Data Communications Networking Devices - 4th Ed.pdf

86 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.44 Control signal timing relationship. The state of the control signals varies bytime based upon the operational characteristics of devices connected as well as thestrapping option settings of those devicessettings of those devices. Figure 1.44 illustrates a common timing relationship ofcontrol signals between a computer port and a modem.At the top of Figure 1.44 it is assumed that the data set is powered on,resulting inthe data set ready DSR) control signal being high or in the ON state. Next,tworing indicator RI) signals are passed to the computer port,resulting in the computerresponding by raising its data terminal ready DTR) signal. The DTR signalin conjunction with the second ring indicator RI) signal results in the modemanswering the call,presenting the carrier detect CD) signal to the computer port.Assuming the computer is programmed to transmit a sign-on message,it will raiseits request to send RTS) signal. The modem will respond by raising its clear tosend CTS) signal if it is ready to transmit,which enables the computer port tobegin the actual transmission of data.Synchronous operationsOne major difference between asynchronous and synchronous modems is thetiming signals required for synchronous transmission.Timing signalsWhen a synchronous modem is used,it puts out a square wave on pin 15 at afrequency equal to the modem's bit rate. This timing signal serves as a clock fromwhich the terminal would synchronize its transmission of data onto pin 2 to themodem. Thus,pin 15 is referred to as transmit clock as well as its formal designatorof transmission signal element timing DCE),with DCE referencing the fact thatthe communications device supplies the timing.Whenever a synchronous modem receives a signal from the telephone line it putsout a square wave on pin 17 to the terminal at a frequency equal to the modem's bitrate,while the actual data is passed to the terminal on pin 3. Since pin 17 providesreceiver clocking,it is known as `receive clock' as well as its more formal designatorof receiver signal element timing.