Q. What is a Modem?

Modems are generally used to interface digital circuits to transmit information on analogue channels such as telephone systems. Modem (from modulator-demodulator) is a device which modulates an analogue carrier signal to encode digital information and also demodulates such a carrier signal to decode transmitted information. The goal is to produce a signal which can be transmitted easily and decoded to reproduce original signal data. Modems can be used over any means of transmitting analog signals. Two main modulation schemes are currently being used to implement ADSL: carrier less amplitude/ phase (CAP) a single carrier modulation scheme which is based on quadrature amplitude modulation (OAM); and discrete multi-tone (DMT), a multichannel modulation scheme. Choice between them naturally relies on how well they perform in the presence of impairments on existing copper twisted -pair access cabling (see side bar), as these can limit the transmission capacity. Furthermore high bit rate services carried by ADSL mustn't interfere with other services. Essentially multicarrier modulation superimposes a number of carrier modulated waveforms to represent input bit stream. Transmitted signal is the sum of these sub-channels(or tones)that have same band width and equally spaced centre frequencies. The number of tones should be large enough to make sure good performance. Practically a value of 256 provides near optimum performance while ensuring manageable implementation complexity.