FAQ - Com overruns

Reducing or Eliminating Comm Overruns

If you have a 386 or 486 IBM Compatible computer or an older MacIntosh with a 14.4, 28.8, 33.6 or 56K modem and you notice performance on the Internet is slow, sporadic, starts and stops and/or you lose your modem connection easily, you could be experiencing comm overruns which are a common problem when high-speed modems are paired with an older computer.

When comm overruns are present, often 28.8 or faster modems will seem to perform even worse than the slower 14.4 modems. An overrun results when a modem moves data faster than the computer's serial port, processor or hard drive can handle it. When this happens the modem must stop, then re-send the same data more than once. Performance of your connection under these conditions will seem either slow or it will work in bursts, starting, then stopping, sometimes for long periods. If the condition is bad enough, eventually the modem may give up and hang up your connection altogether. The problem gets even more serious when the modem is an external model and is plugged into an older serial port. The serial ports on most computers manufactured prior to 1994 not only are slower; not capable of transmitting data faster than about 19200 kbps, but they don't have a buffer to hold data temporarily if the computer's processor or hard drive is busy with other chores.

MacIntosh Computers

MacIntosh computers most affected by comm overruns are typically the LC series or older. On these older Macs I've found just one solution and that's to slow your modem speed settings in your PPP Dialer. This is done by starting your Config PPP or Free PPP Control Panel and clicking on the "Config" button in the lower left corner of the window. Make sure "Flow Control" is set for either "CTS Only" or "CTS & RTS". Change the Port Speed or Baud Rate to 19200. If this doesn't help you may need to lower it to 14400.

Also refer to your modem manual and make certain that the Modem Init String shown on this screen in the space below the phone number includes an AT command for Hardware Handshaking, also known as Hardware Flow Control or CTS & RTS Flow Control. Your Modem Manual or User's Guide should have a section detailing the various AT commands supported by the modem. Try to find the command for Hardware Flow Control or one of the terms above and include it on the "Modem Init" line on the configuration screen of your PPP dialer program.

For most Mac modems the Modem init string, AT&F1&K3 will work. If your modem will not dial with this setting try AT&F&K3 In other words leave out the 1 after &F. Exceptions are US Robotics modems and their clones made by other manufacturers. These modems use a different set of commands and the modem init string AT&F1 or occasionally just AT&F will usually work for these modems.

IBM Compatibles - Windows 3.1 or 3.11 for Workgroups

If you are using Trumpet Winsock to dial in to your Internet account, comm overruns will be reported on Trumpet's screen as they happen and you will see messages documenting them when you use Trumpet to disconnect from your Internet session.

Since most other Internet Dialers do not diagnose comm overruns, I recommend using Trumpet Winsock if you suspect you may be having this problem.

Following are some steps you will want to take to correct comm overrun problems. I've found that taking all these steps will help improve high-speed modem performance on almost all machines running either Windows 3.1 or Windows 3.11 For WorkGroups:

1. If your modem is an external model, check to see if your modem's comm port has 16550 UARTs. In Windows 3.1 and 3.11 For WorkGroups you can check by going to the DOS prompt and typing MSD to start Microsoft Diagnostics, then hitting your "Enter" key. After MSD has started look for the Comm Ports Button and either click on it with your mouse or, if your mouse doesn't work in DOS mode, press the letter C on your keyboard. You will see your comm ports listed and at the bottom of each column the type of UART will be shown.

   If the number shown for the UART is 8250 rather than 16550 or some variation, your serial ports are the older, slower type without buffering and you can expect to have comm overruns if you use an external modem on one of these ports. For this reason, internal modems are recommended for these machines since not only are they usually less expensive than external models, but they have the faster, buffered 16550 UART chip built into the modem card.

   If you already own an external modem and don't want to replace it, you should install a high-speed serial card with 16550 UARTs and plug your modem into it rather than the built-in ports that came with your computer. Most computer shops stock these and the cost should be $20 to $30. The card will fit into one of the expansion slots on your computer's mother board. If you're not comfortable installing such components yourself, the cost to have the computer shop do it for you when you purchase it there is usually nominal.

2. Make the following changes to your Windows system.ini file. You can open this file in your Windows Notepad (usually found in your "Accessories" program group).

   Scroll down through this file until you find the [386enh] section. Somewhere in this section of the file add the following lines (IMPORTANT! Each place in these lines where I have shown the n in bold italics, substitute the number of your modem's comm port (this will be 1,2,3 or 4) for the letter n:

   MinTimeslice=20
   WinTimeslice=100,50
   COMnFIFO=1
   COMnBuffer=1024

   If your version of Windows is 3.11 For WorkGroups add these additional lines:

   COMnRxTrigger=4
   ForceLazyOff=C

   Save this file, close Notepad, exit from Windows and re-start your computer for the changes to take effect.

3. If your computer with its current configuration will run with 32 bit disk / file access turned on, I've found that turning this option on eliminates comm overruns on many computers when combined with the other measures in this document.

