 the line represents a 4 bit digital sequence. The CCITT V.29 standard uses this modulation technique over 4 wire lines at a baud rate of 2400. Since each baud represents 4bit/s, the resulting speed of operation is rated at 4x2400 or 9600bits/s. Having outlined how a digital signal can be encoded into a signal suitable for transmission over the telephone line, what happens when the modem has used its dial up telephony capabilities to establish a transmission path? (Figure 39.5; Scott, 1980). The digital Figure 39.4 Phase modulation: (a) carrier; (b) modulating equipment originating the call uses the interface signals presented waveform; (c) modulated waveform to it by the modem to raise the Request To Send (RTS) signal. If the Modem standards 39/5 MODEM MODEM COMPUTER INTERFACE TERMINAL INTERFACE RINGING DTR ON DTR ON DSR ON RTS ON CTS ON TRANSMIT DCD ON RTS OFF CTS OFF RECEIVE DCD OFF RTS ON CTS ON TRANSMIT DCD ON RECEIVE Figure 39.5 Simplified timings for data transmission between a data terminal and a remote computer over the telephone network. The terminal initiates a PSTN call to the computer which responds to set up a two way dialogue ing 2 bit digital signal is output at the Receive Data (RD) line at the characters. The extra circuitry and logic to achieve this provides modem interface to the receiving digital equipment. greater throughput but at a price premium compared to asyn- So far data has been serially transmitted from source to destina- chronous modes. tion. But data only went one way, and while the source transmitted the destination listened. This is half duplex operation and is ineffi- cient since the line has to clear before transmission in the opposite Modem standards direction can take place, limiting the speed of operation. The use of four wire lines, instead of two, reduces the turnaround time. Standardisation in modem operation is important to guarantee that Modems that operate full duplex permit simultaneous operation in a user can connect to a modem attached to the same link or network bo