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Computer Networks · Class 12 · Data Communication · Measuring Capacity
networksdata communication⏱️ 7 min read

Data Transfer Rate

“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

Data transfer rate
Data transfer rate is the number of bits sent from the sender to the receiver in one second. It is also called the bit rate, and its unit is bits per second (bps).

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.

UnitStands forIn bits per secondStep up (NCERT writes 1024)
bpsbits per second1—
Kbpskilobits per second1,0001 Kbps = 1000 bps
Mbpsmegabits per second1,000,0001 Mbps = 1000 Kbps
Gbpsgigabits per second1,000,000,0001 Gbps = 1000 Mbps
Tbpsterabits per second1,000,000,000,0001 Tbps = 1000 Gbps
1000 or 1024?
Network equipment and internet companies use 1000: 1 Mbps is exactly 1000 Kbps. Your NCERT book writes 1 Kbps = 210 bps = 1024 bps, the step used for memory sizes. In the exam, write the textbook’s 1024 unless the question gives you a figure to use.

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.

One file, three speeds

Pick a file and press Start. All three computers download it at the same moment. Drag sideways to turn.

⏩ The clocks show the real time each download takes; the race itself is shown 75 times faster so you are not waiting 20 minutes.
The working: 150 MB × 8 = 1,200 megabits
LinkMegabits ÷ MbpsTime
1 Mbps1,200 ÷ 1 = 1,200 s20 minutes
10 Mbps1,200 ÷ 10 = 120 s2 minutes
100 Mbps1,200 ÷ 100 = 12 s12 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:

  1. Turn the file size into bits: megabytes × 8 = megabits.
  2. Divide by the rate: megabits ÷ Mbps = seconds.
  3. Turn the seconds into minutes if it helps (÷ 60).
Worked example
How long does a 150 MB video take to download at 10 Mbps?
150 MB × 8 = 1200 megabits.
1200 ÷ 10 = 120 seconds = 2 minutes.

5Bandwidth vs data transfer rate

Both are in bits per second, so they are easy to mix up. Bandwidth is how much a channel can carry. Data transfer rate is how much is actually being sent. Eight lanes on a highway is its capacity; the number of cars really passing each minute is its rate.

BandwidthData transfer rate
SaysThe most a channel can carry each secondHow many bits are actually sent each second
Unitbps (and Hz, as a frequency range)bps, Kbps, Mbps, Gbps, Tbps
ExampleYour plan: up to 100 MbpsThe download right now: 40 Mbps, because others share the line
Which is bigger?Sets the limitCan reach the bandwidth, but never goes above it

6Check yourself

Quick Check

Which list is in order from slowest to fastest?

Quick Check

How long does a 4 MB photo take to download at 1 Mbps?

Quick Check

A connection has a bandwidth of 50 Mbps. Which data transfer rate is NOT possible on it?

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