Network performance: bandwidth, transfer rate, latency and range

Definitions

• Bandwidth: the maximum amount of data that can be carried by a connection per second, measured in bits per second (bit/s). It is the capacity of the link.
• Transfer rate (speed): the amount of data actually transferred per second. It is usually less than the bandwidth because of other traffic (congestion), interference, protocol overheads and the speed of the devices.
• Latency: the delay between data being sent and starting to arrive, usually in milliseconds (ms). Caused by distance, the number of devices (hops) passed through and processing at each. Low latency matters for gaming and video calls.
• Range: the maximum distance over which a signal can be sent reliably, for example about 100 m for copper Ethernet, tens of metres for Wi-Fi indoors, and kilometres for fibre.

Calculating transfer time

time (s) = file size (bits) / transfer rate (bits per second)

Remember:

1. File sizes use binary prefixes and bytes: convert to bits (x 8, and KiB = 2¹⁰, MiB = 2²⁰ bytes).

2. Rates use decimal prefixes: Mbit/s = 10⁶ bit/s.

3. Total time for a message can be estimated as latency + size / rate.

Expressions are fine without a calculator: for example time = (5 x 2²⁰ x 8) / (20 x 10⁶) seconds.

Worked example

A 15 MiB file is sent over a connection with a transfer rate of 30 Mbit/s. Write an expression for the time and evaluate it.

1. Size in bits = 15 x 2²⁰ x 8
2. Rate = 30 x 10⁶ bit/s
3. time = (15 x 2²⁰ x 8) / (30 x 10⁶)
4. = 4.194 s

Answer: About 4.19 s

Check yourself

1. A 4 MiB file is sent at 8 Mbit/s. How long does it take, in seconds, to 2 decimal places?
   Show answer

   4.19 s. Bits = 4 x 2²⁰ x 8 = 33 554 432; time = 33 554 432 / 8 000 000 = 4.19 s.

2. A 600 KiB image is sent at 2 Mbit/s. How long does it take, in seconds, to 2 decimal places?
   Show answer

   2.46 s. 600 x 1024 x 8 = 4915200 bits; / 2 000 000 = 2.46 s.

3. A connection transfers data at 50 Mbit/s for 4 seconds. How many megabits are sent?
   Show answer

   200 Mbit. 50 x 4 = 200 Mbit.

4. A 10 MB (decimal) file takes 16 seconds to download. What is the transfer rate in Mbit/s?
   Show answer

   5 Mbit/s. 10 MB = 80 Mbit; 80 / 16 = 5 Mbit/s.

5. Which term describes the delay between a packet being sent and starting to arrive?
   • Bandwidth
   • Range
   • Transfer rate
   • Latency
   Show answer

   Latency. Latency is a time delay, measured in ms.

6. Why is the transfer rate usually lower than the bandwidth?
   • Bandwidth is measured in bytes and transfer rate in bits
   • Other traffic, interference and protocol overheads reduce the data actually carried
   • The transfer rate includes the range
   • Bandwidth only applies to wireless networks
   Show answer

   Other traffic, interference and protocol overheads reduce the data actually carried. Bandwidth is the theoretical maximum; real transfers share and lose capacity.

7. Explain why low latency matters more than high bandwidth for online gaming.
   Show answer

   Games send small amounts of data that must arrive quickly, so the delay (latency) affects how responsive the game is more than the total capacity does.. Gaming data is small but time critical.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.