Most digital data processing devices generate simple digital signals (e.g., unipolar), which are limited of data transmission capability. Consequently, it is rare for such a device (terminal, computer) to attach directly to a transmission or networking facility. For example, communicating between two PCs via a telephone line (connected between subscriber's home and local office, then between local offices, and finally between local office and another subscriber's home). The telephone line is an analog system. That means it can not be used for digital transmission. If we try to transmit digital signals over the analog system, our bit rate or bandwidth will be limited to a minimum. On a 4 kHz analog band limited system we can only transmit at a maximum rate of roughly 1 kHz. Because a 1 kHz digital signal needs a bandwidth of 5 kHz in its poorest shape (only transmitting the fundamental, 3[SUP]rd[/SUP] harmonic and the 5[SUP]th[/SUP] harmonics). To maximize the transmission rate we convert the digital signal to analog first and then do an analog transmission. To achieve that we need a communication circuit equipment, called a MODEM as follows.