Data storage

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)

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

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

• 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.

• 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

• 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

• 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.

• 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

• 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

• 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

• 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