When data is to be sent from one computer to another, it is broken up into individual characters and sent in sequence. Such transmission is called an synchronous transmission when the receiving computer uses a clock that is synchronized with the clock of the sending computer. The clock establishes a rhythm. When the clocks of the receiving and sending computers are not synchronized, the form of transmission is referred to as asynchronous transmission.
A message to be transmitted from one computer to another is composed of textual and non-textual characters. Characters, in turn, are composed of bits. These bits are sent across electrical wires, and are indicated by signal levels. For the sender and the receiver to successfully transmit bits, a rhythm must be agreed upon by the two so that the receiver can determine the correct end of one bit and the start of another bit.
In asynchronous transmission the rhythm or the clock rate is determined before the start of any communication. Thus, one computer or computing device may declare that it will operate at 9600 bits per second. Another device can properly communicate with this device only if that speed is matched.
When one character has to be transmitted from the sender to the receiver, the receiver must be alerted to receive the character. This is done by using a “wake up” signal at the start of the character. This signal is called the START bit. In order to ensure that one character does not get confused with the next character, STOP bits are used to indicate the end of each character. In addition, to make sure that the signals did not get mangled by stray electrical noise, there is a provision for error checking via a PARITY bit.
A character is often composed of seven or eight bits. So, when transmitting one character, the START bit is sent, and then the bits of the character are sent one after another. After which the PARITY bit is sent and finally the STOP bit is sent. A seven or eight bit character requires three bits as overhead to be properly sent from the sending device to the receiving device. This overhead in asynchronous transmission can limit the amount of useful information that could be sent.
Among devices, there can be variations in speed, parity methods, number of useful bits as well as the number of STOP bits. After one character is received, the next character may come in after any amount of time. For example, characters being typed at a keyboard are generated and sent after relatively long intervals of time compared to the speed of the computer. A web page being sent for display could have many characters sent quickly in sequence.
Asynchronous transmission is usually preferred when the data is sent less frequently. It may be used when communicating through telephone landlines and for simple printers. Asynchronous transmission is considered to be simpler and cheaper compared to synchronous transmission, the latter, however, can provide much faster data speeds.