OSI model seven layers in Computer Networks - Step-by-Step Execution

Concept Flow - OSI model seven layers

Application Layer

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Presentation Layer

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Session Layer

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Transport Layer

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Network Layer

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Data Link Layer

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Physical Layer

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Transmission

Data starts at the Application layer and moves down through each layer to the Physical layer for transmission, then back up on the receiving side.

Execution Sample

Computer Networks

User types message -> Application Layer processes
Application Layer passes data -> Presentation Layer encodes
Presentation Layer passes data -> Session Layer manages connection
Session Layer passes data -> Transport Layer segments data
Transport Layer passes data -> Network Layer routes packets
Network Layer passes data -> Data Link Layer frames data
Data Link Layer passes data -> Physical Layer sends bits

Shows how data moves step-by-step down the OSI layers before transmission.

Analysis Table

Step	Layer	Action	Data State	Next Layer
1	Application	User data created	Raw user data	Presentation
2	Presentation	Data encoded/encrypted	Encoded data	Session
3	Session	Connection established	Session data	Transport
4	Transport	Data segmented and error-checked	Segments	Network
5	Network	Packets addressed and routed	Packets with addresses	Data Link
6	Data Link	Frames created with error detection	Frames	Physical
7	Physical	Bits transmitted over medium	Bits on wire	Transmission
8	Transmission	Data sent to receiver	Bits received	Physical (receiver)
9	Physical (receiver)	Bits converted to frames	Frames	Data Link (receiver)
10	Data Link (receiver)	Frames checked and passed	Frames	Network (receiver)
11	Network (receiver)	Packets routed to transport	Packets with addresses	Transport (receiver)
12	Transport (receiver)	Segments reassembled	Segments	Session (receiver)
13	Session (receiver)	Connection managed	Session data	Presentation (receiver)
14	Presentation (receiver)	Data decoded/decrypted	User data	Application (receiver)
15	Application (receiver)	Data presented to user	Readable data	End

💡 Data reaches Application layer on receiver side, completing transmission.

State Tracker

Layer	Data State Start	After Processing
Application	User input	Raw user data
Presentation	Raw user data	Encoded data
Session	Encoded data	Session data
Transport	Session data	Segments
Network	Segments	Packets with addresses
Data Link	Packets with addresses	Frames
Physical	Frames	Bits on wire
Physical (receiver)	Bits on wire	Frames
Data Link (receiver)	Frames	Frames
Network (receiver)	Frames	Packets with addresses
Transport (receiver)	Packets with addresses	Segments
Session (receiver)	Segments	Session data
Presentation (receiver)	Session data	User data
Application (receiver)	User data	Readable data

Key Insights - 3 Insights

Why does data change form at each layer?

Each layer adds or removes information needed for its specific task, as shown in the execution_table where data changes from raw to encoded, segmented, addressed, framed, and finally bits.

What happens if an error is detected at the Data Link layer?

The Data Link layer can request retransmission or correct errors before passing data up, as indicated in step 6 and 10 of the execution_table where frames are checked.

Why is the Physical layer last in sending data but first in receiving?

Physical layer sends raw bits over the medium and on receiving side it first converts bits back to frames, shown by steps 7 and 8 for sending and 8 and 9 for receiving in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the data get segmented?

A Step 2 - Presentation Layer

B Step 4 - Transport Layer

C Step 6 - Data Link Layer

D Step 1 - Application Layer

💡 Hint

Check the 'Action' column in execution_table row 4 where segmentation is mentioned.

At which step does the Physical layer send bits over the medium?

A Step 7

B Step 5

C Step 3

D Step 9

💡 Hint

Look for 'Bits transmitted over medium' in the 'Action' column of execution_table.

If the Presentation layer did not encode data, what would happen to the data state at step 3?

A Data would be segmented

B Data would be framed

C Data would still be raw user data

D Data would be bits on wire

💡 Hint

Refer to variable_tracker for Presentation layer showing data changes after encoding.

Concept Snapshot

OSI Model Seven Layers:
1. Application - user interface
2. Presentation - data encoding
3. Session - connection management
4. Transport - data segmentation
5. Network - routing packets
6. Data Link - framing and error detection
7. Physical - bit transmission
Data flows down layers to send, up layers to receive.

Full Transcript

The OSI model has seven layers that data passes through when sent over a network. Starting at the Application layer, user data is created and passed down. Each layer changes the data form to add necessary information: Presentation encodes it, Session manages connections, Transport segments it, Network routes packets, Data Link frames data with error checks, and Physical sends bits over the medium. On the receiving side, data moves up these layers in reverse order, being decoded and reassembled until it reaches the user. This step-by-step flow ensures reliable and organized communication between devices.