OSI Model Overview

Introduction

• OSI Model: Open Systems Interconnection reference model
  • Describes how traffic moves from one part of the network to another
  • Model is a guideline and very broad
  • Common language for IT professionals
  • Primarily use TCP/IP today, but the OSI model is still a reference standard

OSI Layers and Protocols

• Seven Layers of the OSI Model
  • Layer 7: Application
  • Layer 6: Presentation
  • Layer 5: Session
  • Layer 4: Transport
  • Layer 3: Network
  • Layer 2: Data Link
  • Layer 1: Physical

Mnemonic for OSI Layers

• Mnemonic: "All People Seem To Need Data Processing"
  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Layer Details

Layer 1: Physical

• Deals with signaling and physical transmission media
  • Cables, fiber runs, physical network connections
• Troubleshooting: Check cabling, punch down connections, connectors
  • Use loopback tests and replace cables/adapters

Layer 2: Data Link

• Responsible for data transfer between adjacent network nodes
  • Known as Data Link Control (DLC) layer
  • Uses protocols like MAC addresses (Media Access Control)
  • Referred to as the switching layer
• Switches forward decisions based on MAC addresses

Layer 3: Network

• Focuses on IP addresses and routing
  • Known as the routing layer
  • Fragmenting data across different networks
• Devices make forwarding decisions based on IP addresses

Layer 4: Transport

• Manages data delivery and error checking
  • Known as the post office layer
  • Uses protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)
• Ensures data is sent and reassembled correctly

Layer 5: Session

• Manages sessions between networked devices
  • Start, stop, and manage communication sessions
  • Uses control and tunneling protocols

Layer 6: Presentation

• Ensures data is in a usable format
  • Deals with data encoding, encryption, and formatting
  • Often combined with the application layer

Layer 7: Application

• Closest to the end user
  • Handles network services like file transfers, email, and other network software services
  • Protocol examples: HTTP, FTP, DNS

Practical Examples

Layer 1 (Physical)

• Working with cables or signal analysis

Layer 2 (Data Link)

• Examining MAC addresses, working with switches

Layer 3 (Network)

• Handling IP addresses, routers

Layer 4 (Transport)

• Managing TCP/UDP protocols and port numbers

Layer 5 (Session)

• Establishing and managing sessions, tunneling

Layer 6 (Presentation)

• Handling encryption and data formatting

Layer 7 (Application)

• Interaction with web browsers, application data processing

Application in Tools (Wireshark Example)

• Wireshark: Captures data across the network with views representing different OSI layers
  • Top window: Summary view (overview of frames)
  • Middle window: Detailed view (OSI layers broken out)
    • Example: Frame 88 details physical, data link, network, and transport layers

Applying OSI Model Concepts

• Example Application: Google Mail
  • Layer 7: Interacting with Gmail interface
  • Layer 6: SSL encryption
  • Layer 5: Session management
  • Layer 4: TCP communication, port 443
  • Layer 3: IP address communication
  • Layer 2: Ethernet frames
  • Layer 1: Physical transmission via signals

Using the OSI Model for Troubleshooting

• Common framework for diagnosing network issues
• Allows precise communication among IT professionals

Summary

• OSI model as a foundational tool in IT
  • Describes data transmission from physical transmission to application use
  • Important for troubleshooting and understanding network communication