Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 421Ping-pong modemsAnother type of modem operation that simulates full-duplex transmission is the`ping-pong' or fast-turnaround modem. With this transmission method a modemsends data in one direction and then signals the remote modem when its transmissionis completed. This signal informs the remote modem that it can nowtransmit data, because the originating modem has placed itself in the receive mode.The faster the two modems are able to turn off their transmitter and turn on theirreceiver, the closer it appears that full-duplex transmission is occurring. Modemsthat employ a ping-pong transmission scheme include RAM buffers to hold data asthe direction of transmission changes. This permits terminal devices connected toping-pong operating modems to appear to continuously transmit data to themodem for modulation, although the modem is actually operating as a half-duplexdevice.Modem handshakingModem handshaking is the exchange of control signals necessary to establish aconnection between two data sets. These signals are required to set up andterminate calls, and the type of signaling used is predetermined according to one ofthree major standards, such as the Electronics Industries Association EIA) RS-232 or RS-449 standard or the ITU V.24 recommendation. RS-232 and ITU V.24standards are practically identical and are used by over 95% of all modems currentlymanufactured. To better understand modem handshaking, let us examine thecontrol signals used by 103-type modems. The handshaking signals of 103-typemodems and their functions are listed in Table 4.20, while the handshakingsequence is illustrated in Figure 4.40.The handshaking routine commences when an operator at a remote terminaldials the telephone number of the computer or uses an intelligent modem to dial aprede®ned telephone number. At the computer site, a ring indicator RI) signal atthe answering modem is set and passed to the computer. The computer then sendsa data terminal ready DTR) signal to its modem, which then transmits a toneTable 4.20 Modem handshaking signals and their functionsControl signalFunctionTransmit dataReceive dataRequest to sendClear to sendData set readyCarrier detectData terminal readyRing indicatorSerial data sent from device to modemSerial data received by deviceSet by device when user program wishes to transmitSet by modem when transmission may commenceSet by modem when it is powered on and ready to transferdata; set in response to data terminal readySet by modem when signal presentSet by device to enable modem to answer an incoming callon a switched line; reset by adaptor disconnecting callSet by modem when telephone rings