Data Communications Networking Devices - 4th Ed.pdf

4.3 INTELLIGENT MODEMS ________________________________________________________ 441Methods of ¯ow controlThere are three primary methods by which ¯ow control can be implemented,including the use of RTS/CTS control signals and the transmission of the characterpairs XON/XOFF and ENQ/ACK. In addition, some devices support the useof a mixture of ¯ow control methods.RTS/CTS signalingWhen a modem receives a RTS request to send) signal from a terminal device andis ready to receive data, it will respond by raising its CTS clear to send) signal. Onemethod to control the ¯ow of data into the modem is thus for the modem to turn offits CTS signal whenever it wants to stop the ¯ow of data into its buffers. Wheneverthe modem wants the ¯ow of data to resume it will then raise or turn on its CTSsignal. This method of ¯ow control is referred to as hardware ¯ow control as thecontrolling signals at the interface govern the operation of the hardware.XON/XOFFThe use of XON and XOFF characters for ¯ow control is referred to as in-bandsignaling as the characters ¯ow over pin 3 receive data) to the attached terminaldevice. In comparison, the use of the CTS control signal which is not a data signalis referred to as out-band signaling.Many asynchronous terminal devices recognize an ASCII code of 19, whichrepresents the CTRL-S character, as a signal to suspend transmission. This XOFFsignal is also known as a DC3 device control number 3) character and is issued bythe modem when it wants an attached DTE to suspend the ¯ow of data to themodem. Once the modem has emptied its buffer to a low level it will transmit anXON character to the DTE, which serves as a signal for the DTE to resumetransmission. The XON character has an ASCII code of 17 and represents theCTRL-Q character. This character is also known as the DC1 device controlnumber 1) character.One of the problems associated with the use of XON/XOFF ¯ow control isthe use of those characters in a data ®le by an application program which, whentransmitted, can cause a DTE to be in®nitely suspended. For example, supposeyou are transmitting a WordStar ®le to another computer system. That wordprocessing program uses DC3s generated by a CTRL-S as markers for the beginningand end of underscoring. If your modem is set to echo characters and you haveDC3s in a ®le, the ®rst DC3 modulated and transmitted to the distant system is alsoechoed back to the attached terminal device. The DTE detects the DC3 as anXOFF request and correctly responds by suspending transmission and waiting foran XON from the modem which will never come. Here the obvious solution to theproblem is to turn echo off for the ®le transfer. Unfortunately, this obvious solutionis not obvious to many modem users, especially when they are transmitting andreceiving binary ®les. For this reason it is highly recommended that you usehardware ¯ow control if you intend to transfer binary ®les.