Data Communications Networking Devices - 4th Ed.pdf

792 ___________________________________________________________ SPECIALIZED DEVICESencoder/decoder between the terminal and the terminal's associated modem orDSU. Clear text originating at the terminal device is then scrambled by the encoderand passed to the modem which transmits the data over the common carrier'sfacility. At the receiving end, scrambled data passes through the modem to adecoder which now produces clear text that is passed to the terminal at this end.In Figure 7.39a), a typical mainframe access application for an encryption deviceis shown. In this example, the terminal device which required the security measuresassociated with encryption devices is connected via a leased line to a time divisionmultiplexer TDM). Since this is a dedicated port connection, there is no problemFigure7.39 Data security for dial access operations. a) Mainframe access encryptionapplication. Terminals connected to a Mainframe system through multiplexers must have adedicated port to effect secure communications. b) Dial access considerations. Totransmit encoded information via the switched network through a multiplexer,a selectormust be employed at the computer site to switch the scrambled output of a TDM channelinto a decoder to produce clear text