📡 How Does Data Travel?

Data Transmission Visualiser

When you send a message or load a webpage, the data has to physically travel from one device to another. But how? Watch it happen.

CS GCSE §1.3 CS A-Level Unit 3 DigiTech A-Level Unit 3

🤔 How does a message get from your phone to your friend's?

Data is sent as electrical signals (through wires), light pulses (through fibre optic), or radio waves (WiFi/4G). But there are rules about how it's sent — one bit at a time? All at once? Can both sides talk at the same time?

1️⃣

Serial vs Parallel Transmission

Should we send bits one at a time, or several at once?

📏 Serial — one lane road

Bits travel one at a time down a single wire. Like a single-lane road — cars go one behind the other.

  • ✅ Reliable — bits arrive in order
  • ✅ Works over long distances — USB, internet
  • ❌ Slower — one bit at a time
  • 📌 Examples: USB, SATA, Ethernet
🛣️ Parallel — motorway

Multiple bits travel at the same time on separate wires. Like a motorway — 8 lanes at once.

  • ✅ Faster — 8 bits at once
  • ❌ Skew — bits may arrive at different times
  • ❌ Short distances only — wires interfere
  • 📌 Examples: Internal CPU buses, old printer cables
2️⃣

Direction of Transmission

Can both sides send at the same time, or do they take turns?

📺
Simplex
One direction only
A → → → B
TV broadcast, keyboard to PC, radio
🔄
Half Duplex
Both directions, but take turns
A → → B
A ← ← B
Walkie-talkies ("over!")
📞
Full Duplex
Both directions at the same time
A ⇄ B
Phone call, video chat
3️⃣

Packet Switching

When you send a large file, it gets broken into small packets. Each packet finds its own route and they're reassembled at the other end. Like sending a jigsaw puzzle — one piece per envelope, different routes, put together when they all arrive.

💻
Your PC
🌐
Internet
🗄️
Server
📦 What's inside a packet?
Source IP
Where it's from
Dest IP
Where it's going
Seq #
Packet order (1 of 3)
Payload
The actual data
📝 Exam tip: Packets may arrive out of order (they took different routes). The sequence number tells the receiving computer how to put them back in the right order. If a packet goes missing, the receiver asks for it to be re-sent — this is part of the TCP protocol.