2.2 Packet Switching
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2.2 Packet Switching Trainer
Understand how data packets travel across a network — different routes, out-of-order arrival, and reassembly. The process Cambridge tests every session.
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§1 · Topic overview
§2 · Learning objectives
By the end of this lesson you should be able to:
- Define packet switching in the words the mark scheme uses
- Describe the packet switching process in ordered steps
- State the role of the router precisely — not just "it sends the data"
- Explain why each packet may take a different pathway
- Explain why packets are likely to arrive out of order
- State when packets are reordered, and what is used to reorder them
- Keep packet switching (2.2) separate from packet structure (2.1)
§3 · Key terminology
| Term | Definition |
|---|---|
| Packet switching | A method of transmitting data packets across a network. Each data packet is able to take an individual pathway across the network. |
| Router | A network component that examines a data packet to obtain its destination address and then forwards the packet to this address. |
| Network | Computers and devices joined together using cables or wireless technology. |
| Pathway (route) | The particular set of links a packet travels along to get from the sending device to the receiving device. |
| Destination address | The address in the packet header that the router examines to decide where to forward the packet. |
| Packet number | The number in the packet header used at the destination to put the packets back into the correct order. |
| Out of order | Packets arriving in a different sequence from the one they were sent in — the normal outcome of packet switching. |
| Reassembly | Reordering the packets by packet number, once they have all arrived, to recreate the original file. |
| Packet | A small unit of data. (From 2.1 — header, payload, trailer.) |
§4 · Core theory
The process, in order
| Step | What happens |
|---|---|
| 1 | The data is split into packets. |
| 2 | Each packet is given a header containing the destination address and a packet number. |
| 3 | Each packet is sent individually across the network. |
| 4 | When a packet reaches a router, the router examines the destination address and decides which pathway to send it along next. |
| 5 | Each packet can travel along a different pathway. |
| 6 | Packets are likely to arrive at the destination out of order. |
| 7 | Once all packets have arrived, they are reordered using the packet numbers to recreate the file. |
What the router does — and doesn't
| ✅ Creditworthy | ❌ Not creditworthy |
|---|---|
| Examines the packet's destination address | "It sends the data" |
| Decides which pathway the packet takes next | "It speeds up the network" |
| Forwards the packet towards that address | "It connects networks together" (true generally, but not its role here) |
Order of arrival
| Question | Answer |
|---|---|
| Why do packets arrive out of order? | Each one takes an individual pathway, so they do not all take the same time to arrive. |
| When are they reordered? | Only after all the packets have arrived. |
| What is used to reorder them? | The packet number in each header. |
§5 · Worked examples
Example 1 — Describe packet switching [5 marks]
Question: A file is sent from one device to another across a network. Describe how packet switching is used to transmit the file.
Answer: The data is split into packets [1]. Each packet is given a header containing the destination address and a packet number [1]. Each packet is sent individually and can take a different pathway across the network [1]. At each router the destination address is examined and the router decides the next pathway [1]. The packets arrive out of order, and once all have arrived they are reordered using the packet numbers to recreate the file [1].
Why it earns the marks: ordered, one idea per mark. Marks are awarded per distinct point, so a vague paragraph covering three ideas in one sentence still only banks what it clearly states.
Example 2 — Role of the router [2 marks]
Question: State the role of a router in packet switching.
Answer: It examines the packet to obtain its destination address [1] and forwards the packet along a chosen pathway towards that address [1].
Why it earns the marks: two distinct actions — read the address and choose/forward the route. "It sends the packets on" is one vague idea and scores 0–1.
Example 3 — Out-of-order arrival [3 marks]
Question: Explain why the packets may arrive at the destination out of order, and describe what happens next.
Answer: Each packet can take a different pathway across the network [1], so they do not all take the same amount of time to arrive [1]. Once all of the packets have arrived they are put back into the correct order using their packet numbers [1].
Why it earns the marks: the cause is different pathways → different journey times, and reassembly is stated as happening after all packets arrive.
§6 · Common misconceptions
No. Reordering happens once all the packets have arrived. Writing "they are reordered on arrival" loses the mark — the timing is the mark.
Too vague to credit. The router examines the destination address and decides which pathway the packet takes next.
The whole point of packet switching is that each packet can take an individual pathway.
"Connects networks together" and "filters traffic" may be true elsewhere, but the question asks about its role in packet switching. Answer that one.
Each packet is sent individually. They are not a convoy.
Header, payload and trailer are packet structure. If the question says switching, describe the journey, not the contents.
§7 · Cambridge exam focus
§8 · Quick knowledge check
Tap each card to reveal the answer. Say it out loud before you tap — that's the whole trick.
🔒 Nearly there
Work through the Quick Knowledge Check above. Once you've reviewed all six, the Activities unlock recommendation appears here.
What is packet switching?
The sat-nav analogy
Imagine 5 friends driving from the same start to the same destination. Each uses a different route based on traffic. They all arrive, but at different times and in a different order. The host waits for everyone, then seats them in the right order.
Packets on a network
Data is split into packets. Each packet is sent independently. Routers direct each packet along the best available path. Packets can arrive out of order and are reassembled at the destination.
The 7-step process (full marks)
Cambridge typically awards 5 marks from these 7 points. Learn all 7 to guarantee full marks.
Packet switching animation
Watch packets travel independently through different routes.
Press Animate to see packets travel independently.
The role of the router
What the mark scheme accepts
• Controls the route/path the packet takes
• Sends/directs each packet toward its destination
• Determines the best available path for each packet
Why packets arrive out of order
Each packet takes a different route through the network. Some routes are shorter, faster, or less congested than others. So packets that were sent later might arrive earlier — like the 5 friends taking different roads.
Quick check
When are packets reordered?
Activity 1 — Order the Steps
Put the packet switching steps in the correct order.
Press New Question.
Activity 2 — Spot the Mistake
Press New Question.
Activity 3 — Router or Not?
Is this the router's role in packet switching?
Press New Question.
Adaptive practice
Press New Question.
Cambridge-style questions
Press New Question.
Review quiz
Press New.
Exam traps
Memory triggers
🧠 "5 friends, 5 routes, 1 dinner table"
Each friend (packet) takes a different road (route). They arrive at different times (out of order). The host (destination) waits for everyone, then seats them in order (reassembly).
🧠 "Split → Header → Route → Arrive → Reorder"
The 5-word summary of packet switching. Hit each one for 5 marks.
🧠 "Router = traffic controller"
A router doesn't carry the packet — it directs it. Like a traffic officer pointing cars down different roads.