When referring to modem transmission, the terms baud rate and bite per second are sometimes used to describe the speed of the channel. The term baud rate is sometimes used in low speed communication channels (usually a modem). Sometimes, it is incorrectly understood that bits per second (bps) and baud rate (bytes per second) have the same meaning. The root of the problem is the fact that as communication channels have become faster and other methods are being used, such as broadband, they generally use bits per second as an indication of channel speed…
Baud rate is a measure of the number of times per second a signal in a communications channel varies, or makes a transition between states (states being frequencies, voltage levels, or phase angles). One baud is one such change. Thus, a 300 baud modem’s signal changes state 300 times each second, while a 600 baud modem’s signal changes state 600 times per second. This does not necessarily mean that a 300-baud and a 600-baud modem transmit 300 and 600 bits per second.
Bits per second are a measure of the number of data bits (digital 0’s and 1’s) transmitted each second in a communications channel. This is sometimes referred to as “bit rate.” Individual characters (letters, numbers, etc.), also referred to as bytes, are composed of several bits. While a modem’s bit rate is tied to its baud rate, the two are not the same.
Depending on the modulation technique used, a modem can transmit one bit, more than one bit or less than one bit with each baud or state change. To illustrate this, first consider a modem with a baud rate of 300, using a transmission technique called FSK.
When using FSK, each baud (state change) transmits a bit; only one change of state is required to send a bit. Therefore, the modem’s bps rate is also 300: 300 baud per second X 1 bit per baud = 300 bps. Similarly, if a modem operating at 1200 bauds would use a state change to send each bit, that modem’s bps speed would be 1200. Now, consider a hypothetical 300-baud modem using a modulation technique that requires two changes of state to send a bit, which can also be seen as 1/2 bit per baud. The bps rate of that modem would be 150 bps: 300 baud per second X 1/2 baud per bit = 150 bps.
In modern industries there is a technique that has a higher bit rate than the baud rate. For example, 1200 bps modems that comply with the Bell 212A standard (which includes most 1200 bps modems used in the US) operate at 300 bauds and use a modulation technique called phase modulation that transmits four bits per baud. These modems are capable of operating at 1200 bps, but not at 2400 bps because they are not 1200 baud modems; They use a baud rate of 300.
(300 bauds) * (4 bit/baud ) = 1200bps