A fast line and a slow notebook
Leo has a very fast internet line. A huge data file downloads in a minute. But Leo works on a remote computer in another country. Every key press shows up a moment late.
The line is fast, so why does typing lag? The answer is that "fast" means two different things.
Key words
| Word | What it means | Example |
|---|---|---|
| Bandwidth | how much data a link can carry each second | 100 Mbps |
| Latency | the delay before data starts to arrive | 20 ms |
| Mbps | megabits per second: a unit of bandwidth | |
| Millisecond | one thousandth of a second | 20 ms |
| Round trip | the time for a message to go and the reply to return | |
| Hop | one router that data passes through | |
| Router | a device that joins networks and passes data on | |
| Network | devices joined so they can share things |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can tell bandwidth from latency and say how each one affects a system.
Starter
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Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Bandwidth is the width, latency is the length
Think of a water pipe. Bandwidth is how wide the pipe is. A wide pipe carries a lot of water each second. It is measured in Mbps.
Latency is how long one drop takes to get through. It is measured in milliseconds. A short pipe has a low latency, a long pipe a high one. A wide pipe can still be long.
Bandwidth compared with latency
| Question | Bandwidth | Latency |
|---|---|---|
| What is it? | Data each second | Delay for one message |
| Unit | Mbps | Milliseconds |
| Good is | High | Low |
| Pipe picture | How wide | How long |
| More bandwidth fixes it? | Yes, for bulk data | No |
The cells students fill in are left blank in this preview.
Extra: Measure the round trip time
ping -c 4 -W 1 127.0.0.1 | tail -1A command for the lesson’s lab terminal. It is shown here and not run.
Check
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3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.