Data Communications Networking Devices - 4th Ed.pdf

1.5 TRANSMISSION TECHNIQUES ___________________________________________________ 271.5 TRANSMISSION TECHNIQUESData can be transmitted either synchronously or asynchronously. Asynchronoustransmission is commonly referred to as a start±stop transmission where onecharacter at a time is transmitted or received. Start and stop bits are used to separatecharacters and synchronize the receiver with the transmitter,thus providing amethod of reducing the possibility that data becomes garbled.Most devices designed for human±machine interaction that are teletype compatibletransmit data asynchronously. By teletype compatible,we refer to terminalsand personal computers that operate similarly to the Teletype 1 terminal manufacturedby Western Electric,originally a subsidiary of AT&T and now known asLucent Technology after this equipment manufacturing arm of AT&T was spunoff during 1996. Various versions of this popular terminal have been manufacturedfor over 30 years and an installed base of approximately one million such terminalsat one time during the late 1970s were in operation worldwide. As characters aredepressed on the device's keyboard they are transmitted,with idle time occurringbetween the transmission of characters. This is illustrated in Figure 1.20b).Although many teletype terminals have been replaced by personal computers,asynchronous transmission pioneered by those terminals remains very popular inuse.Figure 1.20 Asynchronous start-stop) transmission. a) Transmission of one 8-bitcharacter. b) Transmission of many characters. STB = start bit; CB = character bits;SPB = stop bits); idle time is time between character transmissionAsynchronous transmissionIn asynchronous transmission,each character to be transmitted is encoded into aseries of pulses. The transmission of the character is started by a start pulse equal