Journey of a Click
Down the layers, across the network, up the layers.
Your mission
What really happens between your click and the page appearing? Meet the seven layers of the OSI model, then follow your own message down the sender's layers, across the network as bits and back up at the receiver. Finally, put together the whole journey of a click.
Explore
Tap all seven layers of the OSI model to find out what each one does.
Each layer on the sender talks to the same layer on the receiver. The only real connection is at the bottom: the physical medium.
Networking is complicated, so the job is divided into layers. Each layer performs one particular communication task, and the layers work together to move data from one device to another. Tap each of the seven layers to find out what it does.
0 of 7 layers explored
Experiment
Send a message down the layers, across the network and up again. Peek inside the headers, and try UDP.
Layer experiments
Try each one. They tick themselves off.
0 of 6 experiments done
Challenge
Six hops: order the journey of a click, match jobs to layers, build a frame and explain encapsulation.
Traveller's notes
- 7 Application · 6 Presentation · 5 Session · 4 Transport · 3 Network · 2 Data Link · 1 Physical
- Transport: TCP/UDP header · Network: IP header · Data Link: frame header + trailer
- Sender: down 7 → 1, adding headers (encapsulation)
- Receiver: up 1 → 7, removing headers (decapsulation)
Finish all 6 hops to finish this stage.
Understand
Here is what the journey was showing you, in the words you need for the exam.
Networking is complicated, so we divide the job
Sending a parcel takes several separate jobs: write the message, pack it, add the address, choose a delivery route and physically transport it. Computer networking works in a similar layered way. The OSI model (Open Systems Interconnection Reference Model) divides the job into 7 layers. Each layer performs a particular communication task, and the layers work together to move data from one device to another.
| 7Applicatione.g. web, email, file transfer | Provides network services to user applications; creates the data |
|---|---|
| 6Presentatione.g. encryption, compression, encoding | Transforms data into a suitable format |
| 5Sessione.g. start, maintain, synchronise | Establishes, manages and ends communication sessions between applications |
| 4Transporte.g. TCP, UDP | Provides reliable or best-effort delivery; adds a transport header with port numbers |
| 3Networke.g. IP, routing | Routes packets between networks; adds an IP header with source and destination IP addresses |
| 2Data Linke.g. Ethernet, Wi-Fi | Framing, error detection and access to the local network; adds a frame header (MAC addresses) and trailer (e.g. CRC) |
| 1Physicale.g. cables, radio, fibre | Transmits raw bits over a physical medium, as electrical, optical or radio signals |
Sender: encapsulation
Data moves down the layers, from 7 to 1. Each layer adds its own control information: a header, and at the Data Link layer also a trailer.
At layer 1 the whole frame becomes bits: 0100100001100101…
Receiver: decapsulation
Data moves up the layers, from 1 to 7. Each layer reads and removes its own control information and passes the rest up: layer 2 checks for errors and removes the frame header and trailer, layer 3 checks the destination IP address and removes the IP header, layer 4 reorders the data and removes the transport header, and the application finally shows "Hello World!".
The big picture: what happens when you open https://www.hkmu.edu.hk
- 1Your device: the browser is loaded from storage (a hard disk or SSD).
- 2Your network: your device connects to the LAN (Network Builder).
- 3DNS: www.hkmu.edu.hk is translated into an IP address (Internet Address Detective).
- 4Router: packets leave your network through the router.
- 5Internet: data travels as packets across the Internet (Packet Postman).
- 6Protocols: TCP/IP manages communication between your device and the server.
- 7Security: HTTPS/TLS encrypts the connection, protecting your data (The Secret Message Lab).
- 8Website: the server responds and the webpage appears.
Exam tip
Focus on the concept before memorising names: networking tasks are divided into layers, each layer has a specific responsibility, and data moves down the layers → across the network → up the layers. Then learn the seven names in order. A memory aid from layer 7 down: All People Seem To Need Data Processing. Be ready to say which layer adds which header: Transport (TCP/UDP header), Network (IP header), Data Link (frame header and trailer), and that layer 1 sends the bits as signals.