Transfer times from file size and bandwidth, bits against bytes, shared bandwidth, latency and packet counts.
What you will learn
- Calculate how long a transfer takes
- Convert between bits per second units
- Work out packets and header overheads
- Include latency and shared bandwidth
Bits and bytes
Connection speeds are in bits per second (Mbps = megabits per second).
File sizes are in bytes (MB). One byte is 8 bits, so:
time (s) = file size in bits ÷ bandwidth in bits per second
a 25 MB file on a 40 Mbps line: 25 × 8 = 200 megabits, 200 ÷ 40 = 5 s
The most common slip is forgetting to multiply by 8.
The prefixes work as for bytes: 1 kbps = 1000 bps, 1 Mbps = 1000 kbps = 1 000 000 bps, and 1 Gbps = 1000 Mbps = 1 000 000 000 bps. So 3 Gbps is 3 000 000 000 bps.
- Data sent in a time: 16 Mbps for 30 seconds is 16 × 30 = 480 megabits, and 480 ÷ 8 = 60 MB.
- Streaming: a 4 Mbps stream for 20 minutes is 4 × 1200 = 4800 megabits = 600 MB = 0.6 GB.
- Comparing two lines: 1 GB is 1000 MB = 8000 megabits. At 20 Mbps it takes 400 s; at 100 Mbps it takes 80 s. The faster line is 400 - 80 = 320 seconds quicker.
The rest of this lesson and its 7 practice questions are locked.
In this lesson
- Bits and bytes
- Packets, latency and sharing
- Try it: How long will it take?
7 practice questions, marked as you go; about 20 minutes in all.
Key terms
Bits, Bytes
In this unit: Networks and protocols
LANs, WANs and PANs, network hardware, star, mesh and bus topologies, the internet and DNS, IP and MAC addresses, protocols and layers, network security, and transfer time calculations.