Master Thy Modem

If you have Internet access - and you probably do, otherwise you probably 
wouldn't be reading this - chances are that you use a modem and a phone 
line to go online. But what exactly is a modem? How does it work? How do 
you make sure you get the best performance out of your modem? Read on for 
the answers to these and more questions! 

What is a Modem?

The word modem is derived from MOdulator-DEModulator and that pretty well 
describes their function. Standard telephone circuits, known as POTS lines 
for Plain Old Telephone Service, are designed to carry analog information 
like the sound waves of your voice. That is not compatible with information 
between computers which is a digital stream composed of ones and zeros. The 
computers digital signal must be modulated or converted to analog sound 
waves to travel through the phone lines, and demodulated or converted back 
into digital pulses that can be understood by the receiving computer. 

Modems need to have a set of instructions that tell them how to accomplish 
various communication tasks. An early pioneering modem manufacturer, Hayes, 
developed a set of instructions called the Hayes Command Set, which has 
been modified and expanded as the technology advanced. Other manufacturers 
adopted these commands so that modems made by different companies would be 
able to talk to each other. Thus the Hayes set became a de facto standard 
that is usually referred to as the AT command set because commands are 
prefaced with AT to let the modem know that an instruction is to follow. A 
group of instructions is called a Command String. 

Modem Speed

The original modems operated at the blazing speed 300 data bits per second 
(bps). Today the fastest analog modems are rated at 56,000 bps, 
unfortunately the Federal communications Commission (FCC) limits them to a 
maximum speed of 53 Kbps in order to protect the telephone network from 
interference that could affect normal voice communication. If you ever see 
a 56K modems speed reported over 53K, it is reporting the port speed of 
your computer (DTE), NOT the connection speed (DCE). This can occasionally 
happen when any modem is initialized, it is in error, your 56K modem will 
not connect at 115.2K. A modem can be forced to report port speed with an 
AT command so if you consistently see unrealistically high speeds reported 
you can correct your command string to fix it, Unfortunately, not all 
modems use the same command code so consult your modem manual or 
manufacturers web site. 

Modern modems are designed to adapt to phone line and noise conditions, and 
dynamically increase or decrease in speed as needed. When you sign on and 
the computer tells you that you connected at e.g. 48,000, don't worry about 
reconnecting to get a higher speed, that was just the speed at the moment 
you connected and it normally changes constantly as conditions dictate. 

There are other things to be aware of with the 56K modems that most of us 
use today. In order to deliver speeds above 28K there can only be one 
digital conversion between your computer and the telephone companies switch 
to which it connects. Since there is one in your home (your modem), that 
means that the phone company must have a digital rather than an analog 
switch. Unless you are really out in the sticks, nearly all TelCo switches 
are digital today. There is, unfortunately, another limitation, you must be 
located within about 3+ miles (5.6 Kilometers) of the TelCo switch, or line 
loss and noise will make high speeds impossible. 

Modem types

Two different types of 56K modems originally were sold, the X2 modem 
developed by U.S. Robotics, and the Flex developed by Rockwell and Lucent. 
The two different types were unable to speak with each other, and it was 
important to use an Internet service provider (ISP) that used the same type 
of modem that you had. Eventually saner heads prevailed and the two camps 
came up with a system that combined the best features of each. This new 
system was adopted by the International Telecommunication Union (ITU) and 
is now universal under the ITU V.90 Standard. 

There are still different types of V.90 modems. External modems are self 
contained in their own box and have their own power supply that must be 
plugged into a power outlet. They are preferred by some users because they 
are easy to set up, just connect them to an available serial port, or USB 
port, and they have status lights that can give you information about their 
operation and performance. They can be moved easily between different 
computers, and can be turned off when not in use. Internal type modems are 
expansion cards that plug into either ISA or PCI slots inside the computer. 
They cost less than comparable external models since they need neither 
cabinets or power supplies. They, of course, require you to go inside the 
computer to be installed and can't have status lights. They are their own 
serial port and so do not use up one of the ports on your computer that may 
be needed for something else. Most internal modems today are Plug and Play 
(PnP) and are recognized and assigned by a PnP operating system. Make sure 
that your BIOS is NOT configured to set up PnP devices, Windows will handle 
the PnP setup. They require installation of drivers supplied by the 
manufacturer on a diskette or CD ROM. 

