IGCSE COMPUTER SCIENCE PRACTICE
EXAM QUESTIONS AND ANSWERS
How image file like jpeg reduce the file size? - Answer-JPEG is an image file that used
lossy compression to reduce the size of the file. It reduces the range of colour
used/colour depth/bits per pixel and reduces the number of pixels / removes
unnecessary data.
How mp3 reduce the file size? - Answer-The mp3 file should use a lossy compressed
file. It compresses by discarding any unnecessary sounds and sounds that humans
can't hear by reducing the rate of the amplitude of the sounds.
How mp4 reduce the file size? - Answer-The mp4 file is suitable to use lossy
compressed files.
− A compression algorithm is used
− The resolution could be reduced
− Colour depth could be reduced // bits per pixel reduced
− Sounds not heard by human ear could be removed // Perceptual music shaping can
be used
− Repeating frames could be removed
Describe how lossless compression reduces the file size of the video. (3 marks) -
Answer-− Compression algorithm used
− ..., e.g. RLE
− Repeating frames/pixels are identified
− ... and are collated/indexed
− No data is permanently removed
− It just records the changes between frames/pixels
How text file reduce the file size? - Answer-The text file is suitable to use as a lossless
compressed file because if you remove any word it can have an impact.
A compression algorithm is used to find and index repeated words or patterns (or
sections of words) within the data. The indexed words/patterns are then replaced with
numerical values. The index will need to be stored with the data to allow decompression
with no loss of data.
What is the advantage of using lossy compression over lossless compression in photo
images? - Answer--Lossy will reduce the file size more than lossless
-The redundant data can be removed from the files
-Images can still be of similar quality
-There is no requirement for the files to be exactly the same as the original
-Photos can be sent quicker / faster to upload / faster to download
,What is the advantage of using lossless compression over lossy compression in photo
images and video? - Answer--Maintains quality // quality better than lossy
− Original file is retained // Data is not permanently lost − A significant reduction in file
size is not required
The file Michele is sending contains the source code for a large computer program.
Identify which type of compression would be most suitable for Michele to use and
explain why? - Answer-Lossless because the file can be restored/decompressed to the
exact same state it was before compression and because it contains a text, with the use
of lossy compression it can ruin and remove the code and cannot run the code properly.
What are the 2 methods of transmission? - Answer-Serial transmission - sends data one
bit at a time over a single channel. (use a single wire)
Parallel data transmission - when several bits are sent at a time over multiple channels.
What are the 3 directions of transmission? - Answer-Simplex transmission - allows for
data to be sent in only one direction.(unidirectional)(serial)
e.g. a microphone
Half-duplex transmission - allows data to travel in both directions but cannot occur
simultaneously (only one direction at a time) // asynchronous (cannot occur at the same
time) (serial)
e.g. two-way radio (walkie talkie)
Full Duplex - allows data to travel in both directions simultaneously(at the same time)
(parallel)
e.g. telephone
Give an example of method for errors checking? - Answer-1.Parity check
2. Checksums
3.ARQ (Automatic Repeat reQuest)
4.Check digit
5.Echo check
How parity check detect the error? - Answer-The user will clarify if the parity bytes are
going to be even/odd. All the parity bytes have to be added to be either odd or even
depend on what parity set e.g. the user had used even parity 110110100 = 1+1+1+1+1
=5
--> Error founded
If parity bit is incorrect an error is detected like as shown in the example.
Why parity check do not always detect errors? - Answer-If an even number of bits are
changed during transmission (2,4,6,8) then the Parity check is not be able to detect the
error)
,A system used odd parity. Complete these following two bytes of data so they both have
odd parity:
_1111000
_0000111 - Answer-11111000
00000111
How checksum detected the error? - Answer-The sender computers use the block of
data, and a predefined mathematical algorithm, to calculate a checksum value. The
sender sends a checksum with data. The receiver receives data and the checksum is
recalculated. The two checksum values are compared by the receiving computer. Error
detected if checksums are different.
Mark scheme:
• Calculation performed on data (to get the checksum) • Checksum sent with data
• Checksum recalculated after transmission • Comparison made between checksum
before and checksum after transmission
• Error detected if checksums are different
How ARQ detected the error? - Answer-ARQ uses acknowledgement and time out. The
sender transmits a block of data. The sender waits for an acknowledgement. If no
response/acknowledgement or any errors occur(negative acknowledgement is used),
the request(with data) is repeated till the data is sent correctly or the request time out.
