Modem

Modems are important tools in networking. Commonly used modems are low-cost chips. A technique called frequency shift keying is used in modems. Frequency shift keying scheme transmits sinusoidal tones which are effective for analog phone lines. These sinusoidal tones create chirping sound which is characteristic of every modem. For the transmission of audio signals, analog input is sampled and sent to two digital filters. The filter ouput is fed to detectors where the computation of the average value of the signal over the past n samples is carried out. Although the transmitting and receiving modems are isolated and not synchronized, both agree in advance on the length of the time during which a bit will be transmitted. The modem will never implement a hardware interface to telephone line or software for dialing a phone number. The modem is composed of a small subsystem and two major subsystems. The small subsystem deals the interrupt handlers for the samples. Transmitter and receiver are acting as the two major subsystems.