Networks

Links between computers differ in three ways: bandwidth (how much data each second), latency (the delay for each packet) and reliability (how many packets get lost). Cables usually win on all three; Wi-Fi gets slower and less reliable with distance and walls. Which matters most depends on the job.

1. Cable or Wi-Fi?
2. Try: compare the links
3. Worked example: three properties › Time to send a file
4. Worked example: three properties › Find the mistake
5. Worked example: three properties › Why Wi-Fi loses packets
6. Vary: one change at a time › Distance
7. Vary: one change at a time › Walls
8. Vary: one change at a time › Mobile data
9. Vary: one change at a time › Best for games
10. Practise: which property? › Streaming
11. Practise: which property? › Video calls
12. Practise: which property? › File time
13. Practise: which property? › Review: bytes
14. Good-enough Wi-Fi

Data is sent as small numbered packets, not one long stream. You watch a message go to pieces, invent your own fix, then put names to it: routers pass packets on by different routes, numbers put them back in order, and TCP has the receiver confirm each one so a lost packet is sent again.

1. Engage: one message, many pieces
2. Explore: what happens to the pieces? › Watch the pieces
3. Explore: what happens to the pieces? › What did you see?
4. Explore: what happens to the pieces? › Try your own changes
5. Explore: invent a fix › Keep every link
6. Explore: invent a fix › Switches off
7. Explain: what you invented has names › Explain it yourself
8. Explain: what you invented has names › Give it names
9. Explain: what you invented has names › Two fixes
10. Explain: what you invented has names › Find the mistake
11. Elaborate: when the network breaks › Roadworks
12. Elaborate: when the network breaks › Cut off
13. Elaborate: when the network breaks › Video calls
14. Evaluate: look back › Downloads
15. Evaluate: look back › My first guess
16. Evaluate: look back › I can

Packets pass through Wi-Fi and routers you don't control, and anyone along the way can copy them. So anything private must be encrypted on your device before it leaves: scrambled with a key, so only someone with the key can turn it back. Encryption only works because it can be reversed.

1. Who can read it?
2. Try: send it plain
3. Worked example: encrypt, then send › PIN 4821 with key 3
4. Worked example: encrypt, then send › Find the mistake
5. Worked example: encrypt, then send › Why reversible?
6. Vary: one setting at a time › Encryption on
7. Vary: one setting at a time › Key 0
8. Vary: one setting at a time › Key 10
9. Vary: one setting at a time › Too few keys
10. Practise: keep it safe › What needs encrypting
11. Practise: keep it safe › Decrypt
12. Practise: keep it safe › The padlock
13. Practise: keep it safe › Review: packets
14. A safe key

Buying something online uses everything at once: your browser encrypts the data (HTTPS), it is split into packets, the packets cross Wi-Fi and then cables, routers pass them on by different routes, TCP gets any lost packet sent again, and the shop puts them in order and decrypts them.

1. Where is it encrypted?
2. Try: buy something
3. Worked example: the whole chain › Follow the purchase
4. Worked example: the whole chain › Find the mistake
5. Worked example: the whole chain › HTTPS off
6. Vary: change one thing › HTTPS off
7. Vary: change one thing › HTTPS on
8. Vary: change one thing › Slow at the far end
9. Vary: change one thing › Without TCP
10. Practise: the internet › Mobile video
11. Practise: the internet › Which protocol?
12. Practise: the internet › Review: decrypt
13. Put the journey in order

Four flags hidden in the club's own network traffic, each found with an idea from this course: which packets anyone on the Wi-Fi could read, putting packets back in number order, which packet TCP must ask for again, and the quickest route round an overloaded router. Then fix every device that still sends readable data.

1. Engage: who can see it?
2. Explore: four flags › Flag 1: the capture
3. Explore: four flags › Flag 2: out of order
4. Explore: four flags › Flag 3: the lost packet
5. Explore: four flags › Flag 4: the route
6. Explain: what the flags show
7. Elaborate: fix the network
8. Evaluate: look back › The café
9. Evaluate: look back › Look back