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OSI Model Overview and Layers

Jun 5, 2025

Overview

This lecture introduced the OSI (Open Systems Interconnection) model, explaining its seven layers and how it helps IT professionals describe and troubleshoot network data flow.

OSI Model Structure and Purpose

  • The OSI model provides a broad overview of how data traverses networks, not a detailed protocol suite.
  • It is applicable to many protocols, including TCP/IP, and helps standardize network communication discussions.
  • Each OSI layer may have multiple protocols operating within it.
  • OSI layers are commonly referenced by number or name for troubleshooting and communication clarity.

The Seven Layers of the OSI Model

  • Layer 1: Physical Layer — Concerns the transmission of raw signals via cables, fibers, or wireless; troubleshooting includes cable tests and adapter checks.
  • Layer 2: Data Link Layer — Manages direct communication between devices using MAC addresses; associated with switches and ethernet frames.
  • Layer 3: Network Layer — Handles IP addressing, routing, and packet fragmentation; routers operate here.
  • Layer 4: Transport Layer — Oversees the transport of data using TCP or UDP; breaks data into packets and manages delivery.
  • Layer 5: Session Layer — Manages the start, maintenance, and end of communication sessions; handles tunneling and session protocols.
  • Layer 6: Presentation Layer — Translates data formats, character encoding, and encryption/decryption, e.g., SSL/TLS operations.
  • Layer 7: Application Layer — The visible interface for users; where application protocols like HTTP, FTP, and DNS operate.

Real-World Examples and Diagnostics

  • Layer 1: Cables, fiber optics, wireless signals.
  • Layer 2: Ethernet frames, MAC addresses, EUI-48/EUI-64 identifiers, switching.
  • Layer 3: IP addresses, subnet masks, routing.
  • Layer 4: TCP/UDP port numbers.
  • Layer 5: Control protocols for session initiation/termination, tunneling protocols.
  • Layer 6: Application data encryption/decryption (SSL/TLS).
  • Layer 7: Interaction with applications, user-visible messages.

OSI Model in Practice (Wireshark Example)

  • Physical layer: Frame details (bytes on the wire).
  • Data Link layer: Ethernet frame, source/destination MAC addresses.
  • Network layer: IP protocol, source/destination IP addresses.
  • Transport layer: TCP details, port numbers (e.g., port 443 for HTTPS).
  • Top layers (5-7): SSL/TLS encryption and application data (e.g., Gmail frontend).

Key Terms & Definitions

  • OSI Model — A seven-layer conceptual framework for describing network data flow.
  • MAC Address — Media Access Control address, a unique identifier for network devices at layer 2.
  • IP Address — Numerical label assigned to each device involved in a network, used at layer 3.
  • TCP/UDP — Protocols that facilitate transport of data across networks at layer 4.
  • Switch — Network device operating at layer 2, forwarding data based on MAC addresses.
  • Router — Device operating at layer 3, directing data based on IP addresses.
  • SSL/TLS — Protocols for encrypting data, typically at layer 6.

Action Items / Next Steps

  • Review the seven OSI layers and their primary functions.
  • Practice identifying which OSI layer is involved in common networking scenarios.
  • Explore Wireshark or a similar tool to observe real network traffic and map protocols to OSI layers.

Full transcript