Some internal modems are Winmodems, they off load some functions to the 
computer CPU that are normally handled by onboard processors on a 
traditional modem. This obviously uses some of the computational power 
otherwise available to run applications. Manufacturers are not always up 
front in identifying winmodems, so look for the fine print on the box. A 
winmodem will always specify a minimum speed requirement for the computer's 
CPU. A winmodem also must be used only with the Windows operating system 

Modem Installation

There are differing setup requirements for modems so the best advise is to 
carefully read the installation instructions that come with the modem. 
Internal modems cause the most confusion and aggravation so let's look at 
some considerations. 

Most computers come with two Com (Serial) ports. If you use a Serial mouse, 
it is normally installed on Com1, you could disable Com2 in your BIOS and 
use Com2 for your modem, but that is throwing away a port that you could 
use for other purposes, you would use Com2 for an external modem. If you 
use a USB type mouse Com1 would be open and available Remember we said 
earlier that an internal modem is it's own port, so it can become either 
Com3 or Com4. If you have a mouse on Com1, do not use Com3 for the modem. 
Com1 normally shares the same IRQ as Com3. Com2 shares an IRQ with Com4. We 
said normally because although Com1 & 2 are sometimes hardwired, com3 & 4 
can also be placed on IRQ's 2, 5, 9, and 11. Of course, you only have 15 
IRQ's in total and most will be needed for other devices. The following are 
the usual hex addresses and ports used for internal modems: 

Port Hex IRQ 
Com1 3F8 IRQ4 
Com2 2F8 IRQ3 
Com3 3E8 IRQ4 
Com4 2E8 IRQ3 

Most modems have jumpers or switches to set the Com port, Plug and Play 
modems will also have a PnP setting that will let Windows assign the port 
and hex address. Some modems are software configurable. If your modem and 
OS are Plug and Play it is usually OK to let Windows assign the IRQ and 
port, otherwise set the modem for the IRQ and port you want to use. If your 
mouse is not on Com1, use Com3 or Com4. If your mouse is on Com1, use Com4. 
Most modems come already set to Com4 as a default. 

Choosing either a PCI modem or an ISA modem has no effect on modem speed, 
either slot is more than fast enough for a modem. An advantage to a PCI 
modem is that PCI devices can sometimes share the same IRQ. 

As with any add in card, once you open the computer case you will need to 
select the slot you will use. Turn OFF the power and unplug the power cord 
before working inside the case. ISA slots are usually black and in the rear 
left hand corner of the motherboard, PCI slots are usually white and 
shorter than the ISA and slightly farther away from the rear panel. Metal 
slides in the rear panel line up with each slot. Remove the slide for the 
slot you intend to occupy, it will either be held in by a screw, or the 
breakout type which can be removed by prying them with a screwdriver blade 
and then flexing back and forth until it snaps off. 

Carefully line up the card edge with the slot and firmly press it straight 
down into the slot, you should not need excessive force. When the entire 
edge of the card is evenly seated, secure the card with a retaining screw 
at the top of the metal bracket. There will be two RJ11C modular telephone 
jacks exposed on the rear of the card where it shows through the rear 
panel, one is marked "LINE" and you will run a telephone cord from it to a 
nearby telephone wall jack. The other jack will be marked "SET" or "PHONE" 
and you may plug a standard analog telephone into that jack if you wish. 

Unlike some devices for which you can use the drivers in Windows, you will 
want to use the drivers that came with your modem. When you turn on the 
computer and windows finds the modem select the "Have Disk" option and put 
the driver CD or diskette into the proper drive and follow the instructions 
that came with the modem to install the driver inf file. Most manufacturers 
frequently release updated drivers so it pays to check their web sites for 
new versions even with a brand new modem. 

Testing the Modem

To test your modem, go to Control Panel and double click on the Modem icon. 
A window will open that should show your modem by name, click on the 
Properties button and a new window will show what port it is on, there is 
also a slider to set the volume level of the speaker mounted on the modem 
card. Below that is a box for maximum speed, set the speed to 115,200 for a 
56K modem and click OK. Next click on the Diagnostics tab and in the window 
that opens click on the Driver button, a box will tell you the driver that 
is installed. Click OK and then click on the More Info button, a box will 
say it is communicating with the modem and a results box should report the 
test results. 

