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
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
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