es to the receiving terminal that a character is being transmitted. The receiving device detects the start bit and the data bits that make up the char- acter. At the end of the character transmission, the line is returned to a mark condition by one or more stop bits, and is ready for the next character. (An asynchronous character varies in length depend- ing on the information code employed.) This process is repeated Optional Start bit parity bit State Stop bits Data bits Space Frame Frame Data bits SYNC character Data bits Space Frame Frame Frame Fig. 1-5. Asynchronous and synchronous transmission. Choice of proper timing mode depends directly on application. Asynchronous transmission (a) is used mostly with man-machine in- terfaces; synchronous transmission (b) offers high speed necessary for machine-machine com- munication. THE MEDIUM AND THE MARKET 9 character by character until the entire message has been sent. The start and stop bits permit the receiving terminal to synchronize itself to the transmitter on a character by character basis. Synchronous transmission (Fig. 1-5b) uses an internal clocking source within the modem to synchronize the transmitter and receiver. Once a synchronization character (SYN) has been sensed by the re- ceiving terminal, data transmission proceeds character by character without the intervening start and stop bits. The incoming stream of data bits is interpreted on the basis of the receive clock signal sup- plied by the modem. The receiving device accepts data from the mo- dem until it detects a special ending character or a character terminal count at which time it knows that the message is over. The message block usually consists of one or two synchronization characters, a number of data and control characters (typically 100 to 10,000), a terminating character, and one or two error control characters. Be- tween messages, the communication line may idle in SYN characters or be held to mark. Note that synchronous modems can be used to transmit asynchronous data, and, conversely, 