GCSE · Computer Science · Edexcel · Spec 1CP2

Need for data compression

Your holiday video is too large to email. Post a memory stick? Or ask a better question: what if the file simply had fewer bits?

Computer Science · Data compression

Same information, fewer bits: who benefits?

Compression gives a file fewer bits. Tick every advantage each situation shows. Some situations show more than one.

A Less storage space
B Less bandwidth needed
C Faster transfer

Fitting more photos on a phone
Cutting the cost of cloud storage
Streaming a video with less buffering
Leaving bandwidth free for other devices on a home network
Sending a large file as an email attachment instead of posting a memory stick
A website loading faster
Uploading a folder of holiday photos to the cloud

Bandwidth, step by step

Reason it through

Why does a compressed file need less bandwidth, and what does that do for everyone else on the network?

Link 1 of 4

First link · your turn

What does a compression algorithm actually do to the data in a file?

Locked — reveal the link above first

Computer Science · Units

Megabits are not megabytes

A home network is rated at 100 megabits per second (100 Mbps). About how many megabytes per second can it transfer?

Your estimate

50 megabytes per second

A home network is rated at 100 megabits per second (100 Mbps). About how many megabytes per second can it transfer? Estimate between 0 megabytes per second and 100 megabytes per second.

0 megabytes per second100 megabytes per second

Watch out: Mbps counts megabits per second. Divide by 8 to count megabytes per second.

Computer Science · What is compression for?

Sam explains compression

Sam is explaining compression to a friend and has a few ideas about it.

Which idea is closest to what you think right now?

Compression is only for pictures. JPEG is what it's for.Compression makes the connection itself faster.Compressed files take up less space, but they need just as much bandwidth to send.Compression gives a file fewer bits for the same information, and it works on images, video, audio and documents.

How sure are you?

I'm sureI think soI'm guessing

Compression is only for pictures. JPEG is what it's for.

This is a common misconception

It's an easy one to believe because JPEG is so well known. But JPEG is just one method, suited to images. Audio, video and documents are commonly compressed too, with methods such as MP3 for audio and MP4 for video, and ZIP combines several files into one smaller file.

Compression makes the connection itself faster.

This is a common misconception

The connection isn't what changes. The file is. A compressed file has fewer bits, so less bandwidth is needed to transfer it and it takes less time to send and receive.

Compressed files take up less space, but they need just as much bandwidth to send.

This is a common misconception

Half right. Fewer bits does mean less storage. But fewer bits also means less bandwidth is needed to transfer the file. The advantages come from the same cause.

Compression gives a file fewer bits for the same information, and it works on images, video, audio and documents.

This matches the evidence

That's the idea. Fewer bits is the cause, and the advantages follow from it: less storage space, less bandwidth needed and faster transfer, whatever the type of file.

Compression is about bits, not about pictures or about the connection: fewer bits, so less storage, less bandwidth needed and a shorter transfer time.

Computer Science · Explain it in your own words

Too big to email

A file is too large to send as an email attachment. Explain two advantages of compressing it before it is sent. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Names one advantage clearly, e.g. less bandwidth needed, faster transfer (less time to send), or less storage space. (1 mark)
Gives a reason for it that links to the compressed file having fewer bits. (1 mark)
Names a second, different advantage clearly. (1 mark)
Gives a reason for the second advantage. (1 mark)

Model answer

Compressing the file gives it fewer bits. This means less bandwidth is needed to send it, which leaves more bandwidth for other devices. A smaller file also takes less time to send and receive, so the transfer is faster and the file is small enough to share over the internet instead of being posted on a memory stick.

