Communication is called asynchronous if the transmitter and receiver do not need to synchronise before each transmission. A sender can wait arbitrarily long between transmission and receiver must be ready to receive data when it arrives most PC serial devices such as mouse, keyboards and modems are asynchronous.
As the name implies, asynchronous communication is performed between two or more devices which operate on independent clocks. Thus there is no guarantee that when Point A begins transmitting, point B will begin receiving, or that point B will continue to sample at the rate point A transmits. In asynchronous transmission, data are transmitted one character at a time, where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.
Asynchronously transmitted data is preceded with a start bit which indicates to the receiver that a character is about to begin. The end of a character is followed by a stop bit, which tells the receiver that the character has come to an end, that it should begin looking for the next start bit, and that any bits it receives before getting the start bit should be ignored. To avoid confusion with other bits, the start bit is twice the size of any other bit in the transmission. To ensure data integrity, A party bit is often added between the last bit of data and the stop bit.
The party bit is set by the transmitter such that the total number of ones in the character, including the party bit, is even or odd, depending on the convention being used. The receiver uses this bit for error detection. The parity bit makes sure that the data to be received is composed of the same number of bits in the same order in which they were sent