1.1 and 1.2 – OSI Reference Model Notes

1.1 Introduction to the OSI Reference Model

Purpose of the OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework for understanding how network systems communicate.
Created by the International Organization for Standardization (ISO).
Ensures interoperability between products and software from different vendors.

Why It’s Important

Provides a common language for network communication.
Helps isolate network problems to specific layers.
Encourages modular design and easier upgrades.

The 7 Layers of the OSI Model

Layer Number	Layer Name	Main Function	Examples
7	Application	Interfaces with user applications and services	HTTP, FTP, SMTP
6	Presentation	Translates, encrypts, and compresses data	SSL/TLS, JPEG, MP3
5	Session	Establishes and maintains communication sessions	NetBIOS, RPC
4	Transport	Provides reliable or unreliable delivery	TCP, UDP
3	Network	Routes packets between devices	IP, ICMP
2	Data Link	Handles error detection and physical addressing	Ethernet, MAC addresses
1	Physical	Transmits raw bits over a physical medium	Cables, Hubs, NICs

1.2 Key OSI Model Concepts

Encapsulation and Decapsulation

Encapsulation means wrapping data with protocol information at each layer before transmission.
Decapsulation means the reverse process at the receiving end, removing headers as data moves up the layers.

Example: An email message (Application Layer) becomes bits on a wire (Physical Layer) after passing through all layers.

Data Units by Layer

Layer	Data Unit Name
Application, Presentation, Session	Data
Transport	Segments (TCP) / Datagrams (UDP)
Network	Packets
Data Link	Frames
Physical	Bits

Peer-to-Peer Communication

Each layer communicates with its peer layer on the receiving device using protocols.
Example: TCP on one computer communicates directly with TCP on another.

OSI vs. TCP/IP

OSI Model	TCP/IP Model
7 layers	4 layers
Conceptual and detailed	Practical and widely implemented
Transport layer uses TCP/UDP	Same protocols
Examples: IP, ICMP, HTTP	Same, but grouped differently

Benefits of the OSI Model

Standardization: Encourages universal communication protocols.
Interoperability: Ensures devices from different vendors can communicate.
Troubleshooting: Helps isolate network issues layer-by-layer.
Flexibility: Allows updates to one layer without affecting others.