Modems are designed to operate on standard, analog telephone lines like those found in most homes (sometimes called a "POTS" line for Plain Old Telephone Service). However, many telephones that are installed in businesses and hotels are PBX (Private Branch Exchange) phones. A PBX system installed in a business or hotel provides a "mini telephone company" within the building, and allows for special features such as hold, transfers, voice mail, etc. There are many advantages to PBX systems. However, there is at least one disadvantage: most PBX phone lines terminate in a standard RJ-11 jack, making it impossible to tell the difference between a POTS line and a PBX line.

Why is this a problem? PBX manufacturers are not required to adhere to the same restrictions on their inside PBX lines as the phone companies mandate for standard analog lines.   Several PBX manufacturers have designed phone systems that deliver much more current to the phone than does the phone company on a POTS line. In fact, some PBX systems can deliver up to an amp of current to the inside telephones (which are designed by the PBX manufacturer, of course, and are made to handle such current).

Your modem was designed to see no more than about 120 mA, or about 1/8 of the amount of power put out by some PBX systems. As a result, if you plug your modem into a PBX line, thinking that it's a POTS line, you could damage your modem with too much current and not even know it. Suddenly, your modem just doesn't work. If you're lucky, it will smoke a little bit, giving you a sign that something got burnt up. But most often it just dies a quiet death, leaving you without a working modem and no idea what happened.

Here's a typical scenario.  You're in a customer's conference room and need to access your email.  There is a phone jack in the wall. Can you use it to dial-up and get your email?

First, check to see if a phone is already plugged into the jack. Much can be told about a phone jack based on the type of phone that's plugged into it. Pick-up the handset and listen. Is it a standard "outside line" dial-tone? Careful, since many foreign countries have strange-sounding dial tones. Ask someone how to get an outside line, usually by dialing "9", and compare what you hear.

If the initial dial tone is significantly different from the outside line dial tone, which comes from the phone company, chances are the phone system at your customer's site is generating a "system dial tone." This means that when you pick-up the phone in the conference room, a PBX system somewhere else in the building is generating the initial dial tone that you hear. What does that mean? Simply that the phone jack in question goes to a PBX, and is not a standard POTS line. In other words, find another jack.

If you get an outside line immediately upon picking up the phone, look carefully at the phone. Does it have lots of lights, trick features, or a digital display? Most standard phones do not have such features, and so the presence of "hold" or "voice mail" or other buttons would again indicate a PBX phone system, even though you're getting a phone company dial tone immediately upon picking up the handset.

If there is no phone plugged into the jack in question, you can test the jack by plugging in a standard analog phone. Of course that means that you have to have a standard analog phone with you, but there are several compact travel phones that make this a reasonable option if you don't have, or don't wish to have, a Modem Saver type line tester. If you try a standard phone and get nothing, which would be typical with a PBX phone system, that's probably what you have... a PBX phone system. If you get a phone company dial tone, congratulations, you probably have a standard POTS line on that jack and can feel fairly confident that you'll be able to connect your computer to get that important email.

Now, what happens if you find the only jacks in the room are connected to a PBX system? Simply put, there is no way to plug directly into a wall jack to get your email at this point. However, even with a PBX system there are ways around this problem. Note that the handset of the phone, since it has a microphone and a speaker, plugs into a modular jack that must have analog signals. You can use these analog signals with other products to get a connection.

The best method of connecting at this point is using a "PBX adapter" sometimes called a "Digital Line Converter". The latter name is technically incorrect since it doesn't convert digital to analog, but just converts an existing analog signal on four wires into an analog signal on two wires that's usable by a modem or fax machine. To use such a device you would plug the phone's handset plug (usually an RJ-9, which is a bit smaller than the familiar RJ-11) into the digital line converter and then plug the digital line converter into the phone's handset jack RJ-9 jack. Then you'd plug your modem's RJ-11 plug into the digital line converter's jack (which is just a simulated POTS line at this point) and connect. You might have to dial a "9" or some other key to get an outside line before having the modem dial, since the PBX system typically isn't expecting touch tones to originate from the handset and will most likely ignore any that your modem produces. However, the phone company will be expecting touch tones and will respond accordingly.

The analog signals being produced by the speaker and microphone are converted back and forth between acoustic signals (sound waves) into analog electric signals that your modem can understand. The advantage of such an acoustic coupler is that it can be used with most phones, even pay phones or phones that have the handset hard wired to the desk unit. You'll find the later case in some hotels, since they don't want their phone's handsets disappearing as souvenirs. The disadvantage with the acoustic coupler is that it tends to be slower than a direct electrical connection, and so you won't want to surf the latest graphics art museums on the 'Net with an acoustic coupler.

So we have seen that there are relatively easy methods for first discovering what type of phone systems are available in different environments, and connecting in spite of them. However, without the right equipment from the start, you'll have a difficult challenge connecting. Conversely, if you have certain key items in your bag when you leave home, you should have no trouble making all of your connections... at least the electronic ones. Bon Voyage!

This Tech Note is excerpted from copyrighted materials courtesy of Road Warrior International