There are several different general factors that can cause a modem to malfunction, including phone-line problems, incorrect settings, hardware conflicts, and damaged hardware. Of these, phone-line problems are one of the most common modem trouble spots. To help you keep a clear connection, I’ll examine two potential phone-line bugs: a faulty phone line and a noisy one.

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Testing, testing…one, two, three
I can’t begin to count the number of times that I’ve seen someone attempt to plug an analog modem into a digital phone line, such as an ISDN line or a line used with a digital PBX system. To put it simply, analog modems cannot be used on digital phone lines. In some cases, merely plugging an analog modem into a digital phone line can severely damage the modem because of differences in the voltage flowing through the line.

Let’s assume that you’re using an analog phone line with your analog modem, and there is still a malfunction. If you suspect a problem with the line, you’ll need to verify that the phone line is working properly. There are a couple of different ways to do this.

The first method is to use a phone-line tester, such as the one shown in Figure A. You can get a tester like this from a hardware store for about $10. The light on the tester will be green if the phone line is wired correctly and red if the phone line is wired backwards. The light won’t illuminate at all if the phone line isn’t live.

Figure A
A phone-line tester is a great tool for diagnosing modem problems.

Once you’ve used a phone-line tester, I recommend plugging a standard telephone into the line. Try making and receiving calls. About a year ago, I was troubleshooting a phone jack, and the tester said that the line was good. However, when I checked the line with a standard telephone, I found that, because of something strange that the phone company had done, the line could receive but not place calls. Using a standard telephone to troubleshoot the line can help you to quickly spot such problems.

Too much noise
Some phone lines may have too much noise on them to support modem communications. As you already know, modems communicate by passing tones over the phone line at specific pitches and frequencies. Noise on the line can disrupt some of these tones and make communications impossible.

With a noisy line, you can usually hear static through a standard telephone. However, you can’t always hear disruptive line noise; sometimes the line interference, while inaudible, can still interfere with communications. This is particularly true with phone lines that pass through an excessive number of switches. For example, big companies often have trunk lines that feed into a central office and are then distributed out to other offices. These types of setups tend to be really bad for modem communications because of the inherent latencies and line noise caused by so many phone switches.

DSL modems are also a common cause of line noise. DSL is designed to carry data at an inaudible frequency and support simultaneous voice conversations over the line. However, if you attempt to make modem calls over a line that is also used for DSL, the DSL signal will often be disruptive.

What can you do about line interference? Try communicating at a lower speed. Initiate communications at 9600 bps and then increase the speed from there. You can also try using line filters, such as the one shown in Figure B. Heavy duty line filters are especially helpful when it comes to filtering out a DSL signal.

Figure B
Line filters can be very effective at reducing line noise.

Finally, you can try to eliminate radio interference from your phone line by using wire ties to attach magnets or iron blocks to the outside of the line. You can get the parts you need for this simple solution from any electronics store, such as Radio Shack.

Are you a modem master?

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