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.
Gamers plug in a cable; everyone else uses Wi-Fi. Is the cable really better, and better at what?
A network joins computers so they can send data to each other. The link between two of them can be wired (a cable) or wireless (Wi-Fi, mobile data). Three properties describe how good a link is:
Bandwidth: how much data it carries each second, in megabits a second (Mb/s).
Latency: the delay before each packet arrives, in milliseconds (ms).
Reliability: how many packets get lost on the way and have to be sent again.
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Cable or Wi-Fi?
TryTry it on the board and compare the result with your prediction.
Try: compare the links
Was your prediction right? Try Mobile too.
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Note the cable's numbers. Then switch to Wi-Fi and add two walls. How long does the file take now?
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: three properties
Worked exampleBefore each step, say what you think comes next.
50 MB×8=400 megabitsSpeeds are in megabits a second, so change bytes to bits.
400÷1000=0.4 sOver a cable at 1000 Mb/s.
400÷100=4 sOver Wi-Fi at 100 Mb/s: ten times as long.
latency→games, video callsJobs that go back and forth need a short delay.
bandwidth→streaming, downloadsJobs that move a lot of data need a high speed.
reliability→every jobLost packets must be sent again, which costs time.
Now work out a file's time at 100 Mb/s.
A link carries 100 Mb/s. How many seconds does a 50 MB file take? (A byte is 8 bits, so 50 MB is 400 megabits.)
Sam plans to fix a laggy game. Click the line where the reasoning first goes wrong.
ModifyChange the problem on purpose and see what happens.
Vary: one change at a time
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Switch to Wi-Fi with no walls. Move the distance to 40 m.
Use the board. This ticks when you get there.
Come back to 5 m and add three walls.
Use the board. This ticks when you get there.
Switch to Mobile. Which property is much worse than Wi-Fi close by?
Use the board. This ticks when you get there.
PractiseA few on your own, to check you have it.
Which property?
How many seconds does a 100 MB file take over a cable at 1000 Mb/s?
Review: how many bits is one byte?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Good-enough Wi-Fi
Then: where in your house or school is the worst place to sit, and why?
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Need a hint?
Distance and walls both slow Wi-Fi.
Another hint
Keep the wall, but move the laptop closer to the router.
Another hint
Within about 15 m with one wall is fast enough.
The laptop must stay on Wi-Fi behind at least one wall. Change the setup so the 50 MB file takes less than 3 seconds.
Use the board. This ticks when you get there.
For teachers
Students compare cable, Wi-Fi and mobile links on a board to describe bandwidth, latency and reliability, work out file times, and match links to jobs such as gaming, streaming and file transfer (AC9TDI8K02 node 1). About 40 minutes.
I can describe bandwidth, latency and reliability.
I can compare wired and wireless links.
I can choose a link for a job and say why.
Before class
Try the board yourself: switch link types and move the sliders.
The model is simplified and says so: cable 1000 Mb/s, 1 ms, nothing lost; Wi-Fi up to 300 Mb/s, losing speed with distance and walls; mobile 50 Mb/s, 40 ms. Real numbers vary a lot with equipment; the comparisons are what matter.
Hands-on: run a speed test on a school laptop on Wi-Fi and on a cable (if a port is available) and compare download speed and ping.
Running the lesson
Predict: students who chose "about the same" may have fast home Wi-Fi; ask what the walls do.
Try: have students write down all three numbers for cable, then for Wi-Fi.
Worked example: ask why speeds are in megabits but files are in megabytes.
Vary: ask which is worse, 40 m or three walls.
Make: compare setups: who kept the most walls?
The hard part (on purpose)
Bandwidth and latency get treated as one 'speed'. Use the road picture: bandwidth is how many lanes, latency is how long the trip takes.
When students get stuck
A student says a faster plan fixes lag (Sam's mistake). Ask: Is lag about how much data, or how long each packet takes? Fix: Lag is latency; bandwidth is how much data a second.
A student divides megabytes by megabits. Ask: Are the units the same? Fix: Multiply megabytes by 8 first.
Stretch and support
Stretch: Why do some country towns rely on satellite or mobile internet? What does that mean for latency and for school video calls?
Support: A card with the three words and a job for each: bandwidth (streaming), latency (gaming), reliability (everything).
Where it leads
Next: Packets (how a message actually travels, and what happens to lost packets).
Where it leads: the internet journey lesson uses both wired and wireless links in one trip.
What each step is for
cable or Wi-Fi?: Committing to which link sends a big file faster tests the new move: link type changes bandwidth, latency and reliability, so the same job takes different times.
Try: compare the links: Switching the board from cable to Wi-Fi through two walls settles the prediction: every property gets worse and the file takes several times longer.
Worked example: three properties: Working out a file's time from bandwidth and naming which property each job needs models the new move. Sam mixes up bandwidth and latency, the misconception planted after the method.
Vary: one change at a time: Each task changes one thing about the link: the type, the distance, the walls, then mobile data, then choosing the link for a job.
which property?: Matching jobs to the property they need most, a time calculation, and a review of bytes and bits.
good-enough Wi-Fi: Finding a Wi-Fi setup that meets a time target reverses the process: students start from the requirement and adjust the conditions.