300 baud modems used frequency shift keying, meaning they generated two different sounds for 0 and 1 bits. The sound frequencies were high enough that the recei

300 baud modems used frequency shift keying, meaning they generated two different sounds for 0 and 1 bits. The sound frequencies were high enough that the receiving modem didn't need a full 1/300 of a second to decide if it was getting a 0 or a 1. Based on that, if you had a good phone connection it was possible to reconfigure the UARTs on both ends to operate at 450 baud instead of 300. This gave you a 50% increase in speed!