Data storage

Question 1

What is data validation?

List 8 different validation checks

Answer 1

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)

Question 2

What is data verification?

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

Answer 2

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

Question 3

List 4 methods of error detection and correction

Answer 3

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

Question 4

Describe how a parity check works

Answer 4

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.

Question 5

Describe how a check digit works

Answer 5

• 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

Question 6

Describe how a checksum works

Answer 6

• 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

Question 7

Describe how an Automatic Repeat reQuest (ARQ) works

Answer 7

• 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.

Question 8

What are the following file formats?

• MIDI
• jpeg
• MP3
• MP4

Answer 8

• 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

Question 9

What is data compression

Answer 9

• 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

Question 10

What is lossy compression?

Answer 10

• 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

Question 11

What is lossless compression?

Answer 11

• 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