High Speed Tips

Using a V.FC/V.34 or Faster Modem

The following document provides information which may be valuable for troubleshooting v.34 connections and x2/v.90 connections. Additional information for x2/v.90 users is also available. (Adobe Reader is required to view the file).

Very few modem installations can reach a consistent 28,800(v.34 modem), 33,600(33.6k modem), or 56k(x2 modem) bps connection. Speeds of 28,800 bps or faster, require perfect, almost ISDN quality, line conditions along the entire length of the connection. However, V.FC, V.34, and 33.6k modems are capable of pushing the limits of analog phone lines, commonly offering connect speeds of 21600, 24000, and even 26400bps or higher.

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Line Impairment
Variations in line quality are typically the cause of low connection rates. At one point or another, everyone will experience “a bad line” connection, and have to hang up and call again. However, if you find that you never or rarely connect at rates above 19200 bps, you will need to investigate the line quality of your connections. Begin with the following:

Before going through the process of checking your phone line, make sure the software you are using with your modem is set to a port rate of 38400 or higher. If you have your port rate set to 19200, that's the maximum connection you will receive.
Try calling a different location. Line quality differs from region to region and it may be a problem with the lines, or the modem at the other end of the system you are calling.
If you are calling long distance, try making a local connection. Sometimes the connections within a long distance call can cause impairments (if this isolates the problem, you can try switching long distance companies or request they try to fix the problem.).
Try plugging the modem into a different phone line or wall jack.
If you know someone else in your area with a high speed modem, ask what type of connections they make. Try making the connection from their location.
Be sure that your modem firmware is up to date and that you have installed any new modem drivers, INF files, or flash ROM upgrades available from your modem manufacturer.

If you encounter the same low connection rates, the problem may be resulting from impairments along the lines running to the local telephone company or within your home or office. Your telephone company or a private consultant may be able to help. Information on how to troubleshoot a line or specific problem is provided at the end of this document.

Software Setup and Modem Configuration
In addition to line impairments, modem configuration and software setup can affect connection rates and throughput. Our high speed modems default configuration allows for 28,800 or 33,600 connections with other 33.6k, V.34 and V.FC modems.

Dropped Connections
Dropped connections can occur when there is a sharp decrease in line quality during a call. V.34 modems will switch to rates as low as 4800 bps to compensate for these changes. If the loss of quality is extremely severe, the modem will drop the connection.

Dropped v.FC Connections
V.FC connections can only switch rates down to 14,400 bps. If you connect using v.FC, and line quality drops below that allowable for a 14,400 connection, the modem will disconnect. If this occurs frequently for a particular call, you will want to disable v.FC before calling that modem again. A different modulation protocol(v.32bis, for example) will be established and will allow the modems to switch to lower bit rates as line quality warrants. If you want to disable v.FC, and your modems maximum S-Register settings are in the thirties, send ATS32.1=1 to the modem . If your modem has S-Registers that go into the fifties, send ATS56.7=1 to the modem.

Remember to return the modem to it’s original configuration after the call is completed by resetting the modem, or entering ATS32=2 or ATS56=0 depending on the model you have as stated above.

Some v.FC modems made by other manufacturers, do not support rate switching. These connections are more likely to drop. For these calls, you can force a lower connect speed by locking the modem to a lower link rate via the &N command, or disable v.FC by entering ATS32.1=1 or ATS56=128. Remember to reset the modem or return it to its original configuration after the call is completed(AT&N0 or ATS32=2/ATS56=0).

Source: US Robotics/3Com Technical Support

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