Name the advantage, then say why: the compressed file has fewer bits.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Compression cuts the number of bits needed to represent the same information. Its aim is never to increase it.
Algorithms do this by removing unnecessary data or by rearranging the information more efficiently.
Have a goYour classmate Sam compresses a file and proudly announces it now has MORE bits than before. Has the compression done its job?
No. Compression aims to use fewer bits for the same information, never more.
The whole point is to reduce the number of bits needed to represent the information, so more bits is the opposite of success.
Storage: a smaller file takes up less space, so more files fit and local or cloud storage costs can fall.
Bandwidth is the maximum amount of data a channel can transfer at one time, usually in bits per second (Mbps, Gbps).
Fewer bits means less bandwidth is needed, which reduces the load on the network and leaves more bandwidth for other devices.
To turn bits per second into bytes per second, divide by 8: 100 megabits per second is about 12.5 megabytes per second.
Have a goA network is rated at 80 megabits per second. Before any maths: will its speed in megabytes per second be a bigger or smaller number than 80?
Smaller. Dividing by 8 turns 80 into 10.
Converting bits per second to bytes per second means dividing by 8, so the number shrinks. It does not stay the same, and it does not grow.
Transfer: smaller files take less time to send and receive, so they upload, download and load faster on websites.
Streaming services compress audio and video (MP3, MP4), so playback is smoother, with less buffering, on a range of devices.
Images, video, audio and documents are all commonly compressed, so compression is not only for pictures.
Methods suit file types: JPEG for images, MP3 for audio, MP4 for video. ZIP combines several files into one smaller file.
Have a goYour friend Mo insists, "Compression is a picture thing, full stop." Name two other kinds of file that get compressed.
Any two of: video, audio, documents.
Images are only one of the commonly compressed types, so compression is not only for pictures.

The big picture

Compression gives a file fewer bits for the same information. That one change means less storage space, less bandwidth needed and faster transfer, for images, video, audio and documents alike.

Key points

1Compression gives a file fewer bits for the same information; it never aims to increase them.

2Fewer bits gives three advantages: less storage space, less bandwidth needed and faster transfer.

3Divide bits per second by 8 to get bytes per second.

4Any type of file can be compressed, not only pictures.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

A library's network is rated at 240 megabits per second (240 Mbps). Work out how many megabytes per second it can transfer.

⚠ Watch out

Saying compression makes the connection faster or gives you more bandwidth. The connection stays the same. The compressed file has fewer bits, so it needs less bandwidth and takes less time to transfer.

🧠

Memory hook

Fewer bits, fewer problems: less to store, less to squeeze through the network, less to wait for.

✓

Check yourself

From memory: what one change gives all three advantages of compression, and what are they? Then convert 160 megabits per second to megabytes per second (answer: 20).

Flashcards

(13)

What is the aim of data compression?

To reduce the number of bits needed to represent the same information, never to increase it.

How do compression algorithms make a file smaller?

They remove unnecessary data or rearrange the information more efficiently.

What is the storage advantage of compression?

A compressed file takes up less space, so more files fit in the same capacity and local and cloud storage costs can be reduced.

What is bandwidth?

The maximum amount of data that can be transferred over a communication channel at one time, usually measured in bits per second (Mbps, Gbps).

What is the bandwidth advantage of compression?

A compressed file has fewer bits, so less bandwidth is needed to transfer it. That reduces the load on the network and leaves more bandwidth for other devices.

How do you convert bits per second to bytes per second?

Divide by 8.

What is the transfer advantage of compression?

Smaller files take less time to send and receive, so they upload, download and load faster on websites.

Why do streaming services compress audio and video?

So playback is smoother, with less buffering, on a range of devices.

Is compression only for pictures?

No. Images, video, audio and documents are all commonly compressed.

Which file type does each format suit: JPEG, MP3, MP4?

JPEG suits images, MP3 suits audio and MP4 suits video.

What does a ZIP file do?

It combines several files into one smaller file.

A compressed file is small enough to email. What does it replace?

Posting a memory stick: the smaller file can be shared over the internet instead.

Which is right: compression increases bandwidth, or reduces the bandwidth needed?

It reduces the bandwidth needed. The channel is the same; the compressed file has fewer bits and needs less of it.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Same information, fewer bits: who benefits?

Megabits are not megabytes

Sam explains compression

Too big to email

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