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Asynchronous Transmission
Asynchronous transmission does not use a clocking mechanism to keep the sending and receiving devices synchronized. Instead, this type of transmission uses bit synchronization to synchronize the devices for each frame that is transmitted.
In bit synchronization, each frame begins with a start bit that enables the receiving device to adjust to the timing of the transmitted signal. Messages are kept short so that the sending and receiving devices do not drift out of synchronization for the duration of the message. Asynchronous transmission is most frequently used to transmit character data and is ideally suited to environments in which characters are transmitted at irregular intervals, such as when users enter character data.
Figure 6.1 illustrates the structure of a typical frame used to transmit character data. This frame has four components:
- A Start bit. This component signals that a frame is starting and enables the receiving device to synchronize itself with the message.
- Data bits. This component consists of a group of seven or eight bits when character data is being transmitted.
- A parity bit. This component is optionally used as a crude method of detecting transmission errors.
- A stop bit or bits. This component signals the end of the data frame.
Asynchronous transmission is a simple, inexpensive technology ideally suited for transmitting small frames at irregular intervals. Because start, stop, and parity bits must be added to each character being transmitted, however, overhead for asynchronous transmission is high—often in the neighborhood of nearly 20 to 30 percent. This high overhead wastes bandwidth and makes asynchronous transmission undesirable for transmitting large amounts of data.
Asynchronous transmission is frequently used for PC-to-PC and terminal-to-host communication. Data in these environments is often of the bursty, character-oriented nature that is ideal for asynchronous communication. Asynchronous transmission generally requires less expensive hardware than synchronous transmission.
Further Information