Basically, a modem consists of a power supply, transmitter, and receiver. The power supply provides the voltage necessary to run the modem's circuitry. Depending on the type, the power may be supplied from an external source, or through a connection with the computer's power supply. The transmitter portion contains a modulator, which converts the digital data into an analog signal. Also in the transmitter is circuitry that filters, wave shapes, and signal controls while the signal is being modulated. The receiver had a demodulator and associated circuitry that reverses the modulation process and returns the original digital signal.
The modem connects to the Public Switched Telephone Network (PSTN) through a standard twisted pair phone cable connected to a phone jack. Modems can be either internal or external to the CPU of a computer. Sometimes the modem is attached directly to the logic board. Other times, the connection is made externally through a port. The port configuration is based on the RS-232-C standard, where the connector consists of 25 pins. Frequently, only a subset of the standard pins are actually built into the connectors. An 8- or 9-pin connector will work just fine, simply using the pins for the more frequent commands. As long as there is a wire each for sending, receiving, and ground, the connector will function.
Multiplexing
Multiplexing the method of sharing a channel among users.
The phone system, and thus the modem system, uses Time Division Multiplexing.
This method of multiplexing give specific time slots to each user,
and that user is only allowed to transmit during that time.
Modulation
In order to convert a signal from digital to analog, then back to digital
(which is what modems do), a method of modulation must be used.
To convert from digital to analog, the most efficient form of multiplexing
available is Quadature Amplitude Modulation, which is a combination of
frequencies, amplitudes, and phase shifts. This means that many different
signals can be represented with a small set of frequencies, amplitudes,
and phase shifts. The frequencies and other shifts need to be widely
dispersed, so that it is less likely for two signals to be
confused.
Most modern modems use this form of modulation. Older ones use Frequency modulation, which of course, only changes the frequency.
Network Topology
The topology of the telephone system is switched, or star. This topology
has a central computer that all other devices are connected to. It is
through this central computer that all communication occurs. The phone
system is actually many separate stars that are connected together into
a larger network, but the concept is the same.