What is a Modem?

Modems are fundamentally used to interface digital circuits to transmit information on analogue channels as telephone systems. Modem is from modulator-demodulator only is a device which modulates an analogue carrier signal to encode digital information as well as demodulates a carrier signal to decode the transmitted information. The aim is to produce a signal which can be transmitted easily and decoded to reproduce the original signal data. These can be used over any means of transmitting analog signals. Two major modulation schemes are currently being utilized to implement ADSL: carrier less amplitude/ phase (CAP) a single carrier modulation scheme depends on discrete multi-tone (DMT) and quadrature amplitude modulation (OAM) i.e. a multichannel modulation scheme. The option between them naturally based on how well they perform in the presence of impairments on the existing copper twisted -pair access cabling as side bar, since these can limit the transmission capacity. Additionally, high bit rate services carried with ADSL should not interfere with other services. In essence, multicarrier modulation superimposes a number of carrier modulated waveforms to signify the input bit stream. The transmitted signal is the sum of these sub-channels (or tones), that has similar band width and similarly spaced center frequencies. The number of tones must be large enough to ensure good performance. In practice, a value of 256 gives near optimum performance whereas ensuring manageable implementation complexity.