OSI Model in Computer Networks
Simple notes on the OSI Model explained in easy language for quick understanding.
🌐 OSI Model — Computer Networks (Simple Notes)
🧠 What is OSI Model?
The OSI (Open Systems Interconnection) Model is a standard framework used to understand how data travels from one computer to another over a network.
It was developed by ISO (International Organization for Standardization) and divides the communication process into 7 layers.
Each layer has its own job and helps data move smoothly between devices.
⚙️ Why It’s Important
- Makes communication between different systems possible.
- Helps in troubleshooting network problems.
- Guides protocol design and standardization.
- Ensures interoperability between devices from different vendors.
🧩 The 7 Layers of OSI Model (Simple Explanation)
| No. | Layer Name | Data Unit | Main Work |
|---|---|---|---|
| 7️⃣ | Application Layer | Data | User interacts with network (browsers, emails). |
| 6️⃣ | Presentation Layer | Data | Converts, encrypts, and compresses data. |
| 5️⃣ | Session Layer | Data | Starts and ends sessions between devices. |
| 4️⃣ | Transport Layer | Segments | Ensures reliable data delivery using TCP/UDP. |
| 3️⃣ | Network Layer | Packets | Finds best route using IP addresses. |
| 2️⃣ | Data Link Layer | Frames | Transfers data between two directly connected devices. |
| 1️⃣ | Physical Layer | Bits | Sends raw bits through cables and signals. |
🧱 Short Explanation of Each Layer
1️⃣ Physical Layer
Handles cables, signals, voltage — sends bits (0s and 1s).
Example: Cables, hubs, repeaters.
2️⃣ Data Link Layer
Transfers frames between connected devices and fixes basic errors.
Example: Switches, MAC address, Ethernet.
3️⃣ Network Layer
Chooses the best path for data (routing) and assigns IP addresses.
Example: Routers, IP, ICMP.
4️⃣ Transport Layer
Ensures data reaches correctly and in order.
Example: TCP (reliable), UDP (fast).
5️⃣ Session Layer
Opens, manages, and closes communication sessions.
Example: API connections, Remote calls.
6️⃣ Presentation Layer
Changes data format and adds encryption or compression.
Example: SSL/TLS, JPEG, ASCII.
7️⃣ Application Layer
Provides user services like email, file transfer, and web browsing.
Example: HTTP, FTP, DNS, SMTP.
🔄 Data Flow (Encapsulation & Decapsulation)
When sending data, each layer adds its own header (Encapsulation).
When receiving, each layer removes its header (Decapsulation).
Example:
Application → Transport → Network → Data Link → Physical
(bits travel through cables)
📦 Data Unit Names (PDU)
| Layer | PDU |
|---|---|
| Application–Session | Data |
| Transport | Segment |
| Network | Packet |
| Data Link | Frame |
| Physical | Bits |
🧠 Easy Mnemonics
- Top to Bottom: All People Seem To Need Data Processing
- Bottom to Top: Please Do Not Throw Sausage Pizza Away
🔗 OSI vs TCP/IP
| Feature | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 | 4 |
| Type | Theoretical | Practical |
| Example Protocols | HTTP, FTP, TCP, IP | Same protocols used |
| Creator | ISO | Department of Defense (DoD) |
📘 Summary
- OSI Model has 7 layers, each handling a specific part of data transfer.
- Helps in understanding, designing, and debugging networks.
- Acts as the foundation of modern networking.
✅ In short: OSI Model explains how data travels from one device to another step by step.
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