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iGCSE CS 25 > 1.3 & 2.2 Compression & Error Detection > Flashcards

1.3 & 2.2 Compression & Error Detection Flashcards

1
Q

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

A

Parity Check
Checksum
Echo Check

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2
Q

State 1 method for checking for errors in data input

A

Check Digit

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3
Q

Describe how even parity works?

A

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
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4
Q

Describe how a checksum works

A

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
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5
Q

Describe how ARQ works

A

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.
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6
Q

What is a Check Digit?

A
  • 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.
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7
Q

How is a check digit used?

A
  • 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
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8
Q

Describe 2 types of errors detected by check digits

A

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

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9
Q

Describe how an echo check works

A
  • 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
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10
Q

What is compression?

A
  • A reduction in the number of bits needed to represent data
  • It reduces the size of the file
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11
Q

What are the advantages of compression?

A

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
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12
Q

State 2 different types of compression

A

Lossy and Lossless

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13
Q

State 2 different types of lossless compression

A

RLE (Run Length Encoding) and Dictionary

They can be combined

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14
Q

Describe Run Length Encoding compression (lossless)

A
  • 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
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15
Q

Describe Dictionary compression (lossless)

A
  • 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
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16
Q

Describe lossy compression

A
  • 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)
17
Q

Describe how lossy compression could compress a music file

A
  • 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
18
Q

Describe how lossy compression could compress an image file

A
  • 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
19
Q

Describe how lossy compression could compress a video file

A
  • 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)
20
Q

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

A
  • 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
21
Q

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

A
  • 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
22
Q

State 4 image file formats and their type of compression

A
  • .bmp - uncompresssed file format
  • .png - lossless compression
  • .jpg - lossy compression
  • .gif - lossless compression
23
Q

State 2 audio file formats and their type of compression

A
  • ,mp3 - lossy compression
  • .wav - lossless compression
  • .flac - lossless compression
24
Q

Name a commonly used file format for compressing documents

A

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

iGCSE CS 25 (20 decks)

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