By "speed" we really mean the "data flow rate" but almost everybody incorrectly calls it speed. The speed is measured in bits/sec (or baud). Speed is set using the "stty" command or by a program which uses the serial port.
You need to find out the highest speed supported by your hardware.
As of late 1998 most hardware only supported speeds up to 115.2K bps.
If you have a application program that doesn't show high enough
speeds in its menu, then there are some options you can give to the
setserial command so that a low speed command from the
communication program will actually result in a higher speed. With
these options, when you set the speed for 38400 the actual speed will
be higher. See the man page for "setserial" and search for speed_hi,
spd_cust, baud_base, and divisor. Note that you must set baud_base to
the actual maximum speed of the hardware. This speed is usually lower
than the frequency of the crystal oscillator in the hardware since the
crystal frequency is often divided by 16 in the hardware to give the
maximum clock speed for bit pulses. The reason the crystal
frequency needs to be higher is so that its full clock signal can be
used to take several samples of a bit pulse at the highest speed. If
your software doesn't permit typing in a speed of 230400 to an
application program but your physical serial port supports 230400 then
you could try the following:
setserial /dev/ttyS2 spd_cust baud_base 230400 divisor 1
Then to get a speed of 230400 you must claim to the application program
that the speed is 38400. See the man page for setserial for more
info about this.
If you use setserial test it on the command line first, and then
when you have it working, put it into /etc/rc.d/rc.serial
or /etc/rc.boot/0setserial so
that they are run at startup. Make sure that you are using a valid
path for setserial, and a valid device name. You can check the
settings of a serial port by running:
setserial -a /dev/ttyS3
If you are seeing slow throughput and serial port overruns on a
system with (E)IDE disk drives, you can get hdparm. This
is a utility that can modify (E)IDE parameters, including unmasking
other IRQs during a disk IRQ. This will improve responsiveness
and will help eliminate overruns. Be sure to read the man page very
carefully, since some drive/controller combinations don't like this
and may corrupt the filesystem.
Also have a look at a utility called irqtune that will change
the IRQ priority of a device, for example the serial port that your
modem is on. This may improve the serial throughput on your system.
The irqtune FAQ is at
http://www.best.com/~cae/irqtune.