Flashcards in Data storage Deck (11)
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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)
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
3
Q
List 4 methods of error detection and correction
A
- Parity checks
- Check digits
- Checksums
- Automatic Repeat reQuests (ARQ)
4
Q
Describe how a parity check works
A
- 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
- Odd parity: odd number of 1’s, e.g. 11001110
- Even parity: even number of 1’s, e.g. 11011110
- 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.
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
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
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.
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
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
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
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