Understand that data storage is measured in binary multiples (bit, nibble, byte, kibibyte, mebibyte, gibibyte, tebibyte), construct expressions to calculate file sizes and data capacity, and understand the need for and methods of compression (lossless, lossy).

What this dot point is asking

Edexcel wants you to know the binary units of storage (bit, nibble, byte and the binary multiples kibibyte, mebibyte, gibibyte, tebibyte), to build expressions for file sizes and storage capacity, and to explain why data is compressed and the difference between lossless and lossy compression.

The binary storage multiples

This is an Edexcel-specific point worth stressing: the board uses the binary multiples (kibibyte, mebibyte, and so on), where each unit is $1024$ of the one below, because $1024 = 2^{10}$ fits binary storage exactly. So 1 kibibyte $= 1024$ bytes $= 1024 \times 8 = 8192$ bits. Getting the factor right (1024) is essential for capacity calculations.

File-size and capacity expressions

The reliable method for capacity questions is: (1) work out the size of one file; (2) convert the storage capacity into the same unit; (3) divide. Conversions use the factor 1024 for each binary step (and 8 for bits to bytes). For instance, a 1 gibibyte drive holds $1024$ mebibytes, so it stores $1024 \div 4 = 256$ files that are 4 mebibytes each.

Why and how data is compressed

The need for compression is practical: smaller files cost less to store and stream, and load faster on web pages, which matters for images, audio and video that would otherwise be very large.

The trade-off is size against fidelity. Lossy achieves far smaller files but loses quality and is irreversible, so it is wrong for a program file (a single changed bit could break it). Lossless is reversible and safe for any data, but the size saving is more modest. Choosing the right type for the data, and justifying it, is what compression questions reward.

Try this

Q1. State how many bytes are in a kibibyte. [1 mark]

• Cue. 1024 bytes.

Q2. State one reason data is compressed before being sent over a network. [1 mark]

• Cue. A smaller file transfers faster and uses less bandwidth (and less storage).