:::::::: Troubleshooting288.txt :::::::: Troubleshooting 28,800 Connects _____________________________ Tips for High Speed Connections with V.34/FastClass Modems ____________________________________________________ Very few people can get consistent 28,800 bps connections. Speeds of 28,800bps require pristine line quality along the entire length of the connection. However, VFC and V.34 modems are capable of pushing the limits of analog phone lines, commonly offering connect speeds of 21.600, 24,000 and even 26,400 bps. Line Impairment _______________ Variations in line quality are typically the culprit for low connect 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. The U.S. Robotics BBS (708 982-5092) is a good one to try *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.) *Try plugging the modem to 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 for specific problems is provided at the end of this document. Software Setup and Modem Configuration ______________________________________ In addition to line impairments, modem configuration and software setup can (48 min left), (H)elp, More? 9Daffect connection rates and throughput. The Courier's default configuration allows 28,800 connections with other V.34 and VFC modems. Information on software setup is available through the U.S. Robotics fax-on-demand (FOD) system (800)762-6163 or through your software's support service. New Code (Courier V.34 modems only): Download the latest code from the U.S. Robotics BBS (see Upgrades in your user's manual or FOD document #565). The upgrade is free and the latest code revision can improve performance. 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: VFC connections can only switch rates down to 14,400 bps. If you connect using VFC and line quality drops below that allowable for a 14,400 connection, the modems will disconnect. If this occurs frequently for a particular call, you will want to disable VFC before calling that modem again. A different modulation protocol (V.32 bis, for example) will be established and will allow the modems to switch to lower bit rates as line quality warrants. Send the modem the following command to disable V.FC: ATS56=128 Remember to return the modem to its original configuration after the call is completed by resetting the modem or entering ATS56=0. Some VFC modems from other manufactures 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 VFC by entering ATS56=128. Remember to reset the modem or return it to its original configuration after the call is completed (AT&N0 or ATS56=0). Bandwidth Requirements ______________________ This information is provided for consultants or experienced modem users only. V.34 and VFC connection( rates are based on the available bandwidth over the phone line. The modems test the phone lines 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 V.34, the Courier modem 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. Symbol Carrier Bandwidth Maximum Rate Protocol Frequency Requirements Bit Rate 2400 V.34 1600Hz 400-2800 Hz 21600 V.34/VFC 1800Hz 600-3000 Hz 21600 2743 V.34 1646 Hz 274-3018 Hz 24000 VFC/V.34 1829 Hz 457-3200 Hz 24000 2800 V.34 1680 Hz 280-3080 Hz 24000 67-3267 Hz 24000 3000 V.34 1800 Hz 300-3300 Hz 26400 V.34/VFC 2000 Hz 500-3500 Hz 26400 VFC 1875 Hz 375-3376 Hz 26400 3200 V.34 1829 Hz 229-3429 Hz 28800 VFC 1920 Hz 320-3520 Hz 28800 3429 V.34 1959 Hz 244-3674 Hz 28800 NOTE: Minimum bit rates. V.34 will connect at speeds as low as 4800 bps with any of the above symbol rates. VFC 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 lots 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. (The I11 screen is an undocumented USR AT command). Using the above table, you can determine the approximate bandwidth detected by the modem. V.34 can be disabled by setting S56=64. VFC can be disabled by setting S56=128. (S-Register 56 is an undocumented AT command). Additions to Sportster V.FC and V.34 Command Set and S-Registers ________________________________________________________________ The following command and S-Register settings apply to V.FC and V.34 modems. &Nn Sets forced connect speed. If connection cannot be established at this speed, the modem will hang up. &N0 Variable rate &N1 300 bps &N2 1200 bps &N3 2400 bps &N4 4800 bps &N5 7200 bps &N6 9600 bps &N7 12,000 bps &N8 14,400 bps &N9 16,800 bps &N10 19,200 bps &N11 21,600 bps &N12 24,000 bps &N13 26,400 bps &N14 28,800 bps Register Default Function S39-44 0 Reserved S51 0 Bit mapped register. To set the register see the instructions for S13. Bit Value Result 0 1 MNP/V.42 disabled in V.22. 1 2 MNP/V.42 disabled in V.22bis. 2 4 MNP/V.42 disabled in V.32bis. 3-7 - Reserved. S54 0 Bit mapped register. To set the register see the instructions for S13. Bit Value Result 0 1 Disable 2400 symbol rate. 1 2 Disable 2743 symbol rate. 2 4 Disable 2800 symbol rate. 3 8 Disable 3000 symbol rate. 4 16 Disable 3200 symbol rate. 5 32 Disable 3429 symbol rate. 6 64 Disable Call Indicate (V.34 only). 7 128 Disable V.8 (V.34 only) S55 0 Bit mapped register. To set the register see the instructions for S13. Bit Value Result 0 1 Disable 8S-2D map. 1 2 Disable 16S-4D map. 2 4 Disable 32S-2D map. 3 8 Disable 64S-4D map. 4-7 - Reserved. Register Default Function S56 0 Bit mapped register. To set the register see the instructions for S13. Bit Value Result 0 1 Disasble non-linearing coding. 1 2 Disable TX level deviation. 2 4 Disable pre-emphasis. 3 8 Disable pre-coding. 4 16 Disable shaping. 5 32 Reserved. 6 64 Disable V.34 (V.34 only). 7 128 Disable V.FC. S57 0 Reserved. ------------------------------------------------------------------------------ :::::::: werbe.txt ::::::::