While baud rate is sometimes confused with bit rate, and sometimes the two are numerically the same, they measure different things. The short story is that Bd x (bits/symbol) = data rate in bits per second.
There’s a bit (a byte?) more to it, however, and the history of "baud rate" might surprise you. In fact, the origin of the word dates back to decades before the first electronic computer was even created.
What is baud rate?
"Baud," abbreviated "Bd," refers not to some legendary mother computer’s operation, but to Émile Baudot, who invented the Baudot code in the 1870s. Like the Morse code, this system was used to encode letters over a telegraph line, but instead of a series of dots and dashes, it used 5 bits to represent each character. The shortened version of his name, “baud,” or “baud rate” is used to represent the number of signaling events -- i.e. symbols -- across a transmission medium per second.
Baud rate vs. bit rate: What's the difference?
In serial communication, such as between a computer and an Arduino board, data is transmitted directly by 1s and 0s. This means that for each change in state, a single bit is transmitted from one entity to another. So, in this still very much in-use implementation, baud, or symbols per second, is the same as bits per second.
However, with clever encoding, it's possible to transmit more than one bit of information for each symbol. Consider a situation where you were able to send either 0, 1, 2, or 3 volts over a transmission line once per second. The baud rate (i.e. signals per second) would be 1, however, the bit rate would be twice that, as each signal encodes four possibilities for two bits of information. In this case, you might assign binary signals 00 to 0V, 01 to 1V, 10 to 2V, and 11 to 3V. The choice here is somewhat arbitrary; it can be carefully selected to avoid signal confusion.
Telephone baud rate modem usage
Historically, perhaps the most common place where baud rate was encountered–and confused–is in the context of telephone modems. As of 2021, these devices encode digital signals for transmission over a land phone line. Such modems can be specified by baud rates, as well as their bit rates, generally expressed in kbps, or kilobits per second.
Early modems, like the Bell 103, and the later Bell 202 had bit rates equal to their baud rate. The 202, for example, featured a baud rate of 1200, and a bit rate of 1200 bits per second (1.2 kbps). Later modems, however, would employ methods that allow for multiple bits to be sent on each signaling event.
For example, modems using the V.32 standard were able to encode 4 bits per signaling event, and operate at 2400 baud. This gives a bit rate of 9.6 kbps. A variety of different techniques, along with an increased baud rate of 8000, allowed telephone modems to obtain a theoretical bit rate of 56 kbps. In reality, these speeds were normally lower, as per various communication issues, but a still exhibited a vast improvement over predecessors.
One might consider the baud rate the “floor” bit rate, but by no means the top limit. In a world where the baud rate was limited by land-based telephone lines, multi-bit signaling was key to the flow of information.
Modern baud rate applications
The same concept applies to RF applications in wide use today, where a limited spectrum range is used to transmit information through the airwaves. One obvious application is WiFi, where various modulation schemes allow for increased speeds, while still remaining within the rules of physics (and the FCC). Consider that the original IEEE 802.11 standard specified a rate of 2Mbit/s. Subsequent iterations improved, and today we have the IEEE 802.11ax standard which can reach up to 11 Gbit/s.
Such techniques are used in a wide variety of arenas to increase data. I touch on quadrature phase shift modulation in more detail here. To summarize, baud rate (Bd) is not the same as bit rate, or bits per second. To simplify, given various overhead factors, one is a function of the other, with Bd x (bits/symbol) = data rate in bits per second, or bps.
