The Lazy Networker

Let’s walk through a real-world example:

👉 You send a WhatsApp text message to your friend.

📩 Step-by-Step Flow Through OSI Layers

At Your Side (Sender)

1. Application Layer (Layer 7)
   • You type “Hi!” in WhatsApp.
   • WhatsApp app uses HTTP/HTTPS + its own messaging protocols.
2. Presentation Layer (Layer 6)
   • Message is encrypted (end-to-end encryption).
   • Emojis, fonts, and formats are standardized.
3. Session Layer (Layer 5)
   • A secure session is established between your phone and WhatsApp servers.
   • Maintains your login session.
4. Transport Layer (Layer 4)
   • WhatsApp decides TCP (for reliability) for text OR UDP (for voice/video).
   • Message is divided into segments, with sequence numbers.
5. Network Layer (Layer 3)
   • Adds your IP address (source) and your friend’s IP (destination).
   • Routers decide the best path to the server or receiver.
6. Data Link Layer (Layer 2)
   • Adds MAC addresses (your phone → Wi-Fi router → ISP device).
   • Creates frames for local delivery.
7. Physical Layer (Layer 1)
   • Message is converted into electrical signals or Wi-Fi radio waves.
   • Sent over fiber/copper/wireless.

Across the Network

• The message travels through routers, switches, ISP networks, undersea cables, or satellites.
• At each stop, layers 1–3 (Physical, Data Link, Network) ensure correct forwarding.

At Your Friend’s Side (Receiver)

• The process is reversed:
  • Signals → Frames → Packets → Segments → Sessions → Decryption → WhatsApp app shows “Hi!”

✅ In short:

• Upper layers (7–5): Handle the meaning of your message.
• Middle (4): Handles reliability.
• Lower (3–1): Handle delivery.