Syllabus & Goals 3 min
Cambridge 2.1 · Types and methods of data transmission Paper 1 · Computer Systems
By the end of this lesson you can:
- Describe the structure of a data packet — header, payload and trailer.
- Explain how packet switching sends data across a network.
- State the benefits and drawbacks of packet switching.
Recap / Warm-Up 5 min
Unit 1 was about how data is represented. Unit 2 is about how data is moved from one device to another safely.
Quick starter
To post a 500-page book to a friend, would you send it as one huge parcel or several smaller ones? Give one reason.
Reveal a sensible answer
Several smaller parcels are easier to handle and can take different routes — if one is delayed, the others still arrive. Networks split data up for the very same reasons.
Key Concept 14 min
1 · Data packets
Data sent over a network is broken into small packets (about 64 KiB each). Each packet has three parts:
2 · Packet switching
Packet switching breaks a message into packets and sends each one independently. At every node there is a router that reads the header and chooses the next step toward the destination IP address.
Worked Example 12 min
Scenario: Aisyah in Penang sends a photo to Wei Jie in Johor Bahru. Trace what happens.
- The photo is split into (say) five packets, each with a header, payload and trailer.
- Each packet leaves computer A. At every router, the header's destination IP is read and the next route is chosen.
- Packets travel by different routes — some are busier than others.
- They may arrive out of order (e.g. 1, 3, 2, 5, 4).
- Computer B uses the sequence numbers to reassemble the photo correctly, and the trailer's error check confirms each packet arrived intact.
Benefits and drawbacks
| Benefits | Drawbacks |
|---|---|
| No single line is tied up | Packets can be lost and need re-sending |
| Busy / failed lines are bypassed by re-routing | Poor for real-time streaming (a live match) |
| High transmission rate is possible | Delay at the destination while packets are re-ordered |
Try It Yourself 12 min
Goal: List the three parts of a data packet, in order.
Goal: A web page arrives with its packets in the order 2, 1, 4, 3. Explain how the receiving computer puts them back into the right order.
Goal: Explain why packet switching is a poor choice for a live video call, using two of its drawbacks.
📝 Exam Practice 10 min
Identify two items of data held in a packet's header.
Mark scheme
- Any two of: sender IP address · receiver IP address · sequence number · packet size (1 each, max 2).
Describe the role of a router in packet switching.
Mark scheme
- A router receives each packet (1).
- It reads the header / destination IP address (1).
- It decides / forwards the packet on the next route toward its destination (1).
Explain one benefit of splitting a message into packets that take different routes.
Mark scheme
- If one route is busy or has failed (1)…
- …packets can be re-routed another way so the data still arrives (1).
Recap & Key Terms 3 min
Data is split into packets (header, payload, trailer). Packet switchingsends each packet independently; routerschoose routes from the header's IP address, and the receiver reassembles them in order using the sequence numbers.
- Data packet
- A small unit of data with a header, payload and trailer.
- Packet switching
- Sending a message as packets that travel independently and are reassembled at the destination.
- Router
- A device at a network node that reads a packet's header and forwards it toward its destination.
- Payload
- The actual data carried inside a packet.
Homework 1 min
Task (≤ 15 min): Draw a labelled diagram of a data packet, then write one sentence explaining what each of the three parts is for.
Model answer
- Header — control data: source & destination IP, sequence number, size (so the packet can be routed and reassembled).
- Payload — the actual data being carried.
- Trailer — marks the end of the packet and carries an error check.