Wired or wireless

Classroom resources

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Students will be able to

• 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.

Timing

Predict and Try 6 min · Worked example 12 min · Vary 10 min · Practise and Make 12 min

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

1. Predict: students who chose "about the same" may have fast home Wi-Fi; ask what the walls do.
2. Try: have students write down all three numbers for cable, then for Wi-Fi.
3. Worked example: ask why speeds are in megabits but files are in megabytes.
4. Vary: ask which is worse, 40 m or three walls.
5. 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

Answers to the checks

1. cable or Wi-Fi?: You download a 50 MB game update. Laptop A is plugged into the router by a cable. Laptop B uses Wi-Fi two rooms away. Which finishes first?
   A, by a lot. The cable carries more data each second and loses almost nothing; Wi-Fi through walls carries less and loses packets that have to be sent again.
2. Try: compare the links: Note the cable's numbers. Then switch to Wi-Fi and add two walls. How long does the file take now?
   
   Through two walls, Wi-Fi has far less bandwidth, more delay and more lost packets, so the same 50 MB takes several times longer.
3. Time to send a file: 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.)
   4
   400 megabits ÷ 100 Mb/s = 4 seconds (a little more if packets are lost).
4. Find the mistake: Sam plans to fix a laggy game. Click the line where the reasoning first goes wrong.
   line 3
   Line 3: bandwidth is how much data a second, not how soon each packet arrives. Lag is latency; a cable, or being closer to the router, cuts the delay. A bigger plan mostly helps big downloads.
5. Why Wi-Fi loses packets: Why does Wi-Fi lose more packets than a cable?
   The signal travels through the air, where walls, distance and other devices weaken or interfere with it
   A radio signal spreads out, is blocked by walls and shares the air with other devices, so some packets are damaged or lost.
6. Distance: Switch to Wi-Fi with no walls. Move the distance to 40 m.
   
   Distance alone weakens Wi-Fi: less bandwidth and more packets lost.
7. Walls: Come back to 5 m and add three walls.
   
   Walls block the signal: even close by, three walls cut the speed a lot.
8. Mobile data: Switch to Mobile. Which property is much worse than Wi-Fi close by?
   
   The delay: about 40 ms, because each packet goes through a phone tower and the mobile network first.
9. Best for games: For online gaming, which link is best?
   A cable: the shortest delay
   Games need a short delay (low latency), so a cable is best.
10. Streaming: Streaming a 4K movie needs lots of data every second. Which property matters most?
    Bandwidth
    Lots of data a second is bandwidth.
11. Video calls: A video call keeps freezing and voices arrive late. Which properties are the problem?
    Latency and reliability
    Late voices are latency; freezing is lost packets.
12. File time: How many seconds does a 100 MB file take over a cable at 1000 Mb/s?
    0.8
    800 ÷ 1000 = 0.8 seconds.
13. Review: bytes: Review: how many bits is one byte?
    8
    8 bits make a byte, which is why speeds in megabits look 8 times bigger than file sizes in megabytes.
14. good-enough Wi-Fi: 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.
    
    Close to the router with one wall, Wi-Fi keeps enough bandwidth and loses few packets. Moving closer often helps more than a faster plan.

Solutions

One possible solution for each task. Other correct programs are fine.

Vary: one change at a time › Distance

Open task: any program that does what the step asks.

Vary: one change at a time › Walls

Open task: any program that does what the step asks.

Vary: one change at a time › Mobile data

Open task: any program that does what the step asks.

Vary: one change at a time › Best for games

Open task: any program that does what the step asks.

good-enough Wi-Fi

Open task: any program that does what the step asks.

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).

Exit ticket

Name two properties that differ between wired and wireless links. Which link would you choose for online exams, and why?

Any two of bandwidth, latency, reliability. A cable: it is the most reliable, so the exam is least likely to drop out.

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.

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.