Data storage Flashcards

Syallabus: 1.1.3

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1
Q

What is data validation?

List 8 different validation checks

A

Validation means checking carried out by the computer to make sure that input data are within the limits of what a user might resonably enter.

  • Type check (e.g. to only accept numerals)
  • Range check (e.g. must be >0 and <100)
  • Limit check (e.g. date of birth must not be numerically greater than today’s date)
  • Length check (e.g. password must be longer than 8 characters)
  • Character check (e.g. :!* not accepted for file name)
  • Format check (e.g. checking ID codes)
  • Presence check (e.g. email address required)
  • Consistency check (checking that different fields in the same record correspond correctly)
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2
Q

What is data verification?

List two types of data verification to minimise copying errors by humans

A

Data verification means double-checking for accuracy when data is copied.

  • Visual check - a single operator reads through a document, comparing it with the original
  • Double entry - the data is entered twice, either by two different operators or by the same operator, e.g. when entering a new password
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3
Q

List 4 methods of error detection and correction

A
  • Parity checks
  • Check digits
  • Checksums
  • Automatic Repeat reQuests (ARQ)
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4
Q

Describe how a parity check works

A
  1. A parity bit is added at an agreed position in each byte to make the total number of 1’s in the byte either odd or even
  2. Odd parity: odd number of 1’s, e.g. 11001110
  3. Even parity: even number of 1’s, e.g. 11011110
  4. If the receiver device checks the parity of received data and it has the wrong parity, it may re-read the byte that was sent, output an error message and/or request re-transmission.
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5
Q

Describe how a check digit works

A
  • A check digit is an extra digit that is calculated from all the original digits using an algorithm in order to summarise them
  • The algorithm used depends on the type of code, but relies on assigning a numerical ‘weight’ to each original digit that depends on its position in the number
  • The check digit checks for three types of error reliably:
    • two adjacent digits transposed
    • an incorrect digit entered
    • an omitted or extra digit
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6
Q

Describe how a checksum works

A
  • A checksum is a way of summarising a block of data to check that it is not corrupted
  • Arithmetic is applied to the elements of the block (e.g. the sum of all numerical values)
  • The sum is reduced to a standard number of digits and transmitted with the block
  • The same calculation is performed by the receiving device and the result is compared with the received checksum
  • If the checksums do not math, the data is rejected; else, the intergrity of the data is proven to be maintained
  • Cryptography can be used to try to prevent someone from maliciously substituting different data with the same checksum
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7
Q

Describe how an Automatic Repeat reQuest (ARQ) works

A
  • Also called Automatic Repeat Query
  • An error-control protocol that automatically initiates a call to retransmit any data packet or frame after receiving flawed or incorrect data.
  • When the transmitting device fails to receive an acknowledgement signal to confirm the data has been received, it usually retransmits the data after a predefined timeout and repeats the process a predetermined number of times until the transmitting device receives the acknowledgement.
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8
Q

What are the following file formats?

  • MIDI
  • jpeg
  • MP3
  • MP4
A
  • MIDI - Musical Instrument Digital Interface, used by musicians when creating songs
  • jpeg - Joint Photographic Experts Group, a lossy form of compression for colour images
  • MP3 - lossy form of audio compression
  • MP4 - video format
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9
Q

What is data compression

A
  • Storing data in a format that requires less space than usual
  • Data compression is particularly useful in communications because it enables devices to transmit or store the same amount of data in fewer bits
  • Compression can be either lossy or lossless
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10
Q

What is lossy compression?

A
  • This type of compression applies an algorithm that deletes ‘unecessary’ bits of data to reduce the file size
  • Quality of the file is reduced, e.g. poor picture quality
  • The original file cannot be recreated from the compressed file
  • Creates a smaller file size than lossless compression, so it takes up less space and transmission is faster
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11
Q

What is lossless compression?

A
  • This type of compression applies an algorithm to represent the same data in a shorter number of bits, e.g. by replacing a string of ten repeated digits with a command to repeat the digit ten times.
  • Quality of the file is maintained
  • The original file can be recreated from the compressed file as no data is lost
  • Creates a larger file size than lossy compression, so it takes up more space and transmission is slower
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