--- title: "Wired or wireless" subtitle: "Networks · Moving data · Lesson 1" --- # Wired or wireless **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Gamers plug in a cable; everyone else uses Wi-Fi. Is the cable really better, and better at what? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **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?** ☐ B: no wire to slow it down ☐ About the same: Wi-Fi is as fast as a cable now ☐ A, by a lot ☐ It depends only on the internet plan My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 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. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: three properties :::keep-together ### Time to send a file 1. $50 \text{ MB} \times 8 = 400 \text{ megabits}$ — Speeds are in megabits a second, so change bytes to bits. 2. $400 \div 1000 = 0.4 \text{ s}$ — Over a cable at 1000 Mb/s. 3. $400 \div 100 = 4 \text{ s}$ — Over Wi-Fi at 100 Mb/s: ten times as long. 4. $\text{latency} \to \text{games, video calls}$ — Jobs that go back and forth need a short delay. 5. $\text{bandwidth} \to \text{streaming, downloads}$ — Jobs that move a lot of data need a high speed. 6. $\text{reliability} \to \text{every job}$ — Lost 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.)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 400 megabits ÷ 100 Mb/s = 4 seconds (a little more if packets are lost). ::: :::keep-together ### Find the mistake **Sam plans to fix a laggy game. Click the line where the reasoning first goes wrong.** ``` 1 My online game lags: my moves show up late. 2 Lag is the delay (latency) before each packet arrives. 3 More bandwidth means each packet arrives sooner. 4 So a faster internet plan will fix the lag. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### 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 ☐ Wi-Fi is newer, so it has more bugs ☐ Wi-Fi packets are bigger ☐ Cables send each packet twice ::: :::teacher-note **Answer:** 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. ::: :::typical ## Vary: one change at a time **Distance.** **Walls.** **Mobile data.** **Best for games.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Which property? :::keep-together ### Streaming **Streaming a 4K movie needs lots of data every second. Which property matters most?** ☐ The colour of the cable ☐ The number of routers ☐ Bandwidth ☐ Latency ::: :::teacher-note **Answer:** Bandwidth Lots of data a second is bandwidth. ::: :::keep-together ### Video calls **A video call keeps freezing and voices arrive late. Which properties are the problem?** ☐ None: calls always do that ☐ Only the size of the screen ☐ Latency and reliability ☐ Only bandwidth ::: :::teacher-note **Answer:** Latency and reliability Late voices are latency; freezing is lost packets. ::: :::keep-together ### File time **How many seconds does a 100 MB file take over a cable at 1000 Mb/s?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 0.8 800 ÷ 1000 = 0.8 seconds. ::: :::keep-together ### Review: bytes **Review: how many bits is one byte?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8 8 bits make a byte, which is why speeds in megabits look 8 times bigger than file sizes in megabytes. ::: ## Make: good-enough Wi-Fi Then: where in your house or school is the worst place to sit, and why? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Name two properties that differ between wired and wireless links. Which link would you choose for online exams, and why?** {{lines-3}} ::: :::teacher-note **Answer:** Any two of bandwidth, latency, reliability. A cable: it is the most reliable, so the exam is least likely to drop out. :::