Data Communications Networking Devices - 4th Ed.pdf

8 ________________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.4 Signal conversion performed by modems. A modem converts modulates) thedigital signal produced by a terminal into an analog tone for transmission over an analogfacilitytransmission lines when an analog transmission service is used. Analog transmissionfacilities include both leased lines and switched lines; therefore,modems canbe used for transmission of data over both types of analog line connections.Although an analog transmission medium used to provide a transmission pathbetween modems can be a direct connect,a leased,or a switched line,modems areconnected hard-wired) to direct connect and leased lines,whereas they areinterfaced to a switched facility; thus,a terminal user can communicate only withone distant location on a leased line,but with many devices when there is access to aswitched line.Acoustic couplersAlthough popular with data terminal users in the 1970s,today only a very smallpercentage of persons use acoustic couplers for communications. The acousticcoupler is a modem whose connection to the telephone line is obtained byacoustically coupling the telephone headset to the coupler. The primary advantageof the acoustic coupler was the fact that it required no hard-wired connection tothe switched telephone network,enabling terminals and personal computers to beportable with respect to their data transmission capability. Owing to the growth inmodular telephone jacks,modems that interface the switched telephone networkvia a plug,in effect,are portable devices. Since many hotels and older of®cebuildings still have hard-wired telephones,the acoustic coupler permits terminaland personal computer users to communicate regardless of the method used toconnect a telephone set to the telephone network.Signal conversionThe acoustic coupler converts the signals generated by a terminal device into aseries of audible tones,which are then passed to the mouthpiece or transmitter ofthe telephone and in turn onto the switched telephone network. Information transmittedfrom the device at the other end of the data link is converted into audible