Data Communications Networking Devices - 4th Ed.pdf

24 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.18 Transmission modes. Top: simplex transmission is in one direction only;transmission cannot reverse direction. Center: half-duplex transmission permits transmissionin both directions but only one way at a time. Bottom: full-duplex transmission permitstransmission in both directions simultaneouslyprovides more ef®cient throughput,this ef®ciency was originally negated by thecost of two-way lines and more complex equipment required by this mode oftransmission. The development of low cost digital signal processor chips enabledhigh speed modems to operate in a full-duplex transmission mode on a two-wirecircuit through a technique referred to as echo cancellation. This technique will bedescribed when we discuss the operation of modems in detail in Chapter 5.In Figure 1.18,the three types of transmission modes are illustrated,whileTable 1.3 summarizes the three transmission modes previously discussed.In Table 1.3,the column headed ITU refers to the International TelecommunicationsUnion,a United Nations agency headquartered in Geneva,Switzerland.Previously,the standardization body of the ITU was known as the ConsultativeCommittee on International Telephone and Telegraph CCITT) and,eventhough the renaming occurred several years ago,many people still use the termCCITT when referencing certain standards.Table 1.3Transmission mode comparisonUSHistoricaltelecommunicationsphysical lineSymbol ANSI industry ITU requirementOne-way only Simplex Two-wireTwo-way Half-duplex HDX) Simplex Two-wire! alternateTwo-way' simultaneous Full-duplex FDX) Duplex Four-wire