Modems
A modem (MOdulator/DEModulator) connects a terminal/computer (DTE) to the Voice Channel (dial-up line).
Basic Definition
The modem (DCE - Data Communication Equipment) is connected between the terminal/computer (DTE - Data Terminal Equipment) and the phone line (voice channel). A modem converts the DTE (Data Terminal Equipment) digital signal to an analog signal that the voice channel can use.
A modem is connected to the terminal/computer's RS-232 serial port (25 pin male D connector) and the outgoing phone line with an RJ11 cable connector (the same as on a telephone extension cord). Male connectors have pins, female connectors have sockets.
Digital Connection
The connection between the modem and terminal/computer is a digital connection. A basic connection consists of a Transmit Data (TXD) line, a Receive Data (RXD) line and many hardware handshaking control lines.
The control lines determine whose turn it is to talk (modem or terminal), if the terminal/computer is turned on, if the modem is turned on, if there is a connection to another modem, etc.
Analog Connection
The connection between the modem and the outside world (the phone line) is an analog connection. The voice channel has a bandwidth of 0-4 kHz but only 300 - 3400 Hz is usable for data communications.
The modem converts digital information into tones (frequencies) for transmitting through the phone lines. The tones are in the 300-3400 Hz Voice Band.
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