in asynchronous mode, data are transmitted/received per character of 7 or 8 bits at a time (mostly 8 these days). each char has a startbit, stopbit and sometimes a paritybit. between the transmission of each char, the line may be idle for an undetermined time.

in synchronous mode, data are transmitted/received per frame. each frame consists of a header, body and mostly a checksum. The header contains information about the body (type of data e.g. TCP/IP, ...), source- and destination address, and other information, relevant to the protocol. The body consists of multiple chars. The max number of chars is defined in the protocoldescription itself. This is often a value between 256 and 1500 bytes. The transmission of a frame always starts with a begin of frame indicator (this is to say that a new frame is coming or ending). Once the transmission started at a given baudrate, it must not be interrupted or delayed, or the frame will be completely lost. Examples of synchronous protocols are HDLC, Manchester, Ethernet, ... synchronous modes are always used if transmission and reception are done in half-duplex environments like Ethernet.

kind regards,

joachim