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. See Stty
The top speed of 115.2k has been standard since the mid 1990's. But by the year 2000, most new serial ports supported higher speeds of 230.4k and 460.8k. Some also support 921.6k. Unfortunately Linux seldom uses these speeds due to lack of drivers. These ports behave just like 115.2k ports unless the higher speeds are enabled by special software. To get these speeds you need to compile the kernel with special patches but it seems that the 2.4 kernels not yet supported
The patch software is fairly simple since it only needs to enable the higher speeds by dialog with the hardware. But it's not quite as simple as just putting an enable byte in a hardware register since the registers weren't designed for this. But unfortunately, there is no standard way to enable the higher speeds so the driver needs to support a variety of hardware.
A patch to support high-speed is called shsmod (Super High Speed Mode). There are both Windows and Linux versions of this patch. See http://www.devdrv.com/shsmod. For Linux (as of late 2001), most of the documentation is only in Japanese and the patch is for the old kernel 2.2.x. There is also a module for the VIA VT82C686 chip http://www.kati.fi/viahss/. Using it may result in buffer overflow.
For internal modems, only a minority of them advertise that they support speeds of over 115.2k for their built-in serial ports. Does shsmod support these ??
Here's a list of commonly used divisors and their corresponding speeds (assuming a maximum speed of 115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24 (4.8k), 48 (2.4k), 96 (1.2k), etc. The serial driver sets the speed in the hardware by sending the hardware only a "divisor" (a positive integer). This "divisor" divides the maximum speed of the hardware resulting in a slower speed (except a divisor of 1 obviously tells the hardware to run at maximum speed).
There are exceptions to the above since for certain serial port hardware, speeds above 115.2k are set by using a very high divisor. Normally, if you specify a speed of 115.2k (in your communication program or by stty) then the serial driver sets the port hardware to divisor 1 which sets the highest speed. If you happen to have hardware with a maximum speed of say 230.4k (and the 230.4k speed has been enabled), then specifying 115.2k will result in divisor 1. For some hardware this will actually give you 230.4k. This is double the speed that you set. In fact, for any speed you set, the actual speed will be double. If you had hardware that could run at 460.8k then the actual speed would be quadruple what you set. All the above assumes that you don't use "setserial" to modify things.
To correct this accounting (but not always fix the problem) you may use "setserial" to change the baud_base to the actual maximal speed of your port such as 230.4k. Then if you set the speed (by your application or by stty) to 230.4k, a divisor of 1 will be used and you'll get the same speed as you set.
If you have old software which will not permit such a high speed (but your hardware has it enabled) then you might want to look into using the "spd_cust" parameter for setserial with "divisor 1". Then when you tell the application that the speed it 38,400, it will use divisor 1 and get the highest speed.
There are some brands of UARTs that uses a very high divisor to set high speeds. There isn't any satisfactory way to use "setserial" (say set "divisor 32770") to get such a speed since then setserial would then think that the speed is very low and disable the FIFO in the UART.
Note that the baud_base setting is usually much lower than the frequency of the crystal oscillator in the hardware since the crystal frequency is often divided by 16 in the hardware to get the actual top speed. The reason the crystal frequency needs to be higher is so that this high crystal speed can generate clock ticks to take a number of samples of each bit to determine if it's a 1 or a 0.
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