Modems Past and Future

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             Above we see a configuration in which two computers communicate over a telephone line through the use of modems. Computer data is stored in digital format while telephone wires transmit data in analog format, - a modem transforms Digital data to analog data to enable the data to be sent through the telephone wire and analog data back to Digital data when received through the telephone wire. Modem communication over a telephone line is normally performed in serial fashion, a single bit at a time, in which the bits have an encoding that is appropriate for the transmission medium. There are a number of modulation schemes used in communication, which are encodings of data into the medium. We will look at the following forms of modulation, amplitude modulation, frequency modulation, phase modulation and finally pulse code modulation. .
             Amplitude modulation (AM) uses the strength of the signal to encode 1's and 0's. AM lends itself to simple implementations that are inexpensive to build. However, since there is information in the amplitude of the signal, anything that changes the amplitude affects the signal.
             Frequency modulation (FM) is not nearly as sensitive to amplitude-related problems because information is encoded in the frequency of the signal rather than in the amplitude. .
             Phase modulation (PM) is most typically used in modems, where four phases (90 degrees apart) double the data bandwidth by transmitting two bits at time (which are referred to as dibits). The use of phase offers a degree of freedom in addition to frequency and its appropriate when the number of available frequencies is restricted.
             Pulse Code modulation (PCM), an analog signal is sampled and converted into binary. The original signal is sampled at twice the rate of the highest significant frequency, producing values at discrete intervals. The samples are encoded in binary and catenated to produce the PCM sequence.