The OSI Model is a conceptual framework that standardizes the functions of a networking system into 7 layers, guiding how data moves from one device to another across a network.
OSI Model: A 7-layer reference model developed by ISO to understand and design network systems that communicate over a network.
- Break down network communication into manageable layers
- Help in troubleshooting and protocol design
- Ensure interoperability between different systems and vendors
| Layer No. |
Name |
Function |
| 7 |
Application |
Interfaces with end-user applications |
| 6 |
Presentation |
Data translation, encryption, compression |
| 5 |
Session |
Manages sessions and connections between devices |
| 4 |
Transport |
Ensures reliable data transfer (TCP/UDP) |
| 3 |
Network |
Handles routing, IP addressing (e.g., IP) |
| 2 |
Data Link |
MAC addressing, error detection (e.g., Ethernet) |
| 1 |
Physical |
Transmits raw bits over physical media (e.g., cables, signals) |
-
Sender Side (Top to Bottom):
- Data is created at the Application Layer
- Passed through each lower layer, formatted, segmented, and encoded
- Sent over physical media at the Physical Layer
-
Receiver Side (Bottom to Top):
- Bits are received via Physical Layer
- Each layer decodes, reassembles, and passes it up
- Final data is delivered to the Application Layer
Like sending a letter:
- You write it (Application)
- Put it in an envelope (Presentation)
- Add sender/receiver info (Session)
- Choose delivery type (Transport)
- Add postal address (Network)
- Drop it in a postbox (Data Link & Physical)
The OSI Model can be compared to sending a letter through a postal system. Each layer performs a specific function to ensure successful delivery, just like steps in mailing a letter.
- You compose the content of the message (what you want to say).
- This is the user-facing part, like using email or a messaging app.
- Analogy: You write the actual letter content on paper.
- Translates the data into a format both sender and receiver understand.
- Handles encryption, compression, or data conversion (e.g., text to binary).
- Analogy: You format the letter neatly, translate it if needed, and put it into an envelope.
¶ 3. Session Layer (Layer 5) — Addressing and Session Control
- Manages sessions or conversations between two parties.
- Handles start, maintenance, and termination of the session.
- Analogy: You write the sender and receiver addresses, and agree on delivery tracking or expected delivery time.
- Ensures reliable or fast delivery (TCP = reliable, UDP = faster but no guarantee).
- Breaks data into segments and numbers them for reassembly.
- Analogy: You choose between registered mail (safe and tracked) or regular post (faster, less secure).
- Determines the best path to the destination using IP addresses.
- Handles logical addressing and routing between networks.
- Analogy: The post office checks the address and routes the letter across cities or countries.
- Packages data into frames for transmission across the local network.
- Adds MAC addresses, handles error detection, and ensures proper framing.
- Analogy: The letter reaches your local postman, who reads your building/apartment number and prepares for doorstep delivery.
- Transmits raw bits (0s and 1s) over cables or wireless signals.
- Concerned with the physical medium: electrical signals, fiber optics, radio waves, etc.
- Analogy: The postman physically places the letter into your mailbox — the final delivery step.
Just like every step in mailing a letter ensures it reaches the correct person safely and correctly, each OSI layer plays a critical role in ensuring digital data travels from sender to receiver accurately.
This analogy helps visualize how complex network communication is organized and standardized.
The OSI Model helps understand how data travels through a network, layer by layer. It's a foundational concept in networking, useful for learning, design, and troubleshooting.