parity bit question

i have been readin about the "parity bit" method, and how is is used to check is the "packet" is received correctly.

so using odd parity: (from wiki)

A wants to transmit:          1001
A computes parity bit value:  ~(1^0^0^1) = 1
A adds parity bit and sends:  10011
B receives:                 开发者_JS百科  10011
B computes overall parity:    1^0^0^1^1 = 1
B reports correct transmission after observing expected odd result.

what if during the transmission, instead of "10011", "11001" is received. how will the parity check for that, since it checks only the number of "1"'s ? or is it impossible for bits to change during transmission like i stated before? thx

Parity bit is simplest error detection technique. It works if odd number of bits (including the parity bit) are transmitted incorrectly. So if two bits are corrupt then it will not work.