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CompTIA A+ Certification All-in-One Exam Guide

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Figure 21-6 Analog signals used by a telephone line versus digital signals

used by the computer

Modems enable computers to talk to each other via standard commercial

telephone lines by converting analog signals to digital signals, and vice versa.

The term modem is short for modulator/demodulator, a description of

transforming the signals. Telephone wires transfer data via analog signals that

continuously change voltages on a wire. Computers hate analog signals.

Instead, they need digital signals, voltages that are either on or off, meaning

the wire has voltage present or it does not. Computers, being binary by

nature, use only two states of voltage: zero volts and positive volts. Modems

take analog signals from telephone lines and turn them into digital signals

that the computer can understand (see Figure 21-7). Modems also take digital

signals from the computer and convert them into analog signals for the

outgoing telephone line.

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