When you send a photo across the internet it does not travel as one big lump. It is cut into small packets, each labelled with where it is going, and the packets find their own way there before being put back together.
Data sent over a network is broken down into packets. Each packet has a header (destination address, originator's address and packet number), a payload (the actual data) and a trailer (marks the end of the packet and holds an error check). In packet switching each packet may take a different route, chosen by routers, so packets can arrive out of order; once the last packet arrives they are reordered using their packet numbers.
Fig. 1: every packet has a header, a payload and a trailer
Section 01
Why packets?
A large file is split into many small packets of data before it is sent. Small packets share the network fairly and are quick to resend if one goes wrong.
Smaller units: one long message would block a connection; many small packets can share busy network links.
Only resend what is lost: if one packet is corrupted or lost, only that packet is sent again, not the whole file.
Many routes: packets can avoid a busy or broken part of the network by going another way.
Examiner trap
Packets do not make the data itself smaller. The total amount of data is the same (in fact slightly more, because every packet adds a header and trailer).
Section 02
Inside a packet.
Look at a packet in the lab. Click each part to see what it holds.
Part
What it contains
Header
Destination address (where it is going), originator's address (where it came from), packet number (its place in the sequence)
Payload
The actual data: one piece of the file or message
Trailer
A marker showing the end of the packet and an error check, such as a checksum
Examiner trap
The packet number goes in the header, not the trailer. Learn the three header items exactly: destination address, originator's address, packet number.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
Which part of a packet holds the actual data being sent?
Quick check+10 XP
Which item is NOT stored in a packet header?
Quick check+10 XP
In the lab, why do some packets arrive before packets that were sent earlier?
Quick check+10 XP
What device decides the route a packet takes across the internet?
Section 03
Packet switching.
Packets from one file travel independently. Follow these five steps; each one is a mark point in a "describe packet switching" answer.
1. The data is broken down into packets, and each packet is numbered.
2. Each packet is sent out and could take a different route.
3. At each step a router reads the destination address and decides where to send the packet next, avoiding busy routes.
4. Packets may arrive out of order.
5. Once the last packet has arrived, the receiving device puts the packets back in order using the packet numbers.
Worked example Reordering
Packets of an email arrive in the order 3, 1, 5, 2, 4. Explain what the receiving computer does.
It waits until the last packet has arrived.
It reads the packet number in each header.
It reorders the packets as 1, 2, 3, 4, 5 and joins the payloads to rebuild the email.
Answer: Reorders the packets using their packet numbers once all have arrived
Your turn
Why might packet 2 arrive after packet 4?
Packet 2 took a different route, for example a longer or busier one, chosen by the routers.
Examiner trap
It is the router that controls the route a packet takes. A packet does not "choose" its own path.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 1 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
How does a receiving computer put packets back in the correct order?
Exam style+20 XP
Which is a benefit of sending data as packets?
Exam style+20 XP
Which statement about packet switching is correct?
Exam style+20 XP
What is the purpose of the trailer in a packet?
Skill unlocked
Packets and packet switching, mastered.
This skill is now recorded on your profile. Keep the chain going.