Packet
Physical Layer: Bits = convert Frame to bits

Shows how data is encapsulated and renamed as it moves down the layers for sending.

Analysis Table

Step	Layer	Data Unit Name	Action	Resulting Data Unit
1	Application	Message	Create data to send	"Hello"
2	Transport	Segment	Add transport header to Message	Transport Header + "Hello"
3	Network	Packet	Add network header to Segment	Network Header + Transport Header + "Hello"
4	Data Link	Frame	Add data link header and trailer to Packet	Data Link Header + Network Header + Transport Header + "Hello" + Trailer
5	Physical	Bits	Convert Frame to bits for transmission	Bits on the wire
6	Physical	Bits	Transmission complete	Data sent over medium

💡 Data is fully encapsulated and sent as bits over the physical medium.

State Tracker

Data Unit	Start	After Application	After Transport	After Network	After Data Link	After Physical
Data	"Hello"	"Hello"	Transport Header + "Hello"	Network Header + Transport Header + "Hello"	Data Link Header + Network Header + Transport Header + "Hello" + Trailer	Bits on the wire

Key Insights - 3 Insights

Why does the name of the data change at each layer?

Each layer adds its own header (and sometimes trailer) to the data, so the combined unit has a new name reflecting its encapsulation, as shown in execution_table rows 2-5.

What happens at the Physical layer to the data unit?

The Physical layer converts the Frame into bits for transmission over the medium, as seen in execution_table step 5.

Is the original message lost during encapsulation?

No, the original message is preserved inside the headers added by each layer, visible in the variable_tracker where "Hello" remains inside all encapsulated units.

Visual Quiz - 3 Questions

According to the execution_table, what is the data unit called after the Network layer adds its header?

A Segment

B Frame

C Packet

D Message

💡 Hint

Check execution_table row 3 under 'Data Unit Name' after Network layer.

At which step does the data become bits ready for transmission?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at execution_table step 5 where Physical layer converts Frame to bits.

If the Transport layer did not add its header, what would be missing in the variable_tracker after Transport?

A "Hello" only

B Network Header + Transport Header + "Hello"

C Transport Header + "Hello"

D Data Link Header + Network Header + Transport Header + "Hello" + Trailer

💡 Hint

Refer to variable_tracker column 'After Transport' for what data unit includes.

Concept Snapshot

Protocol Data Units (PDUs) change names at each OSI layer:
- Application: Message
- Transport: Segment
- Network: Packet
- Data Link: Frame
- Physical: Bits
Each layer adds headers/trailers, encapsulating data for transmission.

Full Transcript

Data starts at the Application layer as a message. At the Transport layer, a header is added, creating a segment. The Network layer adds its header, forming a packet. The Data Link layer adds headers and trailers, making a frame. Finally, the Physical layer converts the frame into bits for transmission over the medium. Each step wraps the data with additional information needed for delivery, changing the data unit's name. This process is called encapsulation and ensures data is properly prepared for sending across networks.

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