Network Speed

Network speed is the rate at which data is transmitted from one device to another across a network. It is measured in bits per second (bps), so a link carrying 300,000,000 bps is shifting 300 million bits every second.

Decimal prefixes

Network speed units use decimal prefixes, so each step up is a jump of 1000 (not 1024). The letters in front of the bps tell you which step you are on:

To change a larger unit into bits per second, multiply by 1000 once for every prefix step. A 450 Mbps connection is 450 × 1000 × 1000 = 450,000,000 bps, because Mbps sits two steps above bps. Going the other way, divide by 1000 for each step: 7,000,000,000 bps is 7,000,000,000 ÷ 1000 ÷ 1000 ÷ 1000 = 7 Gbps.

Bits and bytes

Mb means megabit and MB means megabyte, and they are not the same unit. One byte holds 8 bits, so a 64 Mbps connection moves 64 ÷ 8 = 8 MB of data each second. The capital letter is the clue: a capital B is always bytes, a lowercase b is always bits.

Comparing speeds

To compare speeds that are written in different units, convert them to the same unit first, usually bits per second. Otherwise the larger-looking number can win by accident.

For example, a college campus link runs at 2 Gbps while a nearby primary school link runs at 250 Mbps. Converting gives 2 × 1000 × 1000 = 2,000,000,000 bps for the campus and 250 × 1000 × 1000 = 250,000,000 bps for the school, so the campus link is 8 times faster — even though 2 looks far smaller than 250.

Bandwidth is the maximum amount of data a network connection can transmit in a given period of time. People often talk about a connection's "speed", but that word is vague: it could mean how much data is carried each second, or how long a message takes to arrive. Bandwidth is the preferred term for the first of these — the maximum amount of data that can be sent each second.

Bandwidth describes the capacity of a connection, not the rate at which data is flowing right now. A school network link with a bandwidth of 900 Mbps can carry up to 900 megabits every second, but while you are only sending a short email it may be carrying far less than that. Think of a motorway: four lanes allow up to four lanes' worth of traffic to flow side by side, even on a quiet Sunday morning when the road is nearly empty.

Bandwidth is measured in bits per second, written bps. Networks move such large amounts of data that you will usually see Mbps (megabits per second) or Gbps (gigabits per second) instead. Bandwidth is not the same as latency: latency measures the delay before data arrives, and it is measured in units of time such as milliseconds, never in bits per second.

Latency is the delay between data being sent and the same data being received. It is the time you spend waiting for a response after a request leaves your device, so it describes how long the data is held up rather than how much data is moving.

Latency is measured in units of time, usually milliseconds (ms). One millisecond is one thousandth of a second, so a latency of 60 ms means a message is delayed by 60 thousandths of a second before it arrives. It is not measured in bits per second: bits per second describe how much data is carried, not how long you wait.

For example, if you press a button in an online game and your character reacts 80 ms later, the connection has about 80 ms of latency. A lower latency feels more responsive because the delay is shorter. High latency can make a video call feel out of step, even when the connection can carry plenty of data.

Latency is not a measure of reliability or signal range; it only describes the delay. Bandwidth, the maximum amount of data a connection can carry in a given time, is a separate measure.

Transfer time is how long a file takes to travel across a network from one device to another. If you know the size of the file and the transmission rate of the connection, you can work that time out.

Working out the time

Divide the file size in bits by the transmission rate in bits per second:

time in seconds = file size in bits ÷ transmission rate in bits per second

For example, you upload a 36 MB folder of photos over a 9 Mbps connection. Both values need changing into bits first: × 8 turns the bytes into bits (1 byte = 8 bits), and × 1,000,000 turns megabits per second into bits per second (1 Mbps = 1,000,000 bits per second). Here 1 MB is taken as 1,000,000 bytes, so the whole calculation fits in one expression:

(36 × 8 × 1,000,000) ÷ (9 × 1,000,000) = 288,000,000 ÷ 9,000,000 = 32 seconds

Rearranging to find the file size

If you know the rate and how long the transfer lasted, multiply them to get the size in bits:

file size in bits = transmission rate in bits per second × time in seconds

For example, a connection running at 8 Mbps for 45 seconds carries 8,000,000 × 45 = 360,000,000 bits. Dividing by 8 gives 45,000,000 bytes, which is 45 MB.

Why the answer changes

The time depends on the other two values. Raise the transmission rate and a fixed file arrives sooner, because more bits are delivered every second. Make the file larger and it takes longer at the same rate, because there are simply more bits to deliver. The two scale neatly: doubling the file size doubles the time, while doubling the rate halves it.

Any time you work out this way is theoretical — the quickest it could possibly be. Real network conditions such as congestion, interference and the number of users sharing the connection all slow the traffic down, so the file usually arrives later than your answer suggests.