Question : Problem: asynchronous or synchronous

I have been confused by what I have read about modems. They are often described as sending data asynchronously in 8-bit packets. I have also seen referances in information about Zmodem and v.42 that they support synchronous transmission in much longer packets. What are the facts?

Answer : Problem: asynchronous or synchronous

In asynchronous transmission, data is transmitted one character at a time. Each character is bracketed by a start bit and one or more stop bits. Asynchronous transmission is also called start-stop transmission. The asynchronous characters are not evenly spaced along the transmission medium. In gaps between characters, the line is idle; nothing is transmitted. The characters are transmitted independently with regard to timing signals.
In synchronous transmission an extra signal, the clock signal, is added to the RS-232 leads. Usually, the two synchronous modems supply the clock signal, although the DTE (PC) can also supply the clock signal. Configuring a synchronous port for external clock means the modem supplies the clock signal. In synchronous transmission, characters are grouped together in large blocks of data, sometimes 2,000 to 4,000 characters to a block. There are no spaces between characters in the block, whereas there are always spaces between asynchronous characters, and therefore, some idle line time. A control character, called a SYN character, always precedes the data block. Usually two or three SYN characters precede the data. The block starts with a start-of-text (STX) character. An end-of-text (ETX) character terminates the block.
During a transmission stream, blocks are evenly spaced along the medium with respect to time. If no data is being transmitted, SYN characters are transmitted periodically to keep the two modems synched up. Generally, modems can achieve a higher bit rate using synchronous transmission, but such transmission requires more complex and therefore more expensive hardware because of the need for a clocking signal. Also, if you send a large block of data synchronously and an error occurs, there is a greater speed penalty for re-transmitting the large packet, as opposed to just re-transmitting a single corrected packet when using asynchronous transmission. However, in asynchronous transmission, the added need for a start bit and one or more stop bits per character may add 10 to 20 percent to the size of the transmission. Nonetheless, asynchronous modems currently enjoy wider use.