The big idea: Data is cut into small packets, each sent on its own, and joined back together at the far end.
Nothing reserves a line for the whole transfer, so many conversations share the same network at once.
One message split, routed two ways, arriving out of order, and reassembled. Step through it.
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Every packet carries its own instructions: A packet is a piece of the data plus a header.
The header says where it is going, where it came from, and which number in the sequence it is. Without that, a packet arriving alone would be meaningless.
Destination address
- Lets every router along the way decide where to send it next
- No router needs to know the whole path
- Each one only chooses the next hop
Source address
- Says where a reply should go
- Also lets the receiver ask for a missing packet again
- Needed even when nothing goes wrong
Sequence number
- Says where this piece belongs in the message
- Lets the receiver reorder packets that arrived jumbled
- Lets it notice that one is missing at all
Out of order is normal: Different packets take different routes, and different routes take different times.
Arriving jumbled is not a fault. The sequence numbers exist precisely because it is expected.
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Switch — inside one network
- Delivers a frame to the one device it is addressed to
- Works only within a single LAN
- Does not read IP addresses, so cannot route between networks
- Keeps traffic off devices it is not meant for, reducing collisions
Router — between networks
- Reads the destination IP address in each packet's header
- Chooses the next network to forward it to
- Decides per packet, so two packets may go different ways
- Knows only the next hop, never the whole path
Why packet switching beats reserving a line: Reserving a line for one conversation wastes it whenever nobody is talking, and a break in that line ends the call.
Packet switching fills gaps with other people's traffic and routes around breaks — which is why the internet works this way.
What it costs: Every packet carries a header, so some of the capacity goes on addresses rather than data.
Packets can also be delayed, reordered or lost, which is why TCP exists to check and resend.
How this is tested — you must name what the header carries and what each device does with it. It comes up two ways:
Paper 1 Section A
- Explain packet switching, 3-5 marks
- State what a packet header contains
- Explain the role of switches and routers
Paper 1 Section B — case study
- Trace data through the case study's network
- Explain why packets may arrive out of order there
The classic trap: Saying "the data is split into packets and sent" and stopping. The marks are in the header contents and in what happens at the far end — reordering by sequence number, and requesting only what is missing.
Explain how a large file is sent from one computer to another across the internet using packet switching.
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