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Data is sent from one computer to another through synchronous or asynchronous transmission. Characters are made up of bits and are transmitted with a START, PARITY, and STOP bit. Asynchronous transmission is preferred for less frequent data and is simpler and cheaper than synchronous transmission.
When data needs to be sent from one computer to another, it is broken down into individual characters and sent in sequence. Such transmission is called synchronous transmission when the receiving computer uses a clock synchronized with the sending computer’s clock. The clock sets a pace. When the clocks of the receiving and sending computers are not synchronized, this form of transmission is referred to as asynchronous transmission.
A message to be transmitted from one computer to another consists of text and non-text characters. Characters, in turn, are made up of bits. These bits are sent down the electrical wires and are indicated by signal levels. For the sender and receiver to successfully transmit bits, the two must agree on a rhythm so that the receiver can determine the correct end of one bit and the start of another bit.
In asynchronous transmission the rate or clock frequency is determined before any communication begins. Therefore, a computer or computing device can claim that it will run at 9600 bits per second. Another device can only communicate successfully with this device if that speed is matched.
When a character needs to be transmitted from the sender to the receiver, the receiver needs to be notified 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. To ensure that one character is not confused with the next character, STOP bits are used to indicate the end of each character. Also, to ensure that signals are not affected by stray electrical noise, error checking is provided via a PARITY bit.
A character is often made up of seven or eight bits. So when you transmit a character, the START bit is sent, and then the bits of the character are sent one after another. Then the PARITY bit is sent and finally the STOP bit is sent. A seven or eight bit character requires three bits of overhead to be sent correctly from the sending device to the receiving device. This overhead in asynchronous transmission can limit the amount of useful information that could be sent.
There may be variations in speed, parity methods, number of useful bits, and number of STOP bits between devices. After a character has been received, the next character can enter after any length of time. For example, characters typed on a keyboard are generated and sent after relatively long intervals compared to the speed of the computer. A web page submitted for viewing may have many characters sent rapidly in sequence.
Asynchronous transmission is generally preferred when data is sent less frequently. It can be used for landline telephone communication and for simple printers. Asynchronous transmission is considered to be simpler and cheaper than synchronous transmission, the latter, however, can provide much higher data rates.
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