Explain packet switching and packet order
Trace how routers direct packets and how the destination reorders them after they arrive.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 2.1–2.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What device controls the route a packet takes?
What you will learn
- Describe how a message is divided into packets
- Explain that packets may follow different routes
- Explain why received packets are reordered
Routers choose packet routes
Packet switching breaks data into packets. A router directs each packet toward its destination; different packets from one message can take different routes through the network.
Apply and check the rule
Because routes and delays differ, packets may arrive out of order. The destination uses packet numbers to reorder them after all packets have arrived and reconstruct the message.
Pause and explain
What information helps restore packet order?
Worked example
Packets 1, 3 and 2 arrive at a destination in that order. How can the original sequence be restored?
Show the worked solution
- Each packet carries its packet number.
- The receiver stores the packets until the message is complete.
- It reorders them as 1, 2, 3 before rebuilding the message.
Answer Use the packet numbers to reorder the complete message as 1, 2, 3.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A message is sent in packets numbered 1–4. They arrive 3, 1, 4, 2. Give the reconstructed order.
Give me a hint
Sort by packet number rather than arrival time.
Compare my reasoning
- Read the four packet numbers.
- Place them in numerical sequence.
1, 2, 3, 4
Look for these in your work
- Arrival order ignored
- Sequence restored
Explain one reason packet 2 might arrive after packet 3.
Give me a hint
Packets can be routed separately.
Compare my reasoning
- The router may send the packets along different paths.
- One path may be slower or busier.
- Packet 3 can therefore arrive first even though its number is higher.
The packets may have taken different routes with different delays, so packet 3 arrived before packet 2.
Look for these in your work
- Different routes stated
- Delay linked to order
A video call is choppy because a few packets arrive late. Explain how packet switching supports delivery and what the receiver must do before using the complete data block.
Give me a hint
Connect routing to reassembly.
Compare my reasoning
- Routers direct packets toward the destination, potentially using different routes.
- The destination uses packet numbers to place packets in order.
- If packets are missing, the message is incomplete; reordering alone cannot recreate missing data.
Packet switching routes data in packets, which can travel separately. The receiver reorders arrived packets by number; missing packets still need recovery, because reordering cannot restore data that never arrived.
Look for these in your work
- Routing explained
- Reordering explained
- Missing data limitation stated
Common mistakes
- Assuming every packet follows the same route
- Reassembling packets in arrival order instead of packet-number order
What can you explain now?
Why can packets from one message arrive out of order?
Compare with the explanation
They may take routes with different delays
Packet switching allows different routes, so arrival times can differ.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What can the receiver do with out-of-order packets, and what can it not do with a missing packet?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.