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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting Modems | Errors and Download Problems ]

Data transfers over the modem are experiencing errors, data is being lost during download, or downloaded files are being corrupted

Explanation: You are able to connect to the remote host and establish a session. When you download information, however, the communications program you are using is reporting errors, or you may see that some of the data you downloaded is missing or corrupted. You may hear an audible "beep" from the speaker when you encounter the lost data.

Diagnosis: The most likely cause of this problem is what is called a "data overrun". What is happening, simply, is that the modem is being fed data from the remote host faster than it can pump the data through to the PC. If this gets bad enough, the incoming data overwrites the previous data in the modem before the modem gets a chance to send it to the PC, and some of the information is lost. Communications software will often detect this condition using an error-detection algorithm called CRC (cyclical redundancy check) and will report it to you as a "CRC error".

Recommendation:

  • If you are using a high-speed external modem on an older PC, typically meaning a modem faster than 14.4 KB on an older 386 or earlier machine, you will likely experience overrun problems because the PC is not fast enough to handle the input from the modem. This may happen with a 14.4 modem or a 486 or faster machine, but it is less likely. The key component that controls the ability to handle the input is the UART, the key component of the serial (COM) port that handles the transaction of data. You need a 16550 UART to handle higher-speed modems or to tackle the overrun problem, because this UART includes a 16-byte buffer to handle those times when data is running into the port faster than it can be processed.See here if your COM ports don't use 16550 UARTs or you aren't sure if they do or not.

Note: Internal modems include their own UART and are not subject to this problem.

  • Make sure you carefully double-check the flow control settings for your modem setup. Flow control (usually called something like "Xon/Xoff" or "RTS/CTS") refers to the protocol used to regulate the flow of information from the modem to the PC. If it is set up incorrectly, this can lead to overrun problems.
  • If you have power management enabled, try disabling it, since this can interfere with modem operations.
  • Ensure that you don't have a resource conflict with the modem or the serial port it is using.
  • Try using a different COM port, if this is an external modem, if possible. There could be a problem related to the port.
  • Screen savers can sometimes cause overrun problems by kicking in at the wrong time. Some of these can "steal" a large number of clock cycles and make the processor unable to keep up with the modem, especially on slower machines. Try disabling the screen saver. If this fixes the problem, try switching to a simpler screen saver (that doesn't require so much processing time) or just leave it off (since they really are not necessary).
  • If the problem with overruns is seen when the hard disk is in action (which can be hard to discern in any event since the hard disk is usually active while downloading) this could be a problem related to the enabling of block mode for the hard disk. Block mode causes hard disks to perform larger transfers than they normally would on each interrupt. This means that the system bus is "busy" for a long period of time and the modem may overrun waiting for its turn to use it. Block mode is normally controlled by this BIOS setting. Note that disabling this feature can lead to a small decrease in hard disk performance.
  • If you have any automatic virus scanning software running, try disabling it. I have heard reports of possible corruption of downloaded files while automatic virus scanning is running.
  • Try looking in John Navas's modem FAQ for some other possible causes of this problem.
  • The modem may be bad. Double-check its configuration first, then if you can, try swapping another modem into the PC and see if the problem clears up.

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