When signals are transmitted through modems from one computer to another, patterns of bits coded to represent data are sent one bit at a time. How does the receiving device know where one character ends and another starts? In asynchronous transmission, also called start-stop transmission, each string of bits that make up a character is bracketed by control bits (start and stop bits). In effect, each group of digital or analog signals making up the code for one character is individually “wrapped” in an electronic “envelope” made of a start bit, an error check bit one or two stop bits. Because asynchronous communication is inexpensive, it is widely used with microcomputers; however, it also relatively slow, because of the number of error check bits that must be transmitted with the data bits.
In synchronous transmission, characters can be sent much faster because they are sent as blocks, or “packets.” Header and trailer bytes are inserted as identifiers at the beginnings and the ends of blocks. In addition error check bits are transmitted before the trailer bytes. Synchronous transmission is used by large computers to transmitted before the trailer bytes. Synchronous transmission is used by large computers to transmit huge volumes of data at high speeds. Expensive and complex timing devices must be used to keep the transmission activities synchronized. Synchronous transmission is rarely used in microcomputer-based communications lines.