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PC Modems

Modems - MO - modulate - DEM - demodulate

Modulation converts digital data to an analog signal suitable to travel over telephone lines.
Demodulation converts the analog signal back to digital data to be processed by a computer.

Command Function
ATA Answer
ATD Dial
ATZ Reset
ATF1 Half-Duplex Off (echo copy of receiving information)
ATHN Hang up
ATMN Speaker On/Off
ATSR Display register
ATH Hangs up modem

When we say a modem is 33.6, that refers to the transmission speed of the modem given in Kbps (33.6 kilobits per second). The maximum transmission rate is limited to 56.6 Kbps on POTS (Plain Old Telephone Service) because of the way signals are sent over telephone lines. The design cannot support speeds greater than this.

Error correction works by first breaking the data into small packets or frames. Each packet of data is compared to an expected check sum. If they differ, then another transmission of the data is requested.

As a general rule, the port speed should be about 4 times as fast as the modem speed. The UART chip controls communication over serial ports.

Hardware Topics: Basic PC Overview
Preventative Maintenance and Safety -|- Basic Troubleshooting -|- PC Architecture Basics -|- PC I/O and Busses
Processors -|- Memory -|- Drives -|- Monitors and Video -|- Modems -|- Printers -|- Portable Systems -|- Networking

Operating System Topics:
DOS basics -|- Windows 3.1 -|- OS Memory Management -|- Installation of DOS and Windows 3.x
Installation of Windows 9X -|- Diagnosis and Troubleshooting -|- Windows 95 Overview -|- Windows 95 Networking
Windows 95 vs Windows NT

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Please Read -DISCLAIMER: Technology changes very rapidly. The information presented here was believed to be accurate at the time it was gathered. No claim is made that this information is up to date, or that it represents the current technology used today.

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