If your modem is external, you will need to ensure that it is attached to the proper port on the computer. There is usually another port on your computer which has 25 holes, but this is your parallel port and is not designed to run a serial modem. Do not get the two confused. When you purchase the cable to attach your modem, get one that has a male connector on one end and a female connector on the other. Because systems today can come in such a wide variety of configurations, please consult your computer manual for more information on serial and parallel ports.
Line noise is the term used to describe the corruption of data as it travels between two modems.
The amount of line noise you encounter will be dependent upon a wide variety of factors, so it may be difficult to track line noise problems to their source. Even if you could track the problem down, though, it may not be worth your while to try unless the problem is extremely severe. You may wish to contact your local telephone service provider and inquire about checking your interior telephone wiring. If you have an extended maintenence contract, with your telephone company, they may do this without charge. In any case, some line noise is to be expected and will probably be corrected by your modem without you ever knowing it was there.