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Internet Online LapTop Warrior Discussion List
I - L-A-P-T-O-P W-A-R-R-I-O-R D-I-G-E-S-T

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August 12, 1998 Issue # 023
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.....IN THIS DIGEST.....


// -- S-P-E-C-I-A-L D-I-G-E-S-T -- //

    "Why Modems Get Fried, and How to Prevent It"
        ~ Kevin Prince


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// -- MODERATOR COMMENT -- //

In this special I-Laptop Digest, we carrying an article by Kevin Prince of Road Warrior International. Kevin describes some of the ways laptop modems can get "fried" by digital phone systems and what you can do to protect yourself. The article contains a number of commercial plugs for products that Kevin sells, but it also contains much useful information. We thank Kevin for permission to distribute this article.

We would also like to welcome a new sponsor to I-Laptop Warrior. I-Call distributes some innovative software to help you save money on phone charges when you need to connect remotely to your office computer. The company's web site is at: http://www.icall.com/road


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Why Modems Get Fried, and How to Prevent It

By Kevin Prince, Road Warrior VP

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, most 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.

So why is this a problem? Because 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). However, 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. A number of products are on the market to protect you from this situation, including a new line of Road Warrior Modem Saver(R) testers that allow you to detect such dangerous PBX lines before you plug in and try to use your modem.

Three small adapters and a five inch RJ-11 cord are included with the Modem Saver(R) products. These items can help you find (and correct) problems that you may encounter while trying to connect to the phone system while traveling.

The "In-Line" adapter is a straight through adapter and allows you to connect your Modem Saver(R) to the end of an RJ-11 cord without changing the polarity of the lines. The "Polarity Reversing" adapter changes the polarity of the Tip and Ring conductors, and the "Line 1/Line 2" adapter switches line 1 and line 2.

Take a look at the conductors on a standard analog RJ-11 jack, which you can usually find at the end of the "dongle" that connects to your modem, or at the end of an RJ-11 cord. You'll notice that there are four conductors. A standard modem (and a standard telephone for that matter) use only two of the four conductors; in the United States and most other countries, the inner pair of wires is used as "Line 1." If the RJ-11 jack or cord has two lines wired to it, "Line 2" will use the outer pair of conductors.

A telephone line, therefore, uses only two wires and these are referred to in the industry as "Tip" and "Ring." Sometimes Tip and Ring get reversed because of incorrect wiring at the RJ-11 jack, or use of a non-standard RJ-11 extension cord that reverses the lines. This doesn't make any difference as far as a telephone goes, since voice communication is not affected by this problem. However, some modems perform at slower speeds if they're connected to a line with these wires reversed. This is called a "Reversed Polarity" condition and is detected by the yellow light on the Modem Saver(R) product.

(IBM also sells a similar product that also detects reversed polarity, but IBM does not provide a reversing adapter to correct the problem with their device.)

In the United Kingdom, Line 1 is on the outer pair of conductors, and line 2 is on the inner pair of conductors. Therefore, if you plug your modem into a UK-to-RJ-11 phone adapter, you will most likely get no connection. The problem is that your modem is trying to pick-up their line 2, and there might not be a line 2 on that phone jack. Therefore, in the UK you need to use the "Line 1/Line 2" adapter in front of the Modem Saver(R) in order to switch the line that your modem is listening on to the line that's actually active in the wall.

With all of these adapters, the use of a small RJ-11 cord or a retractable RJ-11 cord can help you make the connections you need conveniently.

If you happen to be traveling without a Modem Saver type device, there are still methods of protecting yourself from a dangerous phone line.

Here's the scenario... You're in, say, Singapore, on a Friday afternoon, in a customer's conference room. Your supervisor back home said she would email you the final price and configuration that you can quote to your customer, and now you need to get that email at any cost. There is a phone jack in the wall. Can you use it to dial into the local iPASS server to get your email? You can't call your supervisor, since it's Saturday evening back home. Getting that email is your only option.

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 than 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're in England, before you give up, you might also try the outside pair of wires with a "Line 1/Line 2" switching adapter.) 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 to Analog Adapter." The later name is really mis-descriptive, since it doesn't technically 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.

Road Warrior products such a product under the trademark "Modem Doubler," and there are other companies that produce similar products. 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 Modem Doubler, and then plug the Modem Doubler into the phone's handset jack RJ-9 jack. Then you'd plug your modem's RJ-11 plug into the Modem Doubler's RJ-11 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 other method of connecting in this scenario is to use an acoustic coupler. Many people envision the old 300 baud acoustic couplers that were used in the late 1970's, but have no fear, acoustic coupler technology has improved tremendously since then.. Road Warrior produces the Telecoupler II acoustic coupler, which can connect at 28,800 under ideal conditions, but usually no slower than 14,400 bps on marginal quality phones. That's fast enough for a fax or downloading your email. In use, one side of the Telecoupler II is plugged into the modem, and the other end (which looks like a phone handset) is strapped to the phone handset with a Velcro fastener.

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 strange lands, 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!

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The Road Warrior web site can be found at: http://warrior.com and Kevin can be reached directly at: kevin@warrior.com

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