OSI Model Overview

Definition

• OSI Model: Stands for the Open Systems Interconnection Reference Model.
• Purpose: Describes how data moves from one part of the network to another.
• Use: Enables IT professionals to communicate effectively about network traffic.

Importance

• OSI Model continues to be a reference, even though the OSI protocol suite didn't catch on.
• TCP/IP is primarily used today, but the OSI Model remains a crucial reference.

Structure

• Consists of seven layers.
• Common mnemonic: "All People Seem To Need Data Processing."
• Layers: Application, Presentation, Session, Transport, Network, Data Link, Physical.

Details of Each OSI Layer

Layer 1: Physical Layer

• Focus: Signaling and cables.
• Concerns: Cabling, connectors, punchdowns, loopback tests.
• Identified problems: Physical layer problems relate to signal/cable issues.

Layer 2: Data Link Layer

• Also called: Data Link Control (DLC) layer.
• Protocol Example: Ethernet uses MAC addresses ("layer 2 addresses").
• Role: Forwarding decisions ("switching layer").

Layer 3: Network Layer

• Also known as: Routing layer.
• Focus: IP addresses.
• Role: Fragmentation of frames for different network types (e.g., Ethernet to WAN).

Layer 4: Transport Layer

• Nickname: "Post office layer."
• Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
• Function: Splitting large data into frames, reassembling on the other side.

Layer 5: Session Layer

• Role: Start/stop communication sessions.
• Example: Control protocols, tunneling protocols.

Layer 6: Presentation Layer

• Focus: Data encoding and encryption.
• Often combined with: Application layer.

Layer 7: Application Layer

• User interface layer.
• Examples: HTTP, FTP, DNS.

Real-World Application

Practical Examples

• Layer 1: Holding a cable or analyzing signals.
• Layer 2: Working with MAC addresses or switch forwarding.
• Layer 3: Operating with routers based on IP addresses.
• Layer 4: Dealing with TCP/UDP ports.
• Layer 5: Managing sessions and tunneling.
• Layer 6: Encryption processes (e.g., HTTPS).
• Layer 7: Interacting with browsers.

Tools

• Wireshark: An application showing network traffic at different OSI layers.
  • Displays frames, source/destination MAC addresses, IP addresses.
  • Groups layers 5, 6, 7 together.

Example: Google Mail

• Application Layer (Layer 7): Using the mail interface.
• Presentation Layer (Layer 6): SSL encryption.
• Session Layer (Layer 5): Linking presentation/application to lower layers.
• Transport Layer (Layer 4): TCP 443 communications.
• Network Layer (Layer 3): IP encapsulation.
• Data Link Layer (Layer 2): Ethernet frame encapsulation.
• Physical Layer (Layer 1): Physical signals across the network.

Conclusion

• The OSI Model divides network communication into layers, aiding in troubleshooting and understanding applications.