Troubleshooting

If for any reason the diagnostics fail, exit back to Control panel and 
double click on the System icon then on the Device manager tab. There 
should be a listing for Modem, click on the + in front of it and you should 
see your modem listed. Look also at the listing for Ports, click on the + 
and it should show the port your modem is installed on. If either of these 
are missing, or if there is a question mark (?) or exclamation point (!) in 
front of the item there was a conflict in the installation. Remove all 
references to the modem, and shut down the computer (Start/Shutdown), turn 
off the power for at least 5 seconds and then reboot and go through the 
detection and driver installation again being sure to follow the 
manufacturers instructions exactly. PCI modems can be slot sensitive, if 
computer fails to see or properly set up a PCI modem try putting it in 
another slot. 

Testing your phone line

If you fail to get good connect speeds (at least 40 kbps) and your location 
and TelCo switch meet the requirements we described earlier, you should 
determine the quality of your phone line so you can ask your phone company 
to fix the situation. 
You will need to use a terminal program like HyperTerminal that is included 
in Windows or any of the many programs that are distributed with modems or 
available for download. 

Since HyperTerminal comes with Windows, might as well use it for our test. 

Go to Start/Programs/Accessories/Communications/HyperTerminal. 
In the window double click on hypertrm.exe. 
When the program comes up, type in TEXT for the name, and 1234 for the 
phone number and hit Enter. You will now be in a blank terminal window. 
Type ATZ and hit Enter, it will respond with OK 
Type ATDT###-#### (Where ###-#### is the number to your ISP) and hit Enter. 
After the modem dials, it will reply with "Login", type +++, it will 
respond OK 
Type AT&V1 and press Enter 
You should see something like the following: 

DISC. REASON..... NONE 
LAST TX.......... 26400 BPS 
MAX TX........... 26400 BPS 
LAST RX.......... 50667 BPS 
MAX RX........... 50667 BPS 
PROTOCOL......... LAPM 
COMPRESSION...... V42Bis 
Line QUAL........ 023 
Rx LVL........... 012 
Max Rx State..... 67 
Max TX State..... 67 
EQM Sum.......... 0089 
Min Dist......... 0000 
RBS Ptrn......... 04 
Rate Drop........ 00 
Dig Loss......... 3F07 
Loc Rtrn Cnt..... FF 
Rem Rtrn Cnt..... FF 

If the Line QUAL is more than 25 it indicates the line is noisy. Repeat 
this test several times to get an average. 

Note also in the above test the modem was sending data at 26,400 BPS, and 
receiving data at 50,667 BPS. A 56K modem is asynchronous meaning it is 
faster in one direction than in the other direction. 

Under current regulations connection to the Public Service Telephone 
Network (PSTN) at speeds in excess of 53K is prohibited and modems are 
designed to not violate this rule. If you normally see faster speeds 
reported your modem is showing the port speed at which it communicates with 
your computer, rather than the connect speed at which it communicates with 
your ISP. This can often be corrected by adding a command to your modem 
string. 

Go to Control Panel/ Device Manager and right click on your modem and click 
on Properties, click on the Connection tab and then click on Advanced. In 
the advanced window is a box labeled "Extra Settings", type W2 in that box 
and click OK. The W2 command will take precedence and should force your 
modem to report connect speed. (Rockwell based PCI modems use the command 
MR=2). Some commands can override the W2 command so if that does not fix 
the problem, try adding S95=0, or S95=1 to the init string. 

Modem Strings

There are hundreds of modems on the market from dozens of manufacturers. 
Each modem has an initialization string stored in it by the manufacturer 
that tells it how to function. Even though the AT command set is 
standardized, the combination of commands used in a particular modem may be 
very different than that in another model even from the same manufacturer. 
All command strings must begin with "AT" and end with "^M". They can be 
either upper or lower case, but not mixed, and they are limited to a 
maximum of 39 characters not including spaces. 

There is a "Basic Command Set" and an "Extended Command Set". Extended set 
commands are preceded by an ampersand (&). The command ATZ means to use the 
modem default command string. A more specific command is AT&F1 which means 
to use the modem manufacturers command string, or AT&F0 which means to use 
the chipset manufacturers command string. The latter are extended commands. 

The possible combinations of commands number in the thousands. An init 
string for one model of modem can thoroughly mess up some other model by 
causing command conflicts. Fortunately you don't have to know anything 
about init strings because your modem and its manufacturer know. There are 
many tweaks that can modify how a modem handles certain functions. You can 
make it dial faster, connect only at a certain speed, change the speaker 
volume, and much more. All of this is mainly of little benefit for most 
users, and can create unforeseen problems. Only modify your manufacturers 
settings if you really know what you are doing, or on the advice of someone 
knowledgeable. 


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