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Modems     

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THOUGHTS ABOUT MODEMS

The word modem stands for Modulator / Demodulator. Fundamentally, the modem is responsible for translating a data signal (from an RS-232 interface) into a sound pattern that can be transmitted over a telephone line. The problem being solved by the modem is that, while the phone company has wires all around the world, they are designed to carry voices, and not data bits. The job of the transmitting modem is to make a unique sound pattern to represent bits (that’s called ‘modulation’). That sound is carried across the phone lines. The job of the receiving modem is to convert the sounds back into data bits (that’s called demodulation).

The telephone network is designed to carry the human voice. To make the system reasonable to design, a set of specific constraints was placed on the frequencies that would be considered "normal" for humans. More specifically, the challenge of the telephone system designers was to decide just how little "high fidelity" could be tolerated by the users of the telephones.

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General Overview ] RS-232 Standard ] Bit Encoding ] Character Encoding ] Data Errors ] Physical Circuit ] DTE and DCE ] DB-25 Connection ] Control Signals ] Break-Out Box ] Directional Signals ] Connecting DTE's ] Async & Sync ] Other Signals ] Custom IC's ] RS-423 ] RS-422 ] [ Modems ]

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