Troubleshooting a Modem Connection

Printer-friendly version

Introduction

There are more than 1000 modems, each with their own peculiarities and each with their own options. Luckily most all modems are compatible with each other and with the standards set by Hayes many years ago.

In the last few years, there have been two added issues involving modems:

• Inexpensive modems called WinModems

These modems are internal modems that are totally software-driven (no smarts on the modem card). These require a Windows interface driver to work, and it seems that there have been more updates to this software than there are modems sold.
 

• Standards

At present there are new standards coming out from various vendors trying to get more speed over regular phone lines for Internet access. While most Internet Service Providers can match these standards, when doing a direct dial from your PC to another modem, you can run into incompatible settings.

With the advent of the cheaper modems and the ability to connect to the Internet, three different ways of connecting to the modem and the remote site are also available:

• Dial-up Networking

Modems at the remote end can be set to connect using PPP or SLIP (less common today). These provide a network connection to a remote network for  your local PC (connections to this type of modem are done using the Microsoft Dial-Up Networking software, not Anzio. Anzio would be set for a network TCP/IP connection type of Telnet once the dial-up networking is running).

• Direct Serial

Direct serial connections can still be made direct to a modem. This is how you would connect to an external modem as they simply act like a serial device. Internal modems may or may not have Windows drivers for them (these drivers utilize the TAPI protocol), but they should be able to be connected to as a serial device (if you have problems with this, see TAPI below).

You may also want to see our document on Troubleshooting a Serial Connection for more information.

• TAPI

TAPI is a device connection protocol available with Windows that allows Windows to control the modem interface. Many internal modems support TAPI, and recently, some external modems also support TAPI drivers. This lets Windows completely control the communications with the modem, unlike a serial link, where Anzio has control.

The cheaper internal WinModems should be connected using TAPI. However it is important to know that you have the most recent Windows modem drivers from the vendors. Some vendors have had to do patch releases every few months, so don't depend on the modem driver that came with the PC or on the vendors setup CD.

The Issues

The following should help direct you in tracking down modem problems.

Which version of Windows? Do you know the manufacturer of the modem? What are the modem settings?

You need to know which version of Windows you are using and the name of the modem or the manufacturer. The modem information is usually found under the Control Panel in the Modem dialog. This should also tell you the settings for the modem. Of importance to us here is the communications port. i.e. COM2.

Are you using an internal or external modem?

Does the phone line run right into your computer, or to an external device.

Make sure that you have not plugged the phone line into an ethernet card.

Many machines today ship with both and the connections in the back of the computer look similar (the larger of the two is ethernet).

If you are using an internal WinModem, check to be sure you always have the latest driver when you have modem problems.

Even some external modems can have special drivers, available from the vendor, that may be required and may help in solving problems.

Are you dialing into a dial-up network connection or serial?

Modems connect to the outside world either over a network connection or direct modem-to-modem (serial). Know which way the host  you are connecting to wants you to talk to it.

The two modems connect, but immediately disconnect.

When modems first connect, they negotiate a set of connection rules. If any of these fail, the connection will be dropped. Check to make sure there are no incompatibilities between the modem settings, such as parity, baud rate or the error correction protocol.

The two modems connect, but never go any further.

After the connection between the two modems is made, they then try passing data between the two machines. This may fail for a number of reasons. Check out the Problems in Auto-answer Modems on Unix/Linux Systems section below.

Do you see garbage characters even before login? Do you see garbage characters later, after login?

Garbage characters are an indication of something going wrong with the transmission between the various devices.  Check to be sure there is not phone line noise, that the serial connections are set correctly, etc.

On a direct serial link, garbage characters before or during login, may indicate a very noisy line, a wrong setting (baud or parity or data bits), or the wrong protocol (serial versus networking).

Garbage after login usually points to either the terminal types of Anzio and the host do not match, or you have intermittent noise (this can be created by phone activity such as "call waiting" or just a bad connection).

