Transmisson errors

Data that has been entered correctly in a system can become corrupted when it is

transmitted within a computer or when sent from one computer to another.

This may be due to a poor connection or 'noise' on the line. All data is held within a computer and transmitted in binary coded form - everything is represented by a string of ones and zeros.

Characters are most commonly stored in ASCII format and there are coding systems for storing other types of data such as graphics, numbers and sound.

Data corruption will result in one or more zeros being written as ones, or ones being written as zeros. It is likely that the resulting string of bits would be interpreted as another character, graphic etc.

For example, if just 3 bits were corrupted when transmitting the characters ICT in ASCII the result could become HGV

1001001 (I) 1000011 (C) 1010100 (T)

1001000 (H) 1000111 (G) 1010110 (V)

Parity Check

Transmission errors can be avoided by the recipient computer sending the same message back to the original sender. If the two messages are the same, the data will be correct. Electronic 'noise' can cause the state of bits to be altered - resulting in a 0 being changed to a 1 or a 1 to a 0.

A parity check is an extra bit that is added on to a group of bits. The parity bit is solely used to check that the other bits have not been corrupted.

There are two types of parity:

	even
	odd

When even parity is used, the parity bit is chosen to be 0 or 1 so that the total number of transmitted bits set to 1 (including the parity bit) is even.