What is a Modem? What is the need of MODEM in data communication? Explain at least one modulation technique used for high speed modems.

What is a Modem? What is the need of MODEM in data communication? Explain at least one modulation technique used for high speed modems.
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Modems are essentially used to interface digital circuits to transmit information on analogue channels like telephone systems. Modem (from modulator-demodulator) is a device that modulates an analogue carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the 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 based on quadrature amplitude modulation (OAM); and discrete multi-tone (DMT), a multichannel modulation scheme. The choice between them naturally depends on how well they perform in the presence of impairments on the existing copper twisted –pair access cabling (see side bar), because these can limit the transmission capacity. In addition, high bit rate services carried by ADSL must not interfere with other services. In essence, multicarrier modulation superimposes a number of carrier modulated waveforms to represent the input bit stream. The transmitted signal is the sum of these sub-channels (or tones), which have the same band width and equally spaced center frequencies. The number of tones must be large enough to ensure good performance. In practice, a value of 256 provides near optimum performance while ensuring manageable implementation complexity.

Need of Modem: Modems are used to interface computers, computer networks and other terminal equipment to telecommunication lines and radio channels. The word Modem is a contraction derived from the words modulator and demodulator. The modems at the transmitting station changes the digital output from the computer or other data terminal equipment to a form which can be easily sent via a communication circuit, while the receiving modem reverse the process. Modems differ in rate of data transmission, modulation methods and bandwidth and standards have been developed to provide compatibility between various manufacturers’ equipment and systems. 

FSK – Frequency Shift Keying: In this technique the frequency of the carrier signal is changed according to the data. The transmitter sends different frequencies for a “I” than for a “0” as shown in Fig. The disadvantages of this technique are that again (as it was with amplitude modulation) the rate of frequency changes is limited by the bandwidth of the line, and that distortion caused by the lines makes the detection even harder than amplitude modulation. 

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