Most common and inexpensive mechanism error detection which also
called parity check.
A redundant bit (parity bit) is appended to every data unit so that
the total number of 1s in the unit becomes even, if there is even-parity
check used.
There are even-parity check and odd-parity check. For odd-parity check,
the total number of 1s in the unit is odd.
Suppose we want to transmit the binary data unit 1100001, adding the number
of 1s gives us 3, an odd number. Before transmitting, a parity generator
counts the 1s and appends the parity bit (a 1 in this case) to the end. The
total number of 1 becomes 4 now (even number). The system now transmits the
entire appended unit across the network link.
When the data unit is reached its destination, the receiver puts all
eight bits through an even-parity checking function. If the receiver sees
11100001, it counts and gets four 1s, an even number.
But if the receiver sees 11100101, or total number of 1s is odd. The
receiver knows that an error has been occurred into the data somewhere and
therefore rejects the whole unit.
For the odd-parity checking, the principle is same but the calculation
is different.
The advantages of VRC are it can detect all single-bit errors. It also
can detect burst errors as long as the total number of bits changed is
odd (1,3,5, etc). The same holds true for any odd number of errors.
The limitation is it cannot detect errors where the total number of
bits changed is even, where the two bits of the data unit are changed. In
this case, the total number of 1s is still even. The VRC checker will add
them and return an even number although the data unit contains two errors.
Then the unit will pass a parity check even through the data unit is damaged.
The same holds true for any even number of errors.
