1.3 & 2.2 Compression & Error Detection

Question 1

State 3 methods for checking for errors in the transmission of data

Answer 1

Parity Check
Checksum
Echo Check

Question 2

State 1 method for checking for errors in data input

Answer 2

Check Digit

Question 3

Describe how even parity works?

Answer 3

The sender

• counts the number of ones in the bit pattern
• It adds a 0 if there are an even number of 1’s
• It adds a 1 if there are an odd number of 1’s

The receiver

• counts the number of ones in the bit pattern
• If there are an even number of ones it accepts the message
• If there is an odd number of 1’s it identifies there is an error and requests that the message is resent

Question 4

Describe how a checksum works

Answer 4

The sender

• Calculates the checksum for the block of bytes
• Transmits the checksum with the block of data

The receiver

• Checksum is recalculated from the block of data
• Compares the calculated checksum to the one transmitted
• If same, data transmitted has been received without errors
• If different a request is sent for date to be retransmitted

Question 5

Describe how ARQ works

Answer 5

ARQ = Automatic Repeat Request

• It uses an ACKNOWLEDGEMENT and TIMEOUT
• Message is sent by sender and it waits for a set amount of time
• If message received correctly a message (acknowledgement) is sent back by the receiver indicating that data has been received correctly
• If message not received correctly a message (acknowledgement) is not sent back. Timeout occurs and the message is automatically resent.

Question 6

What is a Check Digit?

Answer 6

• A check digit is calculated from other digits and added to the end of a long code 0 12345 67890 5
• Used to identify errors in data entry caused by mistyping or misscanning a barcode.

Question 7

How is a check digit used?

Answer 7

• Full code is input (manual or scanned) e.g. 0 12345 67890 5
• Check digit is recalculated from inputted digits
• Compares the calculated check digit to the one entered
• If same, check digit entered without errors
• If different error detected and code is not accepted

Question 8

Describe 2 types of errors detected by check digits

Answer 8

2 digits transposed e.g. 280419 becomes 280149
Incorrect digit entered e.g 280419 becomes 250419

Question 9

Describe how an echo check works

Answer 9

• The receiving computer sends a copy of the data immediately back to the sending computer
• The sending computer compares the received data to the sent data
• If they are the same the data is deemed to have arrived without error

Question 10

What is compression?

Answer 10

• A reduction in the number of bits needed to represent data
• It reduces the size of the file

Question 11

What are the advantages of compression?

Answer 11

It reduces the size of the file so …

• the transfer speed is quicker
• uses less bandwidth, reducing internet traffic
• takes up less storage space
• allows large multimedia files to be streamed

Question 12

State 2 different types of compression

Answer 12

Lossy and Lossless

Question 13

State 2 different types of lossless compression

Answer 13

RLE (Run Length Encoding) and Dictionary

They can be combined

Question 14

Describe Run Length Encoding compression (lossless)

Answer 14

• It uses a compression algorithm
• No data is lost
• Looks for repeated patterns
• Replaces consecutive identical values into a code consisting of
  
  • the value of the data e.g. A
  • a count (number of times it repeats) e.g. 4
  • e.g. AAAABBBB = A4B4

Question 15

Describe Dictionary compression (lossless)

Answer 15

• It uses a compression algorithm
• No data is lost
• Looks for repeated patterns
  
  • Maintains a dictionary (index) of repeating patterns within a file.
  • Each pattern is given an ID/value
  • The patterns are replaced by their ID /value

Question 16

Describe lossy compression

Answer 16

• It uses a compression algorithm
• Data is lost and cannot be restored
• Deletes data that is not necessary
• Uses perception shaping (changes that cannot be noticed)

Question 17

Describe how lossy compression could compress a music file

Answer 17

• It uses a compression algorithm
  
  • Removes sounds the human ear cannot hear
  • Uses perception shaping
  • If 2 sounds played at the same time it removes the softer sound
  • Reduces bit rate/bit depth
  • Reduces the sample rate

Question 18

Describe how lossy compression could compress an image file

Answer 18

• It uses a compression algorithm
• Use perception shaping
• Similar colours are made the same
• uses less colours (colour depth reduced)
• reduce number of pixels (image resolution reduced

Question 19

Describe how lossy compression could compress a video file

Answer 19

• It uses a compression algorithm
  
  • reduce frame rate
  • similar colours are made the same
  • uses perception shaping
  • uses less colours (colour depth reduced)
  • reduce number of pixels (image resolution reduced)

Question 20

Explain which compression type you would use when emailing a text document to a friend

Answer 20

• Lossless
  
  • RLE or Dictionary (or both) based encoding would be used
• No data is lost
  
  • File can be restored to it’s original state
• Lossy would remove data
  
  • So cannot be read when uncompressed

Question 21

Explain which compression type you would use to email holiday images to a friend

Answer 21

• Lossy
  
  • would reduce the file size more than lossless
  • removes redundant/unnecessary data
  • images can still be of a similar quality (perception shaping)
  • file does not need to be restored to it’s original state
  • Faster to send / upload / download

Question 22

State 4 image file formats and their type of compression

Answer 22

• .bmp - uncompresssed file format
• .png - lossless compression
• .jpg - lossy compression
• .gif - lossless compression

Question 23

State 2 audio file formats and their type of compression

Answer 23

• ,mp3 - lossy compression
• .wav - lossless compression
• .flac - lossless compression

Question 24

Name a commonly used file format for compressing documents

Answer 24

.zip - A ZIP file is an archive that contains one or more files compressed or “zipped”