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Getting Peak Performance From Your Modem

Very few people can get a consistent 28,800 (V.34 modem) or 33,600 (33.6k modem) bps connection. Speeds of 28,800 bps or faster, require pristine line quality 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.

Line Impairment

Variations in line quality are typically the culprit for low connection rates. Everyone occasionally gets “a bad line” and has to hang up and call again to get a better connection. However, if you find that you never or rarely connect at rates above 19200 bps, you will want to investigate the line quality of your connections. Begin with the following:
  • Try calling a different location. Line quality differs from region to region and it may be a problem with the lines or modem at the other end of a particular call.
  • Try connecting with a local call. 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 that 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.

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, they 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 modem has S-Registers in the thirties, send the modem ATS32.1=1. If your modem has S-Registers that go into the fifties, send the modem ATS56.7=1.

Remember to return the modem to its 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 make 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).

Bandwidth Requirements

This information is provided for consultants or experienced modem users only!!
33.6k, V.34 and V.FC connection rates are based on the available bandwidth over the phone line. The modems test the phone line before establishing a connection rate and will select the highest symbol rate allowable. In general, a higher symbol rate allows greater speeds, but requires greater bandwidth.

The approximate bandwidth requirements for each symbol rate are shown in the chart below. Note that when using 33.6k or V.34, our modems can operate at either of two frequency ranges for any of the given symbol rates. This allows it to select the frequency range of better quality for that call.

V.34 Bandwidth Requirements

Symbol Rate Protocol Carrier Frequency Maximum
Bit Rate
Bit Rate (33.6k)
2400 V.34 1600 Hz 21600 21600
  V.34/VFC 1800 Hz 21600 21600
2743 V.34 1646 Hz 24000 26400
  V.FC/V.34 1829 Hz 24000 26400
2800 V.34 1680 Hz 24000 26400
  V.FC/V.34 1867 Hz 24000 26400
3000 V.34 1800 Hz 26400 28800
  V.34/V.FC 2000 Hz 26400 28800
  V.FC 1875 Hz 26400  
3200 V.34 1829 Hz 28800 31200
  V.FC 1920 Hz 28800  
3429 V.34 1959 Hz 28800 33600

NOTE: Minimum bit rates. V.34 and 33.6k will connect at speeds as low as 4800 bps with any of the above symbol rates. V.FC does not support rates below 14,400 bps. If the bit rate is much lower than the maximum bit rate supported by the symbol rate, the phone line has a lot of noise or other impairments on it.

Determining A Connection's Bandwidth

The symbol rate and carrier frequency used during a connection is displayed on the modem’s I11 screen. Using the above table, you can determine the approximate bandwidth detected by the modem. V.34 can be disabled by setting S32.2=1/S56.6=1. V.FC can be disabled by setting ATS32.1=1/ATS56.7=1.
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