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errors and compression (parity checks (problems with parity (You cannot…
errors and compression
parity checks
- one method used to check whether data has been changed or corrupted following transmission from one device or medium to another device or medium.
- A byte of data, for example, is allocated a PARITY BIT. This is allocated before transmission takes place. Systems that use EVEN PARITY have an even number of 1-bits; systems that use ODD PARITY have an odd number of 1-bits.
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- Parity is agreed by the sender and receiver BEFORE transmission starts as part of the PROTOCOL
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check digits
- a digit is calculated from the other digits and added to the data
- when the data is received the digit is recalculated and they are compared
checksums
- Data is sent in blocks and an additional value, the checksum, is also sent at the end of the block of data.
- If the sum of all the bytes in the transmitted block of data is <= 255 (28 − 1), then the checksum is this value.
However, if the sum of all the bytes in the data block > 255, then the checksum is found using the simple algorithm:
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how it works
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At the receiving end, the checksum is recalculated from the block of data received.
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data compression
Lossy
LOSSY FILE COMPRESSION is very different to lossless file compression. With this technique, the file compression algorithm eliminates unnecessary bits of data as seen in MP3 and jpeg formats.
It is impossible to get the original file back once it is compressed. This is why it is chosen for files where removing certain bits doesn’t detract from the quality.
lossless
With LOSSLESS FILE COMPRESSION, all the data bits from the original file are reconstructed when the file is again uncompressed. This is particularly important for files where loss of any data would be disastrous – for example, a spreadsheet file.
