Data Communications Networking Devices - 4th Ed.pdf

7.4 SECURITY DEVICES ___________________________________________________________ 793in determining what port of the demultiplexer to which a similar encoder/decodershould be interfaced.In Figure 7.39b), the terminal user now dials a rotary over the switchedtelephone network in order to connect to the multiplexer. For this con®guration,the terminal device may be connected to any port on the multiplexer, which meansthat at the other end a method is necessary to determine which port of thedemultiplexer has the scrambled data. The encoder/decoder selector performs thisfunction by sampling the output of each of the demultiplexer's ports and switchingthe port that has the scrambled data to the encoder/decoder, which then decodes thescrambled text and passes it to the computer. This selector in effect performslimited switching and acts as a transparent device, passing the data from the otherports directly to the computer.Before selecting an encryption device you should examine in detail the applicationrequirements and develop a set of speci®cations which will aid in theselection process. Table 7.9 contains a checklist of some common encryption devicespeci®cations which should be considered. Although some devices are code transparent,many are not. Since some devices are manufactured for speci®c applicationsand their character sets to include the crypto and plain text character sets should beexamined.Table7.9 Encryption device speci®cationCharacter set supportedMessage keyTransmission rate supported Key code changeOperational modePower supplyCrypto character setTerminal interfacePlain text character setAlarm circuitryCarrier compatibilitySimilarly, the transmission speed supported should also be examined since somedevices only support teletype operating speeds while other devices can operate atspeeds up to 1 million bps.The operational mode of encryption devices falls into three categories: off-line,on-line asynchronous and on-line synchronous, with the mode supported dependentupon your requirements and the capability of the device.The crypto character set and plain text character set can vary from the Telexcharacter set through the extended binary-coded decimal interchange codeEBCDIC) character set of 256 characters. The character set selected will dependupon the terminal character set, the computer's character set, and/or the characterset supported by the communications facility used. Carrier compatibility is animportant feature when the user wishes to transmit encrypted messages over acommercial network. Since certain control characters perform unique functions,carrier compatibility alleviates problems by the suppression of the generation ofcontrol characters during the encoding process as well as by sending the plain textcontrol characters in the clear.