OSI Model (Open Systems Interconnection Model)

The OSI Model is a conceptual framework used to understand how data flows in a network. It was developed by the International Organization for Standardization (ISO) to standardize network communication.

It consists of seven layers, each with specific functions. These layers work together to send and receive data between devices across a network.

OSI Model Overview

Layer No.	Layer Name	Function
7	Application	Interacts with user applications (e.g., web browsers, email clients).
6	Presentation	Formats, encrypts, and compresses data.
5	Session	Manages sessions (start, maintain, and terminate connections).
4	Transport	Ensures reliable or fast delivery of data (TCP/UDP).
3	Network	Routes data between devices using IP addresses.
2	Data Link	Manages frames and MAC addresses for physical transmission.
1	Physical	Transmits raw bits over cables or wireless signals.

Physical Layer (Layer 1)

- Handles the **physical connection** between devices.  
- Converts data into electrical, optical, or radio signals for transmission.  
- When you connect a LAN cable to a router, the **Physical Layer** ensures that bits are transmitted correctly.

Key Components:
- Cables (Ethernet, fiber optics).
- Wireless signals (Wi-Fi, Bluetooth).
- Network adapters and hubs.

Data Link Layer (Layer 2)

- **Formats data into frames** for transmission.  
- Adds **MAC addresses** (physical addresses of network devices).  
- Detects and corrects errors in data transmission.  
- **Switches, Bridges, MAC Addresses**.  
- When a switch directs network traffic **based on MAC addresses**, it works at the **Data Link Layer**.

Divided into Two Sublayers:
1. LLC (Logical Link Control) – Error checking & flow control.
2. MAC (Media Access Control) – Defines access to the physical network.

Network Layer (Layer 3)

- **Routes data** between different networks using **IP addresses**.  
- Determines the **best path** for data packets.  
- **Routers, IP Addresses (IPv4/IPv6), ARP, ICMP**.  
- When you visit a website, your computer contacts a router, which uses **IP addresses** to find the destination server.

Transport Layer (Layer 4)

- Ensures **end-to-end delivery** of data between devices.  
- Handles **error correction, segmentation, and reassembly**.  
- Uses **TCP (reliable) and UDP (fast but unreliable)**.  
- **Streaming a video** uses **UDP** for speed, while **downloading a file** uses **TCP** for reliability.

Key Protocols:
- TCP (Transmission Control Protocol) – Ensures reliable delivery, error checking, and acknowledgment.
- UDP (User Datagram Protocol) – Faster but doesn’t guarantee delivery.

Session Layer (Layer 5)

- **Establishes, maintains, and terminates** communication sessions.  
- Manages sessions between applications.  
- A **Skype call** maintains a session so both users can communicate.

Key Responsibilities:
- Session establishment (e.g., logging into a remote server).
- Session synchronization (e.g., saving a paused video call).

Presentation Layer (Layer 6)

- **Translates data** into a format the application can understand.  
- Handles **encryption, decryption, compression, and encoding**.  
- **Data format conversion** (e.g., converting text to ASCII).  
- **Encryption and decryption** (e.g., SSL/TLS for secure browsing).  
- **Compression** (e.g., reducing image file size).  
- When you **stream a video**, compression (e.g., MP4) is used to reduce file size.

Application Layer (Layer 7)

- Directly interacts with **user applications**.  
- Provides **network services** like file transfers, web browsing, and email.  
- When you type **"www.google.com"**, the **DNS** resolves the domain to an IP address.

Key Protocols:
- HTTP/HTTPS (Web browsing).
- FTP (File transfers).
- SMTP/POP3/IMAP (Email communication).
- DNS (Domain name resolution).

How Data Moves Through OSI Layers (Encapsulation & Decapsulation)

Encapsulation (Sending Data):
- Data moves from the Application Layer (L7) to the Physical Layer (L1).
- Each layer adds headers (e.g., IP, MAC) before transmission.
Decapsulation (Receiving Data):
- Data moves from the Physical Layer (L1) to the Application Layer (L7).
- Each layer removes headers and processes data accordingly.

OSI Model vs. TCP/IP Model

Feature	OSI Model	TCP/IP Model
Layers	7	4
Developed by	ISO	DoD (Department of Defense)
Main Purpose	Conceptual framework	Practical implementation
Structure	Application, Presentation, Session, Transport, Network, Data Link, Physical	Application, Transport, Internet, Network Access
Example Protocols	HTTP, FTP, TCP, UDP, IP, Ethernet	HTTP, FTP, TCP, IP, Ethernet

Why is the OSI Model Important?

Standardization – Ensures different networks and devices can communicate.
Troubleshooting – Helps identify network issues at specific layers.
Scalability – Allows new technologies to integrate seamlessly.
Security – Provides structured security measures at different layers.

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