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.