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Modems 101

The Basics

Most phone lines were designed to carry tonal (analog) signals. The Public Switched Telephone Network (PSTN) picks up these tonal signals, and usually converts them into digital signals to transfer them from one Central Office (CO) to another. On the other end, the digital signals are converted back into an analog signal.

How does a computer use a phone line?

"Modem" is an acronym for MOdulator/DEModulator. It converts the digital signal from your computer into an analog signal that can be carried by the phone line, and vice versa. It is also possible to get a digital line (ISDN) installed in your home, but you would need an adapter in order to resolve the different digital signals. ISDN is common in Europe and other areas where Cable or DSL access is not available.

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How does a hardware modem work?

A hardware Modem will have three components: The Microcontroller Unit (MCU), the Data Pump Unit (DPU), and the Data Access Arrangement (DAA). The Microcontroller Unit checks data for errors and performs compression. It may also convert a parallel transmission into a serial transmission. It utilizes the AT command set to send and receive signals. It sends the data on to the Data Pump Unit.

What does the Data Pump Unit do?

When turned on, the Data Pump Unit gets instructions and settings from ROM, the unit's permanent memory. The DSP (Digital Signal Processor) uses RAM (Random Access Memory) while performing its calculations. If your modem has instructions in EEPROM (Electronically Erasable Programmable ROM), it may be possible to upgrade the instruction set in a modem, giving it new features. After processing, the output is sent to the Data Access Arrangement.

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What does the Data Access Arrangement do?

The Direct Access Arrangement (DAA) serves as the hardware interface to the Public Switched Telephone System (PSTN). A standard RJ-11 jack connects the modem to the phone line. The interface allows the modem to detect signals on the line: dial tone, busy signal, etc.

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