Find the check digit for this identification number
**-multiply each digit by its digit position
-add up the results of the multiplication
-divided the answers by 11
-if the remainder is 0, the identification number and check digit are valid(correct)**
5 0 2 4 1 __ - Answer-(5*6)+(0*5)+(2*4)+(4*3)+(2*1) = 30+0+8+12+2 = 52
(52+3= 55/11 = 5 remainder 0). Therefore, the check digit is 3.
What is USB? - Answer-Universal Serial Bus. USB uses serial transmission for sending
data externally.
(USB is off-line storage)
Leonard has a new laser printer to print letters for his business. Leonard connects his
printer to his computer using the USB port.
Give 3 benefits of using the USB port to connect the printer to the computer. - Answer--
It is a universal standard
− It can't be inserted the wrong way around
− Supports different transmission speeds
− Automatically detects if correct driver installed
, A Universal Serial Bus (USB) connection can be used to transmit data from a mobile
device to a computer.
Give three benefits of using a USB connection for this purpose. - Answer-− Can
charge/power the mobile device (at the same time)
− (Uses serial transmission so) data less likely to be skewed / corrupted
− Universal / industry standard / connection
− Cable can only be plugged in one way // Cannot be inserted incorrectly
- Fast transmission speed
− Backward compatible
− Supports different transmission speeds
− Automatically detects device // Automatically downloads drivers
What is integrated circuits? - Answer-Integrated Circuits (IC) uses parallel data
transmission for sending data internally.(also RAM)
Due to this, parallel transmission is best suited for situations where data is to be sent
over a very small distance.
What is advantage of serial transmission? - Answer--Cheaper wiring because uses only
a single wire
-Less likely to have interference(error)
-More efficient over long distances.
-Sent a bit at a time, therefore less chance of data being skewed/out of synchronization
What is advantage of parallel transmission? - Answer--Faster rate of transmission than
serial transmission
-Great for a short distance
Give an example of online security threats to a computer system. - Answer-• Hacking
• Cracking
• Virus
• Denial of service
• Malware
• Spyware
What is hacking? - Answer-Hacking is the illegal access to a computer system without
the owner's consent/knowledge.
What is cracking? - Answer-Cracking is the act of illegally editing program sources so it
can be exploited/changed for a specific purpose,
What is viruses? - Answer-Viruses a program that replicates itself to damage / delete
files.
EXAM QUESTIONS AND ANSWERS
How image file like jpeg reduce the file size? - Answer-JPEG is an image file that used
lossy compression to reduce the size of the file. It reduces the range of colour
used/colour depth/bits per pixel and reduces the number of pixels / removes
unnecessary data.
How mp3 reduce the file size? - Answer-The mp3 file should use a lossy compressed
file. It compresses by discarding any unnecessary sounds and sounds that humans
can't hear by reducing the rate of the amplitude of the sounds.
How mp4 reduce the file size? - Answer-The mp4 file is suitable to use lossy
compressed files.
− A compression algorithm is used
− The resolution could be reduced
− Colour depth could be reduced // bits per pixel reduced
− Sounds not heard by human ear could be removed // Perceptual music shaping can
be used
− Repeating frames could be removed
Describe how lossless compression reduces the file size of the video. (3 marks) -
Answer-− Compression algorithm used
− ..., e.g. RLE
− Repeating frames/pixels are identified
− ... and are collated/indexed
− No data is permanently removed
− It just records the changes between frames/pixels
How text file reduce the file size? - Answer-The text file is suitable to use as a lossless
compressed file because if you remove any word it can have an impact.
A compression algorithm is used to find and index repeated words or patterns (or
sections of words) within the data. The indexed words/patterns are then replaced with
numerical values. The index will need to be stored with the data to allow decompression
with no loss of data.
What is the advantage of using lossy compression over lossless compression in photo
images? - Answer--Lossy will reduce the file size more than lossless
-The redundant data can be removed from the files
-Images can still be of similar quality
-There is no requirement for the files to be exactly the same as the original
-Photos can be sent quicker / faster to upload / faster to download
,What is the advantage of using lossless compression over lossy compression in photo
images and video? - Answer--Maintains quality // quality better than lossy
− Original file is retained // Data is not permanently lost − A significant reduction in file
size is not required
The file Michele is sending contains the source code for a large computer program.
Identify which type of compression would be most suitable for Michele to use and
explain why? - Answer-Lossless because the file can be restored/decompressed to the
exact same state it was before compression and because it contains a text, with the use
of lossy compression it can ruin and remove the code and cannot run the code properly.
