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CCNANetworking

The OSI Model

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OSI Model

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🌐 All About the Transition of Data (Not Really, But Kinda): OSI Model Breakdown

📚 OSI Model Origins

  • OSI stands for Open Systems Interconnection model, created by the International Organization for Standardization (ISO).

🧱 PDU’s = Protocol Data Units

🔄 How the Life Cycle of PDU’s Moves Through the OSI Model

First of all… what are the OSI model layers?

  • Here’s a mnemonic to help you remember them: Please Do Not Throw Sausage Pizza Away

🔢 OSI Layers with Breakdown

🧃 Layer 1: Physical = Bits

  • “Please”
  • This includes all physical media: PCs, laptops, switches, routers, hubs, modems, cables, etc.
  • At this layer, data is broken down into its foundation—bits (1s and 0s).

🖧 Layer 2: Data Link = Frames

  • “Do”
  • This layer kicks in once devices are physically connected, particularly switches.
  • It handles MAC addresses (Media Access Control), used to identify physical devices.
  • Past this layer, MAC addresses are no longer used—IP addresses take over.
  • The PDU here is called a Frame.

🌐 Layer 3: Network = Packets

  • “Not”
  • Now we transition from physical to logical addressing.
  • This is where routers and Layer 3 (multilayer) switches come into play to route traffic between networks or out to the internet.
  • The PDU here is called a Packet.

⛓️ Learning Break: TCP vs UDP

At this point—between Layer 3 and Layer 4—we meet TCP and UDP.

🔐 TCP = Transmission Control Protocol

  • Connection-based
  • Begins with the three-way handshake: SYN → SYN-ACK → ACK
  • ✅ Reliable
  • ✅ Guarantees packets arrive in order
  • ✅ Retransmits lost/corrupted packets
  • ✅ Uses window size for flow control
  • ✅ Avoids overwhelming the receiver

Common Uses:

  • 🌍 Web Browsing (HTTP/HTTPS)
  • 📧 Email (SMTP, IMAP, POP3)
  • 📁 File Transfer (FTP, SFTP)
  • 🔐 Remote Access (SSH, Telnet)

🧨 UDP = User Datagram Protocol

  • Connectionless
  • ❌ No delivery confirmation
  • ❌ No order guarantee
  • ❌ No retransmission
  • Just sends data and hopes it lands

Common Uses:

  • 🎮 Online Gaming
  • 📞 VoIP
  • 📺 Video Streaming
  • 🌐 DNS Lookup

📦 Layer 4: Transport = Segments (or Datagrams)

  • “Throw”
  • This is where the three-way handshake happens for TCP connections.
  • The PDU for TCP is called a Segment.
  • For UDP, it’s called a Datagram.

💬 TCP Handshake Breakdown:

  1. Client sends SYN to server
  2. Server responds with SYN-ACK
  3. Client replies with ACK→ Connection is established

🗂️ Layer 5: Session = Data

  • “Sausage”
  • Once a connection is secure, this layer handles interhost communication and reassembles segments into packets, then into data.
  • Manages sessions between apps.

🎨 Layer 6: Presentation = Data

  • “Pizza”
  • This is where raw data becomes meaningful.
  • Think file encoding, encryption, and data formatting.
  • The binary data (1s and 0s) gets translated into what you see—like this web page.

🧩 Layer 7: Application = Data

  • “Away”
  • This is where APIs, user interfaces, and apps operate.
  • It’s the layer closest to the user—like your browser or email client.

📝 Final Thoughts

This is just an introduction. You can go way deeper into each of these subjects.

I wrote this mainly to help myself understand and review, and I’m sharing it in case someone else finds it helpful.

I’m not claiming to know everything—I’m just documenting the journey.

📚 References

  • ✍️ Spelling & Grammar: ChatGPT
  • 🖼️ Infographic: SepioCyber
  • 🎓 Videos: Jeremy’s IT Lab, David Bombal (YouTube)
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