A2.3.3 · Data transmissions
Packet switching
By the end you can
Explain how a message is split into packets, sent independently by different routes and put back together, with encapsulation and decapsulation.
How does a message get from your phone to a server on another continent? Not as one block. The internet uses : the message is cut into many small , each sent independently, and put back together at the far end.
What is in a packet#
Every packet has two parts. The payload is a slice of your actual data. The header is information for the network: the source and destination IP addresses, a (this is packet 4 of 11), error-checking data such as a checksum, and a time to live that stops a lost packet wandering for ever.
The steps#
- Segmentation: the data is split into pieces small enough to send.
- Encapsulation: each piece is wrapped with its headers (A2.1.5).
- Independent transmission: each packet is sent separately. At each router, the destination address is looked up in the routing table to choose the next hop, based on current traffic and which links are working. Different packets of the same message may take different routes.
- Delivery and decapsulation: at the destination the headers are checked and removed.
- Reassembly: the packets are sorted into order by sequence number. Damaged or missing ones are sent again, then the complete message is handed to the application.
Inside a local network, switches forward frames by MAC address; between networks, routers forward packets by IP address.
Click a link to break it. Coloured lines are the routes the packets took.
Packets
6
Different routes
3
Arrived in order?
no
| Arrived | Seq | Payload | Route |
|---|---|---|---|
| 1th | 1 | “Meet m” | A → D |
| 2th | 2 | “e at t” | A → D |
| 3th | 3 | “he nor” | A → D |
| 4th | 6 | “oon.” | A → D |
| 5th | 4 | “th gat” | A → B → C → D |
| 6th | 5 | “e at n” | A → F → E → D |
Put back together by sequence number: “Meet me at the north gate at noon.”
Why it works so well#
- Sharing: packets from many conversations travel over the same links, so no link is reserved and sitting idle (compare the old phone system, which held a whole line for each call).
- Resilience: if a link or router fails, the next packets simply take another route, and the conversation continues.
- Cheap repair: if one packet is damaged, only that small packet is re-sent.
The costs are the extra header on every packet, the work of reassembly, and variable delay: arrival times differ from packet to packet, which matters for live voice and video.
Check yourself
Packet switching
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
5 questions · 18 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.State two items of information held in the header of a packet.
[2]State: Give a short answer, no explanation needed.
2.Explain why packet switching is an efficient way to send data across a network.
[3]Explain: Give reasons: say what happens AND why, or how one thing causes another.
3.Outline how a message is put back together when its packets arrive in a different order.
[3]Outline: A brief account: the main points, without much detail.
4.Describe the roles of routers and switches in packet switching.
[4]Describe: A detailed account: say what happens, step by step if it is a process.
5.Explain how packet switching is used to send data across a network, including the processes of encapsulation and decapsulation.
[6]Explain: Give reasons: say what happens AND why, or how one thing causes another.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| May 2024 TZ1 | 12(c) | 5 | Packet switching steps (same in TZ2) |
| November 2024 | 11(d) | 2 | What a packet header holds |
| November 2023 | 4 | 3 | What a packet contains |
| November 2022 | 10(e) | 4 | Splitting data into packets and rebuilding it |
| November 2018 | 5(a) | 4 | How data is sent by packet switching |
| November 2017 | 11(e) | 6 | Packet switching using a packet layout |
| May 2016 | 8(a) | 3 | How VoIP uses packets across an organisation |
In a nutshell
- Packet switching splits a message into small packets, each with a header (addresses, sequence number, error check) and a payload.
- Wrapping data in headers is encapsulation; removing them at the other end is decapsulation.
- Each packet is routed on its own, hop by hop, using routing tables, so packets can take different routes and arrive out of order.
- The receiver reorders packets using sequence numbers and has any missing ones sent again.
- It shares links between many users, copes with failures by routing round them, and only re-sends the damaged piece.