What are the 2 methods of transmission? - Answer-Serial transmission - sends data one
bit at a time over a single channel. (use a single wire)
Parallel data transmission - when several bits are sent at a time over multiple channels.
What are the 3 directions of transmission? - Answer-Simplex transmission - allows for
data to be sent in only one direction.(unidirectional)(serial)
e.g. a microphone
Half-duplex transmission - allows data to travel in both directions but cannot occur
simultaneously (only one direction at a time) // asynchronous (cannot occur at the same
time) (serial)
e.g. two-way radio (walkie talkie)
Full Duplex - allows data to travel in both directions simultaneously(at the same time)
(parallel)
e.g. telephone
Give an example of method for errors checking? - Answer-1.Parity check
2. Checksums
3.ARQ (Automatic Repeat reQuest)
4.Check digit
5.Echo check
How parity check detect the error? - Answer-The user will clarify if the parity bytes are
going to be even/odd. All the parity bytes have to be added to be either odd or even
depend on what parity set e.g. the user had used even parity 110110100 = 1+1+1+1+1
=5
--> Error founded
If parity bit is incorrect an error is detected like as shown in the example.
Why parity check do not always detect errors? - Answer-If an even number of bits are
changed during transmission (2,4,6,8) then the Parity check is not be able to detect the
error)
,A system used odd parity. Complete these following two bytes of data so they both have
odd parity:
_1111000
_0000111 - Answer-11111000
00000111
How checksum detected the error? - Answer-The sender computers use the block of
data, and a predefined mathematical algorithm, to calculate a checksum value. The
sender sends a checksum with data. The receiver receives data and the checksum is
recalculated. The two checksum values are compared by the receiving computer. Error
detected if checksums are different.
Mark scheme:
• Calculation performed on data (to get the checksum) • Checksum sent with data
• Checksum recalculated after transmission • Comparison made between checksum
before and checksum after transmission
• Error detected if checksums are different
How ARQ detected the error? - Answer-ARQ uses acknowledgement and time out. The
sender transmits a block of data. The sender waits for an acknowledgement. If no
response/acknowledgement or any errors occur(negative acknowledgement is used),
the request(with data) is repeated till the data is sent correctly or the request time out.
Find the check digit for this identification number
**-multiply each digit by its digit position
-add up the results of the multiplication
-divided the answers by 11
-if the remainder is 0, the identification number and check digit are valid(correct)**
5 0 2 4 1 __ - Answer-(5*6)+(0*5)+(2*4)+(4*3)+(2*1) = 30+0+8+12+2 = 52
(52+3= 55/11 = 5 remainder 0). Therefore, the check digit is 3.
What is USB? - Answer-Universal Serial Bus. USB uses serial transmission for sending
data externally.
(USB is off-line storage)
Leonard has a new laser printer to print letters for his business. Leonard connects his
printer to his computer using the USB port.
Give 3 benefits of using the USB port to connect the printer to the computer. - Answer--
It is a universal standard
− It can't be inserted the wrong way around
− Supports different transmission speeds
− Automatically detects if correct driver installed
, A Universal Serial Bus (USB) connection can be used to transmit data from a mobile
device to a computer.
Give three benefits of using a USB connection for this purpose. - Answer-− Can
charge/power the mobile device (at the same time)
− (Uses serial transmission so) data less likely to be skewed / corrupted
− Universal / industry standard / connection
− Cable can only be plugged in one way // Cannot be inserted incorrectly
- Fast transmission speed
− Backward compatible
− Supports different transmission speeds
− Automatically detects device // Automatically downloads drivers
What is integrated circuits? - Answer-Integrated Circuits (IC) uses parallel data
transmission for sending data internally.(also RAM)
Due to this, parallel transmission is best suited for situations where data is to be sent
over a very small distance.
What is advantage of serial transmission? - Answer--Cheaper wiring because uses only
a single wire
-Less likely to have interference(error)
-More efficient over long distances.
-Sent a bit at a time, therefore less chance of data being skewed/out of synchronization
What is advantage of parallel transmission? - Answer--Faster rate of transmission than
serial transmission
-Great for a short distance
Give an example of online security threats to a computer system. - Answer-• Hacking
• Cracking
• Virus
• Denial of service
• Malware
• Spyware
What is hacking? - Answer-Hacking is the illegal access to a computer system without
the owner's consent/knowledge.
What is cracking? - Answer-Cracking is the act of illegally editing program sources so it
can be exploited/changed for a specific purpose,
What is viruses? - Answer-Viruses a program that replicates itself to damage / delete
files.
