Parity
checking:
A method of
error detection in data transmission from one device to another
e.g if a
byte of data is transmitted, it would have a parity bit allocated to it before
it was transmitted
Systems
which use odd parity have an odd number of 1 bits
(adds a 1 to
make amount of 1’s odd or adds 0 to keep odd)
Systems
which use even parity have an even number of 1 bits
(adds a 1 to
make amount of 1’s even or 0 to keep even)
Before data
is transmitted, the sender and receiver both agree on what type of parity to
use.
e.g: Even
parity is used
Which of
these bytes have been transmitted correctly?
11010000
01100110
11110101
10111001
00111111
However, the
receiver doesn’t know which of the bits has been transmitted incorrectly! (odd parity is getting and odd numbers of 1's and even parity is getting an even number of 1's)
One way
around this problem is sending a parity
byte as well
Using this
method blocks of data can be sent.
:byte
2 is incorrect because even parity is used but the 1’s add up to and
odd number
The extra parity byte checks every bit to to apply with the (even in this case) parity
What if an
even number of bits have been incorrectly transmitted? Then there is no way of
the receiver knowing
The limitation of parity check is that both the
original and altered bytes pass the odd parity check
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