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14. Lines and Terminators > Your Modem, the Information Driveway

Your Modem, the Information Driveway

Analog modems convert a stream of digital data into an audio-band analog format that can be sent over Plain Old Telephone Service (POTS) to another analog modem. (The telephone network is also sometimes referred to as the Public Switched Telephone Network, or PSTN.) The increases in modem performance of the last few years have come as modem makers learn to cope better with the difficulties imposed by sending digital data over a quirky analog system.

POTS was built and optimized for voice traffic. There are filters in the phone system that amplify the portion of the audio spectrum humans hear best (300 Hz to 3300 Hz), at the expense of other frequencies. There are also filters for echo cancellation. Many phone lines have poor quality. The analog data is sampled at 64 kbps, losing some information, and is sent in digital form to the remote phone switch, where it is reconstructed into analog form (see


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