Introduction to Data Communications
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13c. Analog Connection

The connection between the modem and outside world (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 the digital information into tones (frequencies) for transmitting through the phone lines. The tones are in the 300-3400 Hz Voice Band.

13d. External/Internal Modems

There are 2 basic physical types of modems: Internal & External modems. External modems sit next to the computer and connect to the serial port using a straight through serial cable.

Internal modems are a plug-in circuit board that sits inside the computer. It incorporates the serial port on-board. They are less expensive than external modems because they do not require a case, power supply and serial cable. They appear to the communication programs as if they were an external modem for all intensive purposes.

Introduction to Data Communications
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