Why compression is needed: shorter transmission time

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

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

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

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

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1transmission time		Aattachment.size = 2 MiB -> compressed to 1 MiB, send time is halved
2compressed file		BThe time a file takes to upload, download or send from one device to another
3Compressing an email attachment makes it quicker to send because the file is smaller		CThis is the wrong way round. A bigger file has more data to send, so it takes longer to transmit.
4Compressing web page images lets the page load smoothly and more quickly		DA smaller version of a file that takes less time to download or send
5A bigger file has a shorter transmission time than a smaller file		Epage.image.size = 5 MiB -> 1 MiB, load time is shorter
6Compression makes a download slower because there is less data to send		FLess data to send makes the download faster, not slower. The reasoning contradicts the conclusion.

Definitions

transmission time

Fill in the Blanks

Complete each line of code by filling in the missing parts.

time ← fileSize _ speed OUTPUT time

____ ← compressedSize / speed OUTPUT time

IF compressedSize _ originalSize THEN OUTPUT "Shorter download" ENDIF

time ← compressedSize / speed ______ time

IF time _ 60 THEN OUTPUT "Compress it" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A size_mb = 60

B speed = 5

C small = size_mb / 2

D secs = size_mb / speed

Question 2

A size_mb == 120

B small_mb = size_mb / 4

C print(small_mb)

Question 3

A email_mb = 20

B small_mb = email_mb * 4

C print(small_mb)

Question 4

A size_mb = 400

B speed = 8

C secs = size_mb * speed

D print(secs)

Question 5

A page_kb = 900

B if page_kb > 500:

C print("Quick to load")

D else:

E print("Quick to load")

Answer Key

Why compression is needed: shorter transmission time

Section A: Quick Questions

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

B — A compressed photo of 500 KiB

A — True - fewer bits to send means less time

A — It loads slowly because lots of data must be downloaded

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

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

time ← fileSize / speed
OUTPUT time

time ← compressedSize / speed
OUTPUT time

IF compressedSize < originalSize THEN
  OUTPUT "Shorter download"
ENDIF

time ← compressedSize / speed
OUTPUT time

IF time > 60 THEN
  OUTPUT "Compress it"
ENDIF

Section E: Spot the Error

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.

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

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

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.

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