OSI Model Overview Lecture Notes

Introduction

• OSI Model: Open Systems Interconnection Reference Model
• Purpose: Describe how traffic moves from one part of the network to another
• Usage: Common language for IT professionals
• Primary Protocol Today: TCP/IP, but OSI model is still used as a reference

OSI Model Layers

1. Physical Layer (Layer 1)

   • Focus: Signaling and transmission of raw data
   • Components: Cables, fiber optics, connectors, adapter cards
   • Troubleshooting: Check cabling, connectors, loopback tests

2. Data Link Layer (Layer 2)

   • Focus: Data transfer between adjacent network nodes
   • Components: Media Access Control (MAC) addresses, switches
   • Protocols: Ethernet, DLC protocols
   • Role: Forwarding decisions based on MAC addresses

3. Network Layer (Layer 3)

   • Focus: Data transfer across different networks
   • Components: IP addresses, routers
   • Functions: Routing, fragmentation of data packets

4. Transport Layer (Layer 4)

   • Focus: Reliable data transfer
   • Protocols: TCP (Transmission Control Protocol), UDP (User Datagram Protocol)
   • Functions: Data segmentation, reassembly

5. Session Layer (Layer 5)

   • Focus: Managing sessions between applications
   • Functions: Establishing, maintaining, and terminating connections
   • Typical Usage: Control protocols, tunneling protocols

6. Presentation Layer (Layer 6)

   • Focus: Data translation and encryption
   • Functions: Encoding, encryption, data formatting
   • Often Combined: With the Application Layer (Layer 7)

7. Application Layer (Layer 7)

   • Focus: Network services for applications
   • Components: Browsers, FTP, DNS, HTTP/HTTPS
   • Role: End-user interaction with network services

Practical Examples

• Layer 1: Holding a cable, analyzing signal
• Layer 2: Switch forwarding based on MAC addresses
• Layer 3: Routers making decisions based on IP addresses
• Layer 4: Using TCP/UDP protocols, port numbers
• Layer 5: Tunneling, setting up communications
• Layer 6: Encryption (e.g., HTTPS)
• Layer 7: User applications and services (e.g., web browsers)

Real-World Application: Packet Analysis with Wireshark

• Wireshark: Tool for capturing and analyzing network traffic
• Screen Sections:
  • Top: Summary view (one frame per line)
  • Middle: Detailed view (breaks down communication by OSI layers)
  • Bottom: Hexadecimal and ASCII representation
• Frame Example (Frame 88):
  • Layer 1: Physical layer, bytes on wire
  • Layer 2: Ethernet frame, MAC addresses
  • Layer 3: IP addresses
  • Layer 4: TCP protocol, port numbers
  • Layers 5-7: Secure Sockets Layer (SSL)

Use Case: Google Mail

• Layer 7: Application layer (mail.google.com interface)
• Layer 6: Presentation layer (SSL encryption)
• Layer 5: Session layer (linking layers)
• Layer 4: Transport layer (TCP 443)
• Layer 3: Network layer (IP addresses)
• Layer 2: Data link layer (Ethernet frames)
• Layer 1: Physical layer (signals)

Conclusion

• Benefits: Facilitates communication and troubleshooting within IT
• Applications: Understanding and managing network communications and applications