Fast internet, but the game lags
Sam pays for a very fast internet line. Films stream with no pause. But in an online game, Sam presses a button and the hero moves a moment later.
The line is fast, so why is the game slow? 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.
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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
sudo ip netns exec h1 ping -c 4 -W 1 10.0.12.1 | tail -1A command for the lesson’s lab terminal. It is shown here and not run.
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3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.