m using an analog-to-digital converter (also known as an A/D converter). Within the phone company's network, the conversation is carried in digital form. When it gets to the receiving telephone's central office, the conversation is re-converted into an analog signal using a digital-to-analog (D/A) converter which the receiving telephone can understand. Generally, any noise picked up by along the way is filtered by the human ear.
             Figure 4. POTS. The POTS telephone system converts analog (voice) signals to digital signals and then back to analog signals again.
             When computer systems communicate over POTS, they must interface with an analog phone line. To accomplish this task, a computer uses a modem (modulator/demodulator) to modulate digital data into an analog signal compatible with the 8 kHz limitation imposed by the telephone company. At the other end, a modem demodulates the analog signal and converts it back to digital form. Since the computer is not as forgiving as the human ear, most modems have built-in error detection to request retransmission. Because of all the analog and digital conversions involved, noise is invariably introduced into the system. According a communication theory, Shannon s sampling theory, the modem to modem technologies are capable of achieving only 35 kbps. Most modern modems are currently designed to achieve 28.8 kbps and even 33.6 kbps under ideal conditions, quite close to the theoretical maximum. [http://www.multitech.com/WHpaper/56Kmodem.htp]
             Figure 5. COMPUTER/MODEM/MODEM/COMPUTER. The modem allows conversion of digital to analog signals, and vice versa.
             Most recently, 56 kbps modem technology has been introduced. One might wonder how it exceeds the 35 kbps theoretical maximum: 56 kbps is achieved because there is only one analog to digital conversion in the process. The digital format used by the telephone company is connected directly to a service provider. Thus, with only one analog-to-di...