Do you get disconnected without notice?

If you are working fine, but after a period of time, you start receiving garbage, or if you work for a while and then the modems disconnect, you are probably experiencing one of two things: noise on the line (again check for call waiting or regular phone noise), or something from the host is causing a shutdown of that serial port (errors on the line or perhaps simply a timeout issue).

Connects Only Sometimes / Use To Connect But Does Not Now

One common problem with serial connections to UNIX / Linux hosts is the way in which the serial lines to the modems are set. Your modem normally expects the baud rate to remain constant and not change.

Within the UNIX world, it is not uncommon for a modem to be attached to a standard serial line. Most UNIX hosts set their regular serial lines to handle "Baud Cycling". The way this works is if there is an error on a line, the baud rate is then cut in half for the next try until it cycles around.
An example would be that you connect fine for weeks, then you get a noisy line. The UNIX system may drop from 19200 to 9600 (and then from 9600 to 4800 before returning to 19200). When you then try to connect again, your baud rate on the PC is now wrong.

This is something that can be taken care of at the host end. Change the host settings for modem port to always connect at a fixed baud rate.

Problems in Modems on Unix/Linux Systems

A frequent scenario: A user sets up a fairly new PC with Anzio, intending to dial directly into a Unix or Linux machine, possibly in order to work from home on the office computer. The answering modem may have worked well with other users on other PCs, perhaps for years. But with the new PC, it doesn't work. The modem sounds like it answers, the two modems link up, Anzio opens its main screen, but all the user sees is gibberish, or nothing.

At its root, this problem is caused by the combination of an improperly configured answering modem and a newer faster modem on the PC. Unfortunately, it is not trivial to fix.

When two modems connect, they negotiate to establish the best modem-to-modem baud rate. Usually this is the fastest rate that both modems support, unless that is limited by the telephone line conditions.

Now the answering modem must decide what baud rate to use to talk to the Unix (or Linux) host system. The host system has not sent any information to its modem yet, so that modem has no hint about a preferred baud rate. But the host's serial port (to which the modem is connected) is generally configured to talk at only one specific rate, such as 9600, 19200, or 38400.

If the answering modem is properly configured, it will always talk to the host at the prescribed rate, REGARDLESS of the modem-to-modem connection rate. All will be well.

However, most modems are set by default to talk at the modem-to-modem rate. If the modem has not been reconfigured, this leads to problems. Other modems may have connected at, say, 19200 for years, causing the answering modem to talk to the host at 19200, which works fine. But the new modem manages to connect at 24000 or 38400 or some other, higher, rate. The answering modem now talks to the host at, say, 38400, and a baud rate mismatch occurs, resulting in gibberish.

The PROPER solution is to reconfigure the host-end modem to always talk to the host at the correct baud rate. The procedure for doing so will vary depending on the kind of modem you have. Generally you will need to temporarily plug the modem into a PC running an emulation program (such as Anzio), or even into a dumb terminal. Then you will give it some commands to set configuration, and save those settings. Then you will plug it back into the host systems.
If your host-end modem is from US Robotics, see

   http://www.usr.com/support/s-modem/s-modem-docs/10563.htm

Short of that, you MAY be able to make the following work, with SOME modems. In Anzio, go to Communicate:Setup. Make sure "Modem" is selected. Then click the "Setup" button opposite "Modem". In there you may be able to set a maximum connection speed. Try settings from 38400 down. Note that many new modems will not respond to this setting, and so your only solution is to configure the host-end modem.

In Anzio, starting with version 12.6, you can go to Diagnose:Debug Communications after you connect with the host modem. One of the debug messages will tell you at what speed the modems actually connected. This may help you narrow down the problem.

Conclusion

Modems are common in almost all PCs. If you are unable to figure out what is up with the modem, always look at the vendors web site for updates. Many modems are unique in their operations and hence a manual is important. Most modems will work flawlessly, but when there's a problem, track down the manual and check for updates before you call.