Chapter 2
Modem Fundamentals
This chapter provides a brief overview of how modems work.
2.1Modem Basics
Modems let computers communicate with other computers using normal telephone lines or leased lines. Your modem sends communications by modulating (converting) data into sound waves that a telephone can transmit and another modem can receive. Your modem receives communications by demodulating sound waves from the telephone into data your computer can recognize. The term modem is an abbreviated way of describing this modulation/demodulation process.
In addition to converting data to sound, a modem performs functions such as dialing, answering calls, and adjusting settings for local telephone conditions. Commands must be issued to the modem to control these functions and settings.
2.2Data Format
In asynchronous serial communications, data bytes are disassembled into individual bits and then transmitted bit by bit along with interval bits and checksum bits, in the order of start bit, data bits, parity, stop bits, and so on.
The interval bits, start bit, and stop bit, signal to the receiving party the beginning and end of a byte. The checksum bit and parity allow the receiving party to check the accuracy of the received data.
The pattern in which a byte is disassembled is called the data format. Modems at both ends of a link should use the same data format. Otherwise, the data may be misinterpreted and appear garbled.
The most commonly used data formats include the following types. Before you dial a bulletin board or connect
Start Bit | Data Bits | Parity | Stop Bit | Total |
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