← IGCSE Computer Science Paper 1

Why compression is needed: shorter transmission time

The time a file takes to upload, download or send. A smaller compressed file has a shorter transmission time, so downloads, email attachments and web pages are quicker.

Part of: IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 32, ACARA Digital Technologies Years 9–10 — Data Compression — Module 14, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 54

Understanding Why compression is needed: shorter transmission time in IGCSE Computer Science Paper 1

What it does: The compressed attachment is half the size, so it takes about half the time to send over the same connection. Smaller images mean less data to transmit, so the web page appears faster for the user. The smaller file has fewer bits to transmit, so the attachment sends more quickly. More data means a longer transmission time. Compressing images helps web pages load smoothly. Transmission time is how long it takes to move a file from one device to another.

Common mistakes: The time should use the compressed file size (small), not the original size. It should be secs = small / speed, so the smaller file gives a shorter transmission time. A single = assigns a value to a variable. == compares two values. It should be size_mb = 120.

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This module's key terms are also in a printable crossword (Modules 86–109) and word search (Modules 86–120).

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Learning

Duo

Why does compressing a file make it quicker to download?

The file is smaller, so fewer bits have to be sentThe file is bigger, so it is sent in one go

Matches

A bigger file has a shorter transmission time than a smaller file

↓

This is the wrong way round. A bigger file has more data to send, so it takes longer to transmit.

Applying

Meanings

Term

transmission time

Meaning

The time a file takes to upload, download or send. A smaller compressed file has a shorter transmission time, so downloads, email attachments and web pages are quicker.

Practice

SizeMiB ← 80

Testing

Gaps

time ← fileSize speed

Check Answer

Spots

3small = size_mb / 2

4secs = size_mb / speed

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Quick Questions

1. Why does compressing a file make it quicker to download?

A. The file is bigger, so it is sent in one go

B. The file is smaller, so fewer bits have to be sent ✓

A compressed file has fewer bits to transfer, so it takes less time to download.

2. Which file will take LESS time to send as an email attachment?

A. The same photo uncompressed at 5 MiB

B. A compressed photo of 500 KiB ✓

The smaller file has fewer bits to transmit, so the attachment sends more quickly.

3. True or false: A smaller file always has a shorter transmission time on the same connection.

A. True - fewer bits to send means less time ✓

B. False - file size has no effect on time

On the same connection, transmission time depends on how much data must be sent, so a smaller file is quicker.

4. A web page has large uncompressed images. What is likely to happen when it loads?

A. It loads slowly because lots of data must be downloaded ✓

B. It loads faster because there is more data

More data means a longer transmission time. Compressing images helps web pages load smoothly.

5. Which statement describes transmission time correctly?

A. The time a file takes to upload, download or send ✓

B. The number of colours a file can store

Transmission time is how long it takes to move a file from one device to another.

Match the Pairs

transmission time → The time a file takes to upload, download or send from one device to another

compressed file → A smaller version of a file that takes less time to download or send

Compressing an email attachment makes it quicker to send because the file is smaller → attachment.size = 2 MiB -> compressed to 1 MiB, send time is halved

Compressing web page images lets the page load smoothly and more quickly → page.image.size = 5 MiB -> 1 MiB, load time is shorter

A bigger file has a shorter transmission time than a smaller file → This is the wrong way round. A bigger file has more data to send, so it takes longer to transmit.

Compression makes a download slower because there is less data to send → Less data to send makes the download faster, not slower. The reasoning contradicts the conclusion.

Key Term

transmission time

The time a file takes to upload, download or send. A smaller compressed file has a shorter transmission time, so downloads, email attachments and web pages are quicker.

Fill in the Blanks

1. time ← fileSize / speed OUTPUT time

Hint: Type the arithmetic operator that divides the file size by the transfer speed.

2. time ← compressedSize / speed OUTPUT time

Hint: Type the variable name that stores how long the transfer takes.

3. IF compressedSize < originalSize THEN OUTPUT "Shorter download" ENDIF

Hint: Type the comparison operator that checks the compressed file is smaller than the original.

4. time ← compressedSize / speed OUTPUT time

Hint: Type the pseudocode keyword that displays a value on screen.

5. IF time > 60 THEN OUTPUT "Compress it" ENDIF

Hint: Type the comparison operator that means the transfer time is longer than 60 seconds.

Spot the Error

Question 1

A size_mb = 60

B speed = 5

C small = size_mb / 2

D secs = size_mb / speed

Line D — The time should use the compressed file size (small), not the original size. It should be secs = small / speed, so the smaller file gives a shorter transmission time.

Question 2

A size_mb == 120

B small_mb = size_mb / 4

C print(small_mb)

Line A — A single = assigns a value to a variable. == compares two values. It should be size_mb = 120.

Question 3

A email_mb = 20

B small_mb = email_mb * 4

C print(small_mb)

Line B — Compression makes the attachment smaller, so the size must be divided, not multiplied. It should be small_mb = email_mb / 4.

Question 4

A size_mb = 400

B speed = 8

C secs = size_mb * speed

D print(secs)

Line C — Transmission time is file size divided by speed. Multiplying would make a faster connection give a longer time. It should be secs = size_mb / speed.

Question 5

A page_kb = 900

B if page_kb > 500:

C print("Quick to load")

D else:

E print("Quick to load")

Line C — A large page (over 500 KB) takes longer to transmit, so it should print "Slow to load". Only small, compressed pages load quickly.

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