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Troubleshooting288.txt
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Troubleshooting 28,800 Connects
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Tips for High Speed Connections with V.34/FastClass Modems
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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
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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
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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.
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