Computer Corner

Modems

Computers can communicate with each other by using telephone lines and modems. The modem is a device that converts information from your computer into a form that can be sent over telephone lines. If you have ever mistakenly dialled a fax number, you have heard the type of signal a modem transmits.

Modems are rated by their baud rate, or maximum speed at which they can move data. A 2,400 baud modem will move about 240 characters per second. Most modems sold today have a maximum rate of 14,400 baud. The Eastman Freenet can accept any rate up to 14,400 baud. Receiving a screen full of text information at 2,400 baud takes about 8 seconds, at 14,400 it takes about 1 second. Modems that move information at 28,800 baud are now available although they are still expensive and most dial-up services do not support the speed. Faster modems are generally more expensive.

Modems can correct transmission errors introduced by noise on the telephone lines. The modem determines if the information received is valid and requests that information be retransmitted if errors are detected. Better quality modems can actually be faster at the same baud rate since information is retransmitted less frequently.

There are two types of modems: external and internal. An internal modem fits inside your computer, using an expansion connector on the main computer board. When using an internal modem, the computer supplies the power for the modem and no cable is required to connect the computer to the modem. The modem has two connectors on the back. One connector is used to connect the modem to the telephone jack, a telephone is plugged into the second. This allows the use of the telephone when it is not being used by the computer.

External modems work with terminals or computers and are connected to a serial port (usually COM 2) on the back of the computer or terminal. They are plugged directly into an electrical outlet, making them more awkward to use because of the extra cables. External modems also cost more than internal modems because power supplies and cables are required.

An external modem is easily shared between different computers. If you have more than one computer or if your computer doesn't have an available expansion slot, an external modem is an alternative.

To effectively use an external modem, you must have a fast communications port on the computer. To check whether you have a fast port, run the DOS diagnostic program on a PC by typing MSD at the DOS prompt. Select the Com Ports item by typing C. The last line tells you what type of UART chip is used for each port. A 16550 UART chip is designed to handle high speed communications reliably. If you don't have a high speed port, use an internal modem if possible as it provides its own high speed serial port.

About the Author:

Dennis Parcey is an Information Technology consultant with Whiteshell Information Technologies, a computer consulting business in Pinawa providing computer services and support to businesses in south-eastern Manitoba.