There are variations to the asynchronous format shown, including 8-bit character, no parity, and two rather than one stop bits. The parity bit is used by the receiver to detect a limited number of errors that can occur when the character is sent over the communication path. Odd parity is shown in the example, which means that an odd number of 1Р’В s are in each transmitted character (ensured by proper setting of the parity bit). In addition to the 7-bit character itself (1101000), a start bit, stop bit, and parity bit are included. Shown is the ASCII character РІР‚СљhРІР‚Сњ being transmitted over a communication line. The general problem being solved is the synchronization of a character of information between the sender and receiver. Generally, these basic synchronization problems are solved by transmitting data in one of two standard ways: asynchronously or synchronously.Р”Р„synchronous data transmission is described logically in Fig. Not knowing this, it is difficult to decompose the block back into the character string. A string of characters forming a data block, for instance, would need to be synchronized so that the receiver knows when the block begins. At a higher level, bits are often grouped together to form characters or bytes (1 byteР’В =Р’В 8 bits) or, higher yet, blocks or frames of data.

Ideally, one would want to sample near the middle of the bit time to stay away from transition levels at the beginning or end. Knowing only the basic bit time, a receiver still does not know when to sample incoming data from the network, i.e., it does not know where an arbitrary bit time starts or ends. The most fundamental requirement for synchronization is for both sender and receiver to agree on the transmission rate, i.e., the bits per second (bps). To transfer information from a sender to a receiver over a network requires that the two entities be synchronized throughout the data transmission time.

Kreager, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 III.A Data Synchronization

Transmit data