Data Transfer Rate
🎬 This chapter has a 7:57 video: Data Communication →“Downloading — 2 minutes left.” Why does the same video take two minutes on one connection and twenty on another? The answer is one number: how many bits the connection sends each second.
1What data transfer rate is
A bit is a single 0 or 1 — the smallest piece of data. A link of 10 bps sends ten 0s and 1s every second. Real links send millions, so bigger units are used.
| Unit | Stands for | In bits per second | Step up (NCERT writes 1024) |
|---|---|---|---|
| bps | bits per second | 1 | — |
| Kbps | kilobits per second | 1,000 | 1 Kbps = 1000 bps |
| Mbps | megabits per second | 1,000,000 | 1 Mbps = 1000 Kbps |
| Gbps | gigabits per second | 1,000,000,000 | 1 Gbps = 1000 Mbps |
| Tbps | terabits per second | 1,000,000,000,000 | 1 Tbps = 1000 Gbps |
2Bits and bytes: b and B
Speeds are measured in bits, but file sizes are measured in bytes, and 1 byte = 8 bits. The letters tell them apart: a small b is a bit, a capital B is a byte.
- 100 Mbps — a speed: 100 megabits every second.
- 100 MB — a file size: 100 megabytes, which is 800 megabits.
3One file, three speeds
The lab sends the same file to three computers at once. Each link is ten times faster than the one before. Watch what that does to the waiting time.
Pick a file and press Start. All three computers download it at the same moment. Drag sideways to turn.
| Link | Megabits ÷ Mbps | Time |
|---|---|---|
| 1 Mbps | 1,200 ÷ 1 = 1,200 s | 20 minutes |
| 10 Mbps | 1,200 ÷ 10 = 120 s | 2 minutes |
| 100 Mbps | 1,200 ÷ 100 = 12 s | 12 seconds |
Ten times the rate means one tenth of the time. A movie that takes 3 hours 20 minutes at 1 Mbps takes 2 minutes at 100 Mbps.
4Working out a download time
Three steps, the same every time:
- Turn the file size into bits: megabytes × 8 = megabits.
- Divide by the rate: megabits ÷ Mbps = seconds.
- Turn the seconds into minutes if it helps (÷ 60).
150 MB × 8 = 1200 megabits.
1200 ÷ 10 = 120 seconds = 2 minutes.
5Bandwidth vs data transfer rate
The two go together, so they are easy to mix up. Bandwidth is how wide a channel’s range of frequencies is, in hertz — it decides how much the channel can carry. Data transfer rate is how many bits are actually sent each second. Eight lanes on a highway is its width; the number of cars really passing each minute is its rate.
| Bandwidth | Data transfer rate | |
|---|---|---|
| Says | The range of frequencies a channel can carry (highest − lowest) | How many bits are actually sent each second |
| Unit | Hz, kHz, MHz, GHz | bps, Kbps, Mbps, Gbps, Tbps |
| Example | A Wi-Fi channel from 5170 MHz to 5190 MHz: 20 MHz | A download running at 40 Mbps |
| How they relate | Higher bandwidth → room for a higher rate | Limited by the bandwidth (and by others sharing the line) |
6Check yourself
Which list is in order from slowest to fastest?
How long does a 4 MB photo take to download at 1 Mbps?
An internet plan is sold as “up to 50 Mbps”. Which data transfer rate will you NOT see on it?