Communicating with Modems and Network Adapters
In this section, you find out about installing modems and network cards that allow computer systems to communicate with other systems on the network.
Working with modems
A modem is a communication device that modulates, or converts, the digital signal from the computer into an analog signal so that it can travel over an analog line, such as telephone wire. The signal is then demodulated back into a digital signal at the modem on the receiving system. The process of modulating and demodulating is where the word modem comes from.
The modem allows a computer to send data over an analog line and is useful when you want to dial up another computer over the phone line or dial into the Internet.
Modem characteristics
A modem is either internal or external:
- An internal modem is installed in an expansion slot within the computer housing as either an older ISA card or, more popular today, a PCI card.
- An external modem sits on the desk beside the computer and plugs into the computer's serial port.
Both internal and external modems plug into a wall telephone jack via a standard telephone cable.
Another very important characteristic of the modem is its speed. The speed of the modem is measured in bits per second (bps) - and like anything else, the more bits the better! When it comes to modem speed, you're looking at 56 Kbps (56,000 bps) as the standard. This is known as a 56K modem.
AT commands
AT is short for ATtention, and AT commands are the modem commands that are called by software to perform communication. You typically don't need to use these commands unless you're troubleshooting why a computer cannot dial up. Your first question is always, "Is it the modem or the application I am using?" By using these low-level AT commands, you can determine whether the modem is working.
The AT commands that you should be comfortable are listed in Table below.
ATM Modem CommandsCommand Description ATDT <number> Dial a number using touch-tone dialing. ATDP <number> Dial a number using pulse or rotary dialing. ATA Instructs the modem to answer. ATH Instructs the modem to hang up. ATL Sets the speaker loudness. ATZ Resets the modem to the default settings.
Notice that each command starts with the AT prefix, and then you have a character such as "D" for dial, "A" for answer, or "H" for hang-up. Because there are two types of dial tones, the dial command has another character representing the type of dial tone to use and requires the telephone number to dial.
