WEEK 5: OSI Model - M199205zn/Datacomm-CS3 GitHub Wiki

Open System Network Model (OSI Model) Explained

The Open Systems Interconnection (OSI) Model is a conceptual framework used to understand how different networking protocols interact in a layered manner. It was developed by the International Organization for Standardization (ISO) to standardize network communications and ensure interoperability between different systems.

🛠️ The 7 Layers of the OSI Model

Each layer has a specific function and communicates with the layers above and below it.

1. Physical Layer (Layer 1) - "The Hardware Layer"

📌 Purpose: Defines the physical connection between devices.
📌 Functions:

Manages voltage levels, data rates, and physical connectors (cables, switches).
Converts digital data into electrical, optical, or radio signals.
📌 Examples: Ethernet cables, Fiber optics, Wi-Fi signals, Bluetooth.

2. Data Link Layer (Layer 2) - "The MAC & Switch Layer"

📌 Purpose: Provides reliable data transfer between two directly connected nodes.
📌 Functions:

Handles MAC (Media Access Control) addresses.
Manages error detection and correction using techniques like CRC (Cyclic Redundancy Check).
Controls frame synchronization in networks.
📌 Examples: Switches, MAC addresses, Ethernet, Wi-Fi (802.11).

3. Network Layer (Layer 3) - "The Routing Layer"

📌 Purpose: Determines how data moves between different networks.
📌 Functions:

Uses IP addresses to find the best path to the destination.
Performs routing (sending data packets across networks).
Manages logical addressing and packet forwarding.
📌 Examples: Routers, IP addresses, IPv4/IPv6, ARP (Address Resolution Protocol).

4. Transport Layer (Layer 4) - "The Reliability Layer"

📌 Purpose: Ensures complete and accurate data transmission.
📌 Functions:

Manages end-to-end communication.
Uses segmentation and reassembly to break large data into smaller packets.
Implements flow control and error handling.
📌 Examples: TCP (Transmission Control Protocol), UDP (User Datagram Protocol).

5. Session Layer (Layer 5) - "The Communication Management Layer"

📌 Purpose: Establishes, manages, and terminates communication sessions.
📌 Functions:

Controls dialog between applications (full-duplex, half-duplex communication).
Handles authentication and re-establishing lost connections.
📌 Examples: Remote Procedure Call (RPC), NetBIOS, Session Initiation Protocol (SIP).

6. Presentation Layer (Layer 6) - "The Translation Layer"

📌 Purpose: Converts data into a format the application layer can understand.
📌 Functions:

Encryption and decryption for secure communication.
Data compression for efficient transmission.
Character encoding and format conversion (e.g., ASCII, JPEG, MP3).
📌 Examples: SSL/TLS (Secure Sockets Layer/Transport Layer Security), JPEG, PNG, MP3, ASCII, Unicode.

7. Application Layer (Layer 7) - "The User Interface Layer"

📌 Purpose: Provides network services directly to end-users and applications.
📌 Functions:

Supports user interfaces for email, web browsing, file transfers, and remote access.
Uses protocols for different applications.
📌 Examples: HTTP (Web Browsing), FTP (File Transfer), SMTP (Email), DNS (Domain Name System).

🌍 Why is the OSI Model Important?

✅ Standardization – Ensures devices from different manufacturers work together.
✅ Troubleshooting – Helps identify network problems layer by layer.
✅ Security – Enhances security by implementing measures at different layers.
✅ Interoperability – Allows multiple network types (wired, wireless, cloud-based) to communicate effectively.

📌 OSI Model vs. TCP/IP Model

Feature	OSI Model	TCP/IP Model
Layers	7	4
Developed by	ISO	DoD (Department of Defense)
Focus	Theoretical, used for learning	Practical, used in real networks
Protocols	Conceptual (e.g., HTTP, FTP, TCP/IP fit into it)	Includes actual protocols (TCP, UDP, IP, etc.)