Can Non-56Kbps Modems Achieve Throughput Speeds Above 115,200bps?

Yes. To achieve speeds above 115,200bps, a better UART chip than the standard 16550-series chip found in modern PC serial ports and internal modems is required. 16650 UARTs have a 32-byte buffer, as opposed to the 16-byte buffer found in the standard 16550 UART. The 16650 is seldom used in PCs' built-in serial ports or as part of an internal modem, but it can be added by installing a high-speed serial port interface card (which might require you to disable your current COM ports).

Thus, to achieve the highest possible speeds, you need the following: an external modem capable of running at 230.4Kbps throughput, a 16650 UART chip in the serial (COM) port connected to the modem, and appropriate software drivers for the modem. Note that a 56Kbps-compatible modem isn't required! 230.4Kbps connections are available with anything from V.34bis to ISDN modems if the device is designed for that speed and you have the correct UART chip and drivers. But, as with the lower-speed throughput rates mentioned earlier, these speeds apply only to data that has not been compressed already.

 

External modems connected to the USB port instead of a serial port are capable of speeds above 115,200bps because of the faster throughput of the USB port. If  possible, use the USB port rather than the serial port if you need to connect an external modem to your computer.