Troubleshooting 28.8 Connections
Tips for High Speed Connections with V.34/V.FC 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, V.FC 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.
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
- 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.
Software Setup and Modem Configuration
In addition to line impairments, modem configuration and software setup
can affect connection rates and throughput.
To begin with, put your 28.8 modem on COM2 if possible because COM2 has a
higher interrupt priority than COM1. This is especially important if you
have a serial mouse, and have a slower system.
If you are using Windows 3.x, there are some additional system
When using ISDN devices that connect to COM ports, it is VERY important to
setup your Windows program properly in order to avoid comm overruns (i.e.,
missing incoming data). The multi-tasking MS Windows can be a hostile
environment for performing high speed communications through a serial
port. Overruns mean that data is not being processed as fast as it is
arriving. This results in data is being lost, characters being
overwritten, and overall communications speed is being slowed due to
retransmission of data. Minimize problems by assuring that the Windows
SYSTEM.INI file, under the [386Enh] section, is properly set up. First,
make sure that the following two lines, if present, read:
These values are the Windows defaults, so the above lines don't need to be
included. But if they are included, use the above values. A smaller value
for MinTimeSlice will make multitasking appear smoother, but will decrease
overall system performance. The WinTimeSlice values specify the relative
amount of processing time (based on the MinTimeSlice setting) given to all
Windows applications compared to non-Windows applications. Both settings,
if improperly set, will cause comm problems with high speed modems.
Also, add the following lines to the [386Enh] section:
Where "n" is the comm port your device is connected to - i.e., COM2FIFO=1
if your device is on Com2. Setting COMnFIFO to 1 enables the FIFO buffer
on your 16550 UART, but doesn't hurt even if you don't have a 16550. Note
that Windows 3.0 and earlier versions do not support the FIFO feature of
the 16550 UART - upgrade to Windows 3.1 or better yet, Windows 95, which
has a better architecture for handling serial devices than Windows 3.1).
The COMnBuffer line specifies the number of characters that the device on
the corresponding serial port will place in the buffer. The Windows
default of 128 bytes is not optimum for high speed modems. Increasing the
buffer to 1024 lets Windows process data in more efficient chunks.
2) COMM DRIVERS
If you run Windows 3.1x (rather than Win95), it may be desirable to
replace the default Windows COMM.DRV in SYSTEM.INI with a third-party
driver. While the Windows 3.1x COMM.DRV is able to support 28.8 modems on
a fast system, it is not written to take best advantage of the FIFO buffer
on 16550 UARTs. If you have comm overrun problems that you can't otherwise
- it contains cybercom.drv and installation instructions.
NOTE: If you run Windows 95, DO NOT replace COMM.DRV -- Win95 has an
improved communications architecture which works fine without 3rd party
TSR's, certain SCSI drivers, certain memory managers, screen savers, and
some S3 video cards -- all can cause comm problems. Try running without
these, switch to standard VGA mode, etc. to see if you can eliminate
problems you are having. Always make sure you have the latest drivers and
programs updates -- many manufacturers have fixed poorly written code that
was causing comm programs.
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 modems 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.32 bis, for example)
will be established and will allow the modems to switch to lower bit
rates as line quality warrants.
Remember to return the modem to its original configuration after the
call is completed by resetting the modem. Some V.FC 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. Remember to
reset the modem or return it to its original configuration after the call