Baud Rate, Signalling Rate and Data Rate

The term signalling rate (or baud rate) is used to describe the number of signalling elements (bauds) that can be transmitted in one second. The baud is named after the inventor of the Baudot telegraph code, J.M.E. Baudot. Signalling elements are generally represented either by a change in voltage on a transmission line (digital signalling) or by changes in the phase, frequency or amplitude of an analogue carrier signal (analogue signalling). The terms baud rate and data-rate (usually expressed as bits per second) do not mean the same thing, and are sometimes confused.

If only one bit of information is encoded in each signalling element, then the baud rate and the data rate (or bit-rate) will be the same. If two signalling levels are used, each element will represent either one or zero. If more than two signalling levels are used, however, it becomes possible to encode more than one bit per signal element. If four signalling levels are used, for example, each signalling level can represent two bits, and the bit-rate will be twice the baud rate.