OSI Model Overview

Overview

This lecture introduces the OSI (Open Systems Interconnection) reference model, explaining its seven layers and their roles in network communications.

Introduction to the OSI Model

• The OSI model provides a standard framework to describe how data moves through a network.
• It is a conceptual model, not a protocol suite; most modern protocols (like TCP/IP) map to its layers.
• Using OSI terminology enables clear communication among IT professionals.

The Seven OSI Layers (Top to Bottom)

• Layer 7: Application — Where users interact with applications (HTTP, FTP, DNS, etc.).
• Layer 6: Presentation — Handles data format, encryption, and decryption so data is readable.
• Layer 5: Session — Manages creation, maintenance, and termination of communication sessions.
• Layer 4: Transport — Ensures reliable data transfer between devices (TCP, UDP).
• Layer 3: Network — Determines routing of data by IP address; fragmentation and reassembly occur here.
• Layer 2: Data Link — Manages direct node-to-node data (MAC addresses, Ethernet, switching).
• Layer 1: Physical — Concerned with physical media, signals, cables, and hardware transmission.

OSI Layer Mapping in Practice

• Layer 1: Issues like bad cables or wireless interference are physical layer problems.
• Layer 2: Switches use MAC addresses; troubleshooting involves Ethernet frames and adapters.
• Layer 3: Routers operate here; problems involve IP addresses, subnet masks, and routing.
• Layer 4: Involves TCP/UDP ports and transport of data between endpoints.
• Layer 5: Session control and tunneling protocols operate here.
• Layer 6: SSL/TLS encryption and data formatting happen at this layer.
• Layer 7: Direct interaction with application interfaces and network services.

Wireshark Example Breakdown

• Physical layer: Shows total bytes on the wire.
• Data Link layer: Displays source and destination MAC addresses.
• Network layer: Shows IP addresses for source and destination.
• Transport layer: Indicates TCP/UDP ports; e.g., port 443 for HTTPS.
• Session, Presentation, Application layers: Secure Socket Layer (SSL) encapsulates these upper layers handling encryption and application data (e.g. webmail).

Key Terms & Definitions

• OSI Model — A seven-layer model for understanding network communication.
• MAC Address — The hardware address used by the data link layer.
• IP Address — Logical address used for routing in the network layer.
• TCP/UDP — Transport layer protocols for reliable (TCP) or fast (UDP) data delivery.
• Session — A communication connection established between two devices.
• Presentation Layer — Translates, encodes, and encrypts data for the application.
• Application Layer — The interface for applications to access network services.

Action Items / Next Steps

• Learn and memorize the seven OSI layers and their functions.
• Review protocol examples for each OSI layer.
• Practice identifying OSI layers in tools like Wireshark.