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.
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