Bauds and Bits

2400 baud. 9600 bps. These are familiar terms for any modem user.

The terms baud and bps are often used interchangably. However, the two are not the same at all.

The carrier signal is characterized by the number of signal intervals, or pulses, that are transmitted per second. Each pulse is called a baud.

Bps stands for bits per second. Bps is a measure of how many bits can be transmitted during one pulse (one baud). So,

bps = baud * number of bits per baud.

The two are often confused because early modems used to transmit only 1 bit per baud, so a 1200 baud modem would also be transmitting 1200 bps.

These days, we need higher speeds. But for two-way communications, the baud limit is 1200 baud. So the technique is to try and "pack" as many bits as you can into 1 baud.

A modem operating at 9600 bps is still only transmitting at 1200 baud. But it is "packing" 8 bits into each baud:

9600 bps = 1200 baud * 8 bits per baud.

The general technique for "packing" bits into a baud is called quadrature amplitude modulation.