Network Speed Calculator

How long will it take?

A download is a division sum: the size of the file, divided by the speed of the connection. Two minutes of work, and every exam board asks it. There are only two ways to get it wrong, and this module is both of them. The first is the unit slip that turns a right method into a wrong number. The second is subtler: the sum gives you the best possible time, and a real transfer is always slower. A grade-9 answer knows both.

Two different units

Almost every lost mark on this topic comes from one fact: file sizes and connection speeds are not measured in the same thing.

Where does it go wrong?

A student divides a file size in megabytes by a speed in megabits per second, and writes the answer down. What has gone wrong?

• The two quantities are in different units, so the file size must be converted to bits first
• Nothing; megabytes and megabits are the same size
• They should have multiplied the size by the speed instead of dividing
• The answer needs converting from seconds into minutes

One relationship, three questions

Everything on this topic comes from a single relationship between file size, speed and time: time = size / speed - how long will this take? speed = size / time - how fast was that connection actually running? size = speed x time - how much could I transfer in that long? Convert before you divide. Get the file size and the speed into the same unit first - almost always by turning megabytes into megabits, multiplying by 8 - and only then do the arithmetic. If the answer comes out in bits and you were asked for bytes, divide by 8 at the end.

How long?

A 24 megabyte file is downloaded over a connection running at 6 megabits per second. How many seconds does it take? Convert the file size before you divide.

How fast was it?

A 45 megabit file transferred in 9 seconds. What speed was the connection actually running at, in megabits per second?

How much fits?

A connection runs at 12 megabits per second for 15 seconds. How many megaBYTES could it transfer in that time? Work out the megabits first, then convert.

Order the method

Put the stages of a network speed calculation into a reliable working order.

• Write down the two quantities you are given, with their units
• Convert so both are in the same unit, usually bits
• Choose which of the three rearrangements answers the question asked
• Do the arithmetic
• Check the answer is in the unit the question asked for

Why the real download is slower

Your calculation gives the best case. Several things make the real transfer take longer, and naming them is what separates a full-mark answer from an arithmetic one:

Why did it take longer?

A calculation says a download should take 30 seconds; it actually takes 50. Select the TWO statements that could explain the difference.

• Other devices in the house were using the same connection at the same time
• Some of the data transferred was addressing and error checking rather than the file itself
• The file must have been larger than stated
• The connection was measured in bits rather than bytes

Diagnose it

A school is deciding how to move large video files between two rooms, and why the current setup is slow. Make each decision.

• A 200 megabyte video needs moving. The advertised speed is in megabits per second. What do you do first?
• Two rooms are joined by a fast fibre link, but the computers connect to it over old Wi-Fi. What limits the transfer?
• Transfers are fine at 8am but slow to a crawl at lunchtime. What is the most likely cause?

Bytes, bits and speeds

File sizes are measured in bytes and connection speeds in _____ per second, and there are _____ bits in a byte. To find a time you divide the size by the _____, having converted both to the same unit first. To find how much could be transferred you _____ instead. And the real transfer is always slower than the calculation, because of latency, protocol overhead and _____, where the bandwidth is shared with other users.

Answer the broadband complaint

A customer complains that their 20 megabits per second broadband "never delivers what was promised" when they download a 50 megabyte file. Write a reply that is fair to both sides.

• Work out how long the download should take in the best case, showing the unit conversion
• Explain why the advertised figure is in megabits while the file size is in megabytes
• Give at least two reasons the real download takes longer than your calculated time
• Say what the customer could reasonably expect, and what would genuinely be a fault