📚 Knowledge Library — Topic 2.2 — Data Transmission

Packet Switching

Understand how packets travel independently across a network, why they may take different routes, and how they are reassembled at the destination.

Start LearningWorked Example
1. Invitation

Packets do not have to travel together.

When a file is sent across a network, it is split into packets.

Each packet can travel independently. They do not need to follow the same route or arrive in the same order.

💡 Remember: packet switching lets many small packets move through the network separately.
Figure 1.1
One File, Many Packets
File
↓
Packet 1
Packet 2
Packet 3
The file is split before it travels.
2. Big Idea

Routers choose the path for each packet.

A router looks at the destination address in the packet header.

It then sends the packet towards the destination using an efficient available route.

💡 Router = the device that directs packets through the network.
Figure 2.1
Router Decision
Packet
↓
Router
↙ ↓ ↘
Route A • B • C
Each packet is routed individually.
3. FutureLogic Bridge

Think of delivery lorries on different roads.

Imagine one large delivery split across several lorries.

Each lorry has the same destination, but the sat-nav may send them along different roads depending on traffic.

💡 Bridge: packets are like numbered lorries. Same destination, different possible routes.
Figure 3.1
Lorry Model
🚚 Packet 1 → Road A
🚚 Packet 2 → Road B
🚚 Packet 3 → Road C
Different routes can still reach the same place.
4. Different Routes

Packets may arrive out of order.

Because packets can take different routes, some may arrive faster than others.

This means packets may arrive out of order and must be put back into the correct sequence.

💡 Packet numbers help the receiver rebuild the original file.
Figure 4.1
Arrival Order
Sent:
1 → 2 → 3 → 4

Arrive:
3 → 1 → 4 → 2
Arrival order is not always the same as sending order.
5. Reassembly

Packets are reordered after the last packet arrives.

At the destination, the receiver uses packet numbers to place the packets back into the correct order.

This can only be completed after all the packets have arrived.

🎯 Exam Tip: say packets are reordered after all packets have arrived. That detail often earns marks.
Figure 5.1
Reordering
3 • 1 • 4 • 2
↓
1 • 2 • 3 • 4
↓
Original file
6. Worked Example

Describe packet switching in exam order.

A good answer follows the journey from sender to receiver.

Packet switching process

1
Split data
2
Add headers
3
Route packets
4
Different paths
5
Arrive out of order
6
Reassemble
Use this order for a 5-mark explanation.
Figure 6.1
Exam Sequence
Split
↓
Route
↓
Arrive
↓
Reorder
Keep the answer chronological.
7. Missing Packets

Missing or corrupted packets can be resent.

If a packet is missing or corrupted, the receiver can request that packet again.

Only the missing or damaged packet needs to be resent, not necessarily the whole file.

💡 Packet switching is efficient because damaged parts can be requested again.
Figure 7.1
Resend Request
Packets received:
1 • 2 • ? • 4
↓
Request Packet 3
The missing packet is requested again.
8. Exam Tip

Describe the router's role precisely.

In packet switching, the router does not simply “connect devices”.

Its job is to direct each packet towards its destination and choose an efficient available path.

🎯 Exam Tip: router = directs packets / controls the path / chooses an efficient route.
Figure 8.1
Router Role
Header checked
↓
Destination found
↓
Best path chosen
9. Common Mistake

Do not say packets travel as one group.

Students sometimes think packets move like a convoy.

In packet switching, each packet is handled independently and may take a different route.

⚠️ Common Mistake: packets do not have to travel together, and they do not have to arrive in order.
Figure 9.1
Not a Convoy
Wrong:
1-2-3 together ✗

Right:
1 ↙ 2 ↓ 3 ↘ ✓
10. Summary

Packet switching in one screen.

Packet switching sends data as separate packets across a network.

Routers direct each packet towards its destination. Packets may take different routes and arrive out of order.

When all packets arrive, they are reordered using packet numbers and reassembled into the original data.

💡 Key idea: split → route independently → arrive → reorder → rebuild.
Figure 10.1
Final Model
Split
↓
Route separately
↓
Reorder
↓
Rebuild file