Over long distances, clocking info must be embedded in the data signal. The receiver must determine the beginning and end of a block of data. Each block begins with a preamble bit pattern and ends with a postamble bit pattern. Data plus preamble, postamble, and control info are called a frame. For large blocks of data, synchronous transmission is far more efficient than asynchronous.
Asynchronous transmission is used for applications with low-speed terminals or pcs.