Data Communications Networking Devices - 4th Ed.pdf

1.4 TRANSMISSION MODE __________________________________________________________ 25Until the 1980s most ITU standards were primarily followed in Europe. Sincethen,ITU modem standards have achieved a worldwide audience of followers,andenable true global communications compatibility. Although most modern modemsare compatible with ITU standards,a large base of modems are still being usedthat were designed to Bell System standards that were popular in North Americathrough the 1980s. In Chapter 5 we will examine some of the more common ITUmodem standards as well as a few Bell System standards that will provide us withan understanding of some of the compatibility problems that can occur whencommunications are attempted between Bell System and ITU compatiblemodems.Terminal and mainframe computer operationsWhen referring solely to terminal operations,the terms half-duplex and fullduplexoperation take on meanings different from the communications mode of thetransmission medium. Vendors commonly use the term half-duplex to denote thatthe terminal device is in a local copy mode of operation. This means that each timea character is pressed on the keyboard it is printed or displayed on the localterminal as well as transmitted. Thus,a terminal device operated in a half-duplexmode would have each character printed or displayed on its monitor as it istransmitted.When one says a terminal is in a full-duplex mode of operation this means thateach character pressed on the keyboard is transmitted but not immediatelydisplayed or printed. Here the device on the distant end of the transmission pathmust `echo' the character back to the originator,which,upon receipt,displays orprints the character. Thus,a terminal in a full-duplex mode of operation wouldonly print or display the characters pressed on the keyboard after the character isechoed back by the device at the other end of the line. Figure 1.19 illustrates theterms full- and half-duplex as they apply to terminal devices. Note that althoughmost conventional terminals have a switch to control the duplex setting of thedevice,personal computer users normally obtain their duplex setting via theFigure 1.19 Terminal operation modes. Top: the term half-duplex terminal mode impliesthat data transmitted is also printed on the local terminal. This is known as local copy.Bottom: the term full-duplex terminal mode implies that no local copy is provided