   Open the Windows Control Panel (in your "Main" program group) and double-click on "386enh" then click on "Virtual Memory", then on "Change". If you have the option(s) at the bottom of this window referring to 32 Bit Disk Access and/or 32 Bit File Access, verify that the boxes next to these item are checked. Make sure on this same screen that the "New" size of the swap file is the same as the "Recommended Setting", then click OK and restart Windows when prompted. (Ignore the message you'll probably get regarding 32-bit Disk Access and some portable computers)

   However, if you receive an error message regarding 32-bit Disk Access when Windows is re-starting you will need to go back to the "386enh" control panel and reverse the changes you just made.

4. Hardware Handshaking, also called Hardware Flow Control or CTS & RTS Flow Control is an important aspect of high-speed modem communications and it is imperative that you be sure your Internet dialer software is using an initialization string that includes commands to activate this feature.

   For the vast majority of modems, recalling the factory default profile will activate Hardware Handshaking so if you can't find your modem's manual you can set your dialer software to recall these defaults. The command for this is most often &F1 although sometimes it's just &F (Although Supra uses &F2)

   The specific command for most Rockwell based modems to activate Hardware Handshaking, Hardware Flow Control or CTS & RTS Flow Control is &K3 The next most common is \Q3

   In Trumpet Winsock it's easy to check and change the Modem Init String. Just start Trumpet, then click on the Dialler menu, then on Edit Scripts, then double-click on login.cmd. This will open the login.cmd document in Windows Notepad. Scroll down a few lines until you see the line that looks something like:

   $modemsetup = "&c1&k3"

   Edit the portion of this line between the two quotation marks to reflect any commands you want to add or delete. Your modem's manual is the best guide for the correct commands but if you can't find the manual, try the following to see which one seems to perform best for you. Try them in order, saving the file, then dialing in to your Internet account and doing a little Web surfing in between changes. Use the setting which gives you the best performance:

   &F1 (If this setting gives you an ERROR message, use just &F, without the 1 when trying all the examples below. Otherwise stick with &F1 for each of these examples)

   &F1&K3

   &F1\Q3

   &F1\N3&K3

   &F1\N3\Q3

Windows 95

1. Double-click on the "My Computer" icon, then on the "Dial-up Networking" folder. Use your right-hand mouse button (rather than the normal left button) to click on the "Local Access" icon, then choose "Properties" from the drop-down menu that will appear. Use the left button to click on "Properties".
2. Click on the "Configure" button, then on the "Connection" tab. Now click on "Port Settings".
3. Make certain the "Use FIFO Buffers" box is checked", then slide the adjustment bars for both the Send and Receive buffers all the way to the left before clicking on "OK".

Last Resort Steps - All Windows Versions

1. If none of the above steps help, try changing your modem's settings to a different Comm Port / IRQ combination. Depending on your modem, this is accomplished with either Jumper settings or Dip Switches on the modem itself or, in the case of some newer Plug and Play modems these settings are modified with a software program that came with the modem. You will also need to make adjustments in Windows and in your dialer software to reflect the new settings. How to do this is beyond the scope of this document but if you'd like to try it you'll need the help of your modem's manual. If you can't figure it out by reading the modem manual try asking for the help of a more technically oriented friend or a computer shop.

   I've found that Com3 or Com4 using IRQ5, IRQ7 or IRQ9 are combinations that work well on many computers. Since your Sound Card or Printer Port may be using one of these IRQ's, changing the IRQ sometimes creates a hardware conflict between your modem and one of these devices so you may have to try several combinations before you find the best settings for your particular computer / modem configuration.

2. This is the last resort. Don't perform this step unless you've tried everything above and all have failed to eliminate your comm overruns.

   Reduce your modem speed by setting your dialer software for a lower port speed or baud rate:

   In Trumpet Winsock this is done by opening Trumpet and clicking on the "File" menu, then on "Setup". Change the "Baud Rate" setting to 19200. Also make sure "Hardware Handshake" and "Van Jacobson CSlip Compression" are checked before clicking "OK" and following instructions to close and re-start Trumpet. If your problem is particularly severe and lowering the speed to 19200 still doesn't fix it, you can try lowering to 14400 or even 9600

   Windows 95 users can double-click on the "My Computer" icon, then on the "Dial-up Networking" folder. Now click once with your right-hand mouse button and choose "Properties" from the drop-down menu that will appear with your normal left mouse button. Click on "Configure", then adjust the speed down to 19200, 14400 or 9600 testing your connection in between each change to see which speed works best.

The above information and much more is covered in detail at the following Internet sites. I recommend these web pages as required reading for anyone wishing to optimize their modem connections whether they think they are now having problems or not:

John Navas' High Speed Modem FAQ

Curt's High Speed Modem FAQ

High Speed Modems and Windows 95

"Ask Mr Modem" - Suggested modem initialization strings for most common brands of modems

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"Overruns FAQ" by Mark Carrier





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