Introduction to Data Communications
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13. Modems

A modem is a Modulator/Demodulator, it connects a terminal/computer (DTE) to the Voice Channel (dial-up line).

13a. 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 RS232 serial port (25 pin male D connector) and the outgoing phone line with an RJ11 cable connector (same as on a phone extension cord). Male connectors have pins, female connectors have sockets.

13b. 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 hand-shaking 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..

Introduction to Data Communications
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