The port is represented to the CPU by means of a UART chip. You can see some of the UARTS settings in the modem-setting box under windows. For instance the ´maximum connect´ speed is not the Modems Max speed but the maximum data rate between the Computer and the UART. The UART acts as a controller for the Comm-port, modern systems having UARTS that support data rates up to 155Kb/S. In my experience it is best to leave this set to 115K so the CPU can dump its data to the modem quickly.