Data Communications Networking Devices - 4th Ed.pdf

124 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.70Basic teletype protocolteletype protocol. In this protocol the terminal operator might ®rst transmit theENQ character,which is formed by pressing the shift and E keys simultaneously.If the call originated over the PSTN,the ENQ character,in effect,tells thecomputer to respond with its status. Since the computer is beginning its servicingof a new connection request,it normally responds with a log-on message. This logonmessage can contain one or more lines of data.The ®rst line of the log-on message shown in Figure 1.70 is pre®xed with acarriage return CR) line feed LF) sequence,which positions the printhead to the®rst column on a new line prior to printing the data in the received log-on messageline. The log-on message line,as well as all following lines transmitted by thecomputer,will have a CR LF suf®x,in effect,preparing the terminal for the nextline of data. Upon receipt of the log-on message the terminal operator keys in hisor her log-on code,which is transmitted to the computer as data followed by theCR LF suf®x which terminates the line entry.