A transmission system is classified as asynchronous if the system enables the physical channel to be useless for an arbitrary time between two transmissions. The asynchronous design of communication is well-appropriated to applications that create data at random (e.g., a user classifying on a keyboard or a user that press on a link to include a web page, reads for a while, and then press on a link to include another page).
The drawback of asynchrony improves from the lack of coordination between sender and receiver while the channel is empty. A receiver cannot know how long the channel will remain idle before more data appears.
Thus, asynchronous technologies generally arrange for a sender to transmit a few additional bits before each data element to inform the receiver that a data provider is starting.
The additional bits enable the receiver's hardware to synchronize with the incoming signal. In some asynchronous systems, the extra bits are known as a preamble; in others, the extra bits are known as start bits.
As an example of asynchronous communication, suppose the transfer of characters across copper wires between a computer and a device, including a keyboard. An asynchronous communication technology standardized by the Electronic Industries Alliance (EIA) has become the most widely accepted for character communication known as RS-232-C and commonly abbreviated RS-232+.
The EIA standard determines the element of the physical connection (e.g., the connection must be smaller than 50 feet long), electrical elements (e.g., the voltage ranges from -15 volts to +15 volts), and the line coding (e.g., negative voltage corresponds to logical 1 and positive voltage corresponds to logical 0).
Because it is designed for use with devices such as keyboards, the RS-232 standard specifies that each data item represents one character. The hardware can be configured to control the exact number of bits per second and to send seven-bit or eight-bit characters.
RS-232 specifies that a sender transmits an extra 0 bit (known as start bit) before sending the bits of a character. RS-232 determines that a sender should leave the line empty between characters for at least the time needed to send one bit. Therefore, one can think of a phantom 1 bit added to each character. In RS-232 terminology, the phantom bit is known as the stop bit.
The figure given below shows how voltage varies when a start bit, eight bits of a character, and a stop bit are sent.