Unit 1: OSI Model Overview

What is the OSI model?

The OSI (Open Systems Interconnection) model is a conceptual framework developed by the ISO (International Organization for Standardization) in 1984 to standardise how different computer systems communicate over a network. It breaks the complex process of network communication into seven layers, where each layer has a specific responsibility.

Even though most modern networks actually run on the TCP/IP model, the OSI model is still the foundation taught in every networking course because it gives a clean mental model for understanding what happens to data as it travels across a network.

Why use a layered model?

Splitting networking into layers brings several practical benefits:

• Modularity — each layer can be developed and updated independently.
• Easier troubleshooting — problems can be isolated to a specific layer.
• Interoperability — different vendors can build hardware/software that work together as long as they follow the layer’s standard.
• Clear abstraction — higher layers don’t need to know how lower layers work.

The seven layers

The OSI model is usually presented from Layer 7 (top) down to Layer 1 (bottom).

Layer	Name	Unit	Example protocols / tech
7	Application	Data	HTTP, FTP, SMTP, DNS, SSH
6	Presentation	Data	TLS, JPEG, MP4, JSON, Protobuf
5	Session	Data	NetBIOS, RPC, PPTP
4	Transport	Segment	TCP, UDP, QUIC
3	Network	Packet	IP, ICMP, BGP, OSPF
2	Data Link	Frame	Ethernet, PPP, ARP
1	Physical	Bit	Cables, fiber, Wi-Fi radio

A common memory aid (top to bottom): “All People Seem To Need Data Processing.”

Data encapsulation

When you send data from one computer to another, it travels down the stack on the sender side and up the stack on the receiver side. As it moves down, each layer adds its own header (and sometimes a trailer) — this is called encapsulation.

Sender                                  Receiver
+----------------+                     +----------------+
| Application    |  Data               | Application    |
| Presentation   |  Data               | Presentation   |
| Session        |  Data               | Session        |
| Transport      |  Segment            | Transport      |
| Network        |  Packet             | Network        |
| Data Link      |  Frame              | Data Link      |
| Physical       |  Bits  ---------->  | Physical       |
+----------------+                     +----------------+

On the receiver side the headers are stripped off one by one (de-encapsulation) until the original data reaches the application.

OSI vs TCP/IP model

The TCP/IP model is the one actually used on the internet. It collapses some OSI layers together.

OSI (7 layers)	TCP/IP (4 layers)
Application
Presentation	Application
Session
Transport	Transport
Network	Internet
Data Link, Physical	Network Access

How this module is organised

The remaining units cover each OSI layer in detail, from the bottom up:

1. Unit 2 — Physical Layer
2. Unit 3 — Data Link Layer
3. Unit 4 — Network Layer
4. Unit 5 — Transport Layer
5. Unit 6 — Session Layer
6. Unit 7 — Presentation Layer
7. Unit 8 — Application Layer

For every layer we will look at its responsibilities, protocols and devices, a worked example, and a real-world scenario showing how large companies (Google, Meta, Netflix, Cloudflare, AWS) actually implement and rely on that layer.