Fast downloads, but the game lags
Your friend says: "My internet is fast. I download a film in a minute. But my online game lags and my video call freezes."
Is the internet fast or slow? It depends on what you measure. A network is a group of computers joined together, and it has two different speeds.
Bandwidth is how much data a connection carries each second. Latency is how long one piece of data takes to arrive.
Key words
| Word | What it means | Example |
|---|---|---|
| Bandwidth | How much data a connection can carry each second. | 100 Mbps |
| Latency | The time one piece of data takes to travel from one device to another. | 30 ms |
| Bit | One 1 or 0, the smallest piece of data. | 1 |
| Byte | A group of 8 bits. | One letter |
| Mbps | Megabits per second. The unit for bandwidth. More is faster. | 80 Mbps |
| Millisecond | One thousandth of a second. Written ms. | 30 ms |
| Ping | A test that sends a small message and times how long the reply takes. | ping 10.0.12.1 |
| Packet | A small piece of data sent over a network, with an address on it. | One part of a photo |
| Router | A device that joins networks and sends packets the right way. | A home router |
| Network | Computers joined together to share data. | The college network |
The cells students fill in are left blank in this preview.
A road picture
| Bandwidth | Latency | |
|---|---|---|
| In the picture | How many lanes the road has | How long the road is |
| Meaning | How much data per second | How long one packet takes to arrive |
| Unit | Mbps or Gbps | Milliseconds (ms) |
| Good is | High (more lanes) | Low (a shorter road) |
| Hurts most | Big downloads and streaming | Games, video calls and remote desktops |
The cells students fill in are left blank in this preview.
Quick check
Have a go
Quick check
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Extra: Bits and bytes
File sizes use bytes. Line speeds use bits. One byte is 8 bits.
So a 100 MB file is 100 times 8, which is 800 megabits. That is why a 100 Mbps line does not download 100 MB in one second.
Extra: Small b or capital B?
data-callouts.warning.label
A small b means bits. A capital B means bytes. So Mbps is megabits each second, and MB is megabytes. Shops and phones often show both, so look closely.
Extra: Worked example: download time
| Step | What to do | Result |
|---|---|---|
| 1 | The file is 100 MB. The line is 80 Mbps. | |
| 2 | Change megabytes to megabits: 100 times 8 | 800 megabits |
| 3 | Divide by the speed: 800 divided by 80 | 10 seconds |
| 4 | Check: 10 seconds at 80 Mbps is 800 megabits | It matches |
The cells students fill in are left blank in this preview.
Extra: Try it: download time
size_mb = 100 # file size in megabytes
speed_mbps = 80 # line speed in megabits per second
size_megabits = size_mb * 8
seconds = size_megabits / speed_mbps
print("The download takes", seconds, "seconds.")
# Change the speed to 160. What happens to the time?