PC Interface : It is interface between PC & MODULATOR circuit, which connects PC & modem together. It trans signals from PC to Modem & vice versa. It also performs CONVERSION of data from ANALOG to digital & vice versa.
Modulator Demodulator Circuit : It performs modulation of signals. Mainly it is based on frequency modulation in order to achieve max trans rate.
Line Interface : It is interface between modulation circuit & telephone line. It connects telephone with modem with the help of twisted pair CABLES. It transmits modulated signals over telephone lines.