Asynchronous and Synchronous Transmission

Asynchronous transmission :

Timing problems are avoided by simply not sending long streams of bits. Data is transmitted one character (byte) at a time. Synchronisation only needs to be maintained within each character, because the receiver can resynchronise at the beginning of each new character. When no characters are being transmitted, the line is idle (usually represented by a constant negative voltage). The beginning of a character is signalled by a start bit (usually a positive voltage), allowing the receiver to synchronised its clock with that of the transmitter. The rest of the bits that make up the character follow the start bit, and the last element transmitted is a stop bit that is typically 1.5 or 2 times as long as the other bits transmitted. The transmitter then transmits the idle signal (which is usually the same voltage as the stop bit) until it is ready to send the next character (see below).

Asynchronous transmission is also known as start-stop mode or character mode. Each character is framed as an independent unit of data that may be transmitted and received independently. Data may also be transmitted as a continuous stream of characters. Most communications systems require a specific number of bits to represent each character, plus a parity bit that is often included to provide simple error detection. Asynchronous data characters normally contain 8 data bits (including the parity bit) plus a start bit and at least 1 stop bit, giving a total of 10 bits. Data can be transmitted in blocks of characters known as transmission blocks. The transmission block may use special control characters to provide control functions and to identify the start and end of a block. Asynchronous transmission is only really suitable for relatively low data rates (up to 3 Kbits). Many of the bits transmitted in each block are control bits, giving a high proportion of overhead. It is used mainly for applications where character data is generated at irregular intervals (e.g. user input from a keyboard).

One example of asynchronous transfer is Asynchronous Transfer Mode (ATM) switching. ATM allows voice, data and video to be transmitted in fixed length cells of 53 bytes.

Synchronous Transmission :

With synchronous transmission, the receiver's clock is synchronised with the transmitter's clock. Data is transmitted in a continuous stream, and the arrival time of each can be predicted by the receiver. This is achieved either by using a separate timing circuit, or by embedding the timing information in the signal itself. The latter can be achieved using bi-phase encoding (e.g. Manchester encoding). An embedded timing signal can be used by the receiver to synchronise with the transmitter using a Digital Phase-Locked Loop (DPLL).

A data frame usually starts with one or more bytes of data that have a unique bit pattern, or flag (sometimes called a preamble), that tells the receiver a block of data will follow. The preamble is followed by various control fields, a variable-length data field, more control fields, and finally a postamble. The control information within the frame will include a length field, which specifies the amount of data to be read.