Increasing Modem Speed 

External modems hook up to the back of your computer via the serial port. With 33.6, 56K, and especially ISDN modems, the data stream can push the limit of the throughput of the serial port. This can happen when you are using highly compressed data. The problem can double if you are using several devices through your serial port.

The problem arises because the maximum throughput of a standard serial port is 115Kbps. Even a 28.8 modem with high compression can transmit data at this rate. With faster modems, the problem shows up. With high speed ISDN modems, the problem is noticeable. Obviously, speed is wasted when using a standard serial port.

There are two main reasons as to why this occurs. First, the serial port has a set physical port controlled by a clock crystal soldered onto your motherboard. The clock runs about 1.84MHz, which limits the throughput to 115Kbps. Second, your serial port's buffer is too slow. You are probably using a 16550 UART, or Universal Asynchronous Receiver/Transmitter. The UART manages the flow of data through your serial ports. The 16550 UART has only a 16-byte buffer. The buffer is used to temporarily store incoming data while your system is tending to other duties. If the machine does not empty this buffer before it fills up, you lose data. Under heavy multi-tasking or high data rates, this data loss occurs quite often. When this happens, you may see the system start data streams over again, or possibly just hang up on a download.

The way around this is to install a high-speed serial port card. What makes them high-speed? Well, first, the clock crystal is located on the card itself and ticks at 7.37MHz or 14.7MHz. This is much faster than the 1.84 MHz motherboard clock. Second, these cards have a faster 16750 UART with much larger buffers. These serial port cards can operate at around 1 MB per second. They come with Windows drivers that show Windows how to use the higher speeds. They also enable "clock multipliers" in order to be compatible with older devices that cannot work with these higher speeds.

Installation is a snap. Plug the card into the slot, have Windows 95 detect it, and when prompted for the drivers, install them. Then restart. If you are installing in another OS, just make sure the card comes with drivers for your OS. In a non-PnP OS, you might have to flip some jumpers.

If you are using an internal modem, you are limited to 115Kbps. But, with an external modem, installing a high speed serial port card can remove the bottleneck you would hit using high speed modems.