Notes on OSI Model

Overview of OSI Model

• OSI stands for Open Systems Interconnection Reference Model.
• Describes how traffic moves across a network.
• Serves as a guideline for IT professionals to communicate about data flow.

Importance of OSI Model

• Common language for IT professionals.
• Used as a reference despite the prevalence of TCP/IP protocols.

OSI Model Layers

1. Application Layer

   • Layer 7
   • Where users interact with applications (e.g., browsers).
   • Protocols: FTP, DNS, HTTP, HTTPS.

2. Presentation Layer

   • Layer 6
   • Responsible for data formatting, encoding, and encryption.
   • Prepares data for the application layer.

3. Session Layer

   • Layer 5
   • Manages sessions between devices.
   • Uses control and tunneling protocols.

4. Transport Layer

   • Layer 4
   • Controls data delivery over the network (e.g., TCP, UDP).
   • Ensures complete data transfer in multiple frames if necessary.

5. Network Layer

   • Layer 3
   • Responsible for IP addressing and routing.
   • Fragmentation of frames occurs here.

6. Data Link Layer

   • Layer 2
   • Deals with MAC addresses and switch forwarding.
   • Sometimes known as Data Link Control (DLC) layer.

7. Physical Layer

   • Layer 1
   • Concerned with physical connections and signaling.
   • Includes cables and network interface cards.

Mnemonic for OSI Layers

• All People Seem To Need Data Processing
  • Corresponds to: Application, Presentation, Session, Transport, Network, Data Link, Physical.

Practical Applications of OSI Model

• Physical Layer: Troubleshooting cables and signals.
• Data Link Layer: Working with MAC addresses and switches.
• Network Layer: Handling IP addresses and routers.
• Transport Layer: Managing TCP/UDP and port numbers.
• Session Layer: Establishing communication links.
• Presentation Layer: Encrypting data (e.g., HTTPS).
• Application Layer: User interfaces and application protocols.

Example: Using Wireshark

• Packet capture application showing OSI layers in traffic.
• Summary view, hexadecimal, and detailed view breakdown:
  • Layer 1: Physical layer information (e.g., byte count).
  • Layer 2: MAC address details.
  • Layer 3: IP information.
  • Layer 4: TCP details with port numbers.
  • Layers 5-7: SSL for secure communication between session and application layers.

Example Case Study: Google Mail

• Application Layer: Logging into Gmail.
• Presentation Layer: SSL encryption for security.
• Transport Layer: TCP protocol used for communication.
• Network Layer: IP addresses facilitate routing.
• Data Link Layer: Ethernet frames guide data transfer.
• Physical Layer: Signal transmission of frames.

Conclusion

• Understanding OSI model layers aids in troubleshooting and communication in IT.
• Each layer plays a crucial role in the overall process of data communication.