Spec & Goals 3 min
AQA Spec 3.5.2 — Packets and packet switching
By the end of this lesson you can:
- Describe what a packet is and what it contains.
- Explain how packet switching moves data across a network.
- Give the advantages of packet switching.
Warm-Up 5 min
In Lesson 7, the Transport layer (TCP) "splits data into packets". Now we look at exactly what a packet is and how it finds its way across the internet.
Quick starter
To move a large wardrobe through narrow doorways, you take it apart and carry the pieces separately. Why might sending data work the same way?
Reveal the idea
Small pieces are easier to route, and different pieces can take different paths. Data is split into small packets, sent independently, and rebuilt at the other end.
Key Concept — small pieces, many paths 14 min
Before sending, data is broken into small chunks called packets. Each packet travels separately across the network and they are reassembled at the destination.
What a packet contains
| Part | What it holds |
|---|---|
| Header | The source and destination IP addresses, the packet number (so it can be reordered), and error-checking data. |
| Payload | The actual piece of data being carried. |
| Trailer / footer | Marks the end of the packet and holds error-checking (e.g. a checksum). |
How packet switching works
- The data is split into numbered packets.
- Each packet is sent independently and routers decide the best path for it at that moment.
- Packets may take different routes and arrive out of order.
- At the destination they are reassembled in order using the packet numbers; missing packets are re-requested.
Worked Example — a message in four packets 12 min
Problem: Wei Jie sends a message split into four packets from KL to a server in Penang. Show how packet switching delivers it.
- The message is split into packets 1, 2, 3, 4, each with the destination IP and its number.
- Routers send packets 1 and 3 by one path, 2 and 4 by another (whichever is best at the time).
- They arrive in any order — say 1, 3, 2, 4.
- The server uses the packet numbers to reassemble them in order; any lost packet is re-requested.
Try It Yourself 12 min
Goal: State two pieces of information held in a packet's header.
Hint: addresses and a number.
Goal: Describe how packets are reassembled into the right order at the destination.
Goal: Explain why packet switching is more resilient than sending a whole message along one fixed path.
📝 Exam Practice 10 min
State two items of data stored in the header of a packet.
Mark scheme
- Any two of: destination IP address; source IP address; packet/sequence number; error-checking data (2).
Describe how packet switching sends a file across the internet.
Mark scheme
- The file is split into packets (1).
- Packets are sent independently / routers choose the best route, so they may take different routes (1).
- They are reassembled in order at the destination using the packet numbers (1).
Explain one advantage of packet switching.
Mark scheme
- If a route is busy or fails, packets can take an alternative route (1).
- So data still gets through / the network is used efficiently / only lost packets need resending (1).
Recap & Key Terms 3 min
Data is split into packets, each with a header (source/destination IP, packet number, error-checking) and a payload. In packet switching, packets travel independently — possibly by different routes — and are reassembled in order at the destination. This is efficient and resilient: if a route fails, packets reroute.
- Packet
- A small unit of data, part of a larger message, sent across a network.
- Header
- The part of a packet holding source/destination IP addresses, the packet number and error-checking data.
- Payload
- The actual data being carried inside a packet.
- Packet switching
- Sending data as packets that travel independently (possibly by different routes) and are reassembled at the destination.
Homework 1 min
Task (≤ 15 min): A 6-packet message is sent and packets arrive in the order 1, 4, 2, 6, 3 — packet 5 never arrives. Explain how the destination handles this.
Model answer
Each packet carries a packet number, so the destination reorders the arrived packets into 1, 2, 3, 4, 6. It notices packet 5 is missing (a gap in the sequence) and requests it again from the sender. Once packet 5 arrives, the full message is reassembled in order 1–6.
Award marks for: reordered using packet numbers (1); missing packet 5 detected (1); packet 5 re-requested/resent (1).