Network Plus Course: OSI Model Overview

Introduction to the OSI Model

• OSI Model: Open Systems Interconnection Reference Model
  • Describes the process data takes as it traverses networks
  • Provides a broad overview, not detailed descriptions
  • Works with many protocols, especially TCP/IP
• Purpose: Universal communication tool among IT professionals

OSI Model Layers

• Layer 7 (Application Layer)
  • Interaction with end-user applications (e.g., HTTP, FTP, DNS)
  • Mnemonic: "All People Seem To Need Data Processing"
• Layer 6 (Presentation Layer)
  • Data formatting for human viewing (character encoding, encryption)
• Layer 5 (Session Layer)
  • Managing sessions and connections (session initiation, termination)
• Layer 4 (Transport Layer)
  • Transporting data across networks (e.g., TCP, UDP)
• Layer 3 (Network Layer)
  • Routing data through networks (IP addressing, routing)
• Layer 2 (Data Link Layer)
  • Data transfer between devices (MAC addresses, Ethernet frames)
• Layer 1 (Physical Layer)
  • Physical transmission of signals (cables, fibers, wireless)

Detailed Layer Descriptions

Layer 1: Physical Layer

• Concerns physical signals and transmission
• Issues: Bad cables, interference, loopback tests

Layer 2: Data Link Layer

• Communication between network devices
• Focus: MAC addresses, Ethernet adapters
• Known as the "switching layer"

Layer 3: Network Layer

• Routing and forwarding data
• Focus: IP addresses, subnet masks

Layer 4: Transport Layer

• Reliable data transfer protocols
• Key protocols: TCP (Transmission Control Protocol), UDP (User Datagram Protocol)

Layer 5: Session Layer

• Session management (start, stop, restart sessions)

Layer 6: Presentation Layer

• Data presentation and transformation (encryption/decryption)

Layer 7: Application Layer

• User interface and application interaction
• Example protocols: HTTP, HTTPS, FTP

Practical Application: Wireshark Use Case

• Wireshark: Tool for capturing and analyzing network data
  • Frame-by-frame breakdown
  • Association with OSI model layers:
    • Physical Layer: Electrical signals
    • Data Link Layer: Ethernet frame info
    • Network Layer: IP address info
    • Transport Layer: TCP encapsulation
    • Application Layers (5-7): SSL/TLS encryption, application data

Conclusion

• The OSI model is a crucial framework for understanding and troubleshooting network communications
• Allows consistent communication among IT professionals about networking issues and processes