OSI Reference Model - aljimenez28/champlain GitHub Wiki

🧠 OSI Reference Model (Open Systems Interconnection)

The OSI Model is a framework that standardizes how data is transmitted and received over a network. It has 7 layers, each with a specific role.

🔟 Layer 7 – Application

  • What it does: Interfaces with the end-user (apps like web browsers, email, etc.)
  • Examples: HTTP, FTP, SMTP, DNS, POP3
  • Think: This is what the user sees and interacts with

💬 Layer 6 – Presentation

  • What it does: Formats and encrypts data for the application layer
  • Functions: Data translation, encryption/decryption, compression
  • Examples: JPEG, SSL/TLS, MPEG
  • Think: Translator for different data formats

📞 Layer 5 – Session

  • What it does: Manages sessions or connections between applications
  • Functions: Start, maintain, and terminate sessions
  • Examples: NetBIOS, RPC, PPTP
  • Think: Phone call manager for app connections

📦 Layer 4 – Transport

  • What it does: Provides reliable delivery of data
  • Functions: Error checking, flow control, retransmission
  • Examples: TCP (reliable), UDP (fast but unreliable)
  • Think: UPS/FedEx – ensures data is delivered properly

🌐 Layer 3 – Network

  • What it does: Handles routing and addressing of packets
  • Functions: IP addressing, routing, fragmentation
  • Examples: IP, ICMP, OSPF, RIP
  • Devices: Routers
  • Think: GPS – picks the best path for data

🔁 Layer 2 – Data Link

  • What it does: Moves frames across the local network
  • Functions: MAC addressing, error detection (via CRC), framing
  • Examples: Ethernet, PPP, ARP, VLAN
  • Devices: Switches, Bridges
  • Think: Post office clerk – adds address labels (MAC)

⚙️ Layer 1 – Physical

  • What it does: Sends raw bits (0s and 1s) over physical medium
  • Functions: Electrical/optical signaling, cabling, pin layouts
  • Examples: Cables, Hubs, NICs, Fiber optics
  • Devices: Hubs, Repeaters, Physical network cards
  • Think: Wires and plugs – the physical stuff

🧠 Mnemonic (Top to Bottom):

"All People Seem To Need Data Processing"

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Or reversed (Bottom to Top):
"Please Do Not Throw Sausage Pizza Away"