A baud rate is the measurement of the number of times per second a signal in a communications channel changes its state. For instance, a 2400 baud rate indicates that the channel can change its state 2400 times per second. When changing its state, the channel can change from 0 to 1 or from 1 to 0, up to x, which is 2400 in this case.
Baud rate could also refer to the actual stage of the connection, such as frequency, voltage, and phase level.
A bit rate is the measure of number of data bits, represented by 0s and 1s, transmitted within one second in a communication channel. For instance, 2400 bits, represented by 2400 0s or 1s, can be transmitted in one second. Individual characters, such as letter or number characters, are made of several bits.
Baud versus Bits per Second
The baud unit took its name from an officer of the French Telegraph Service named Jean Maurice Emile Baudot. In the late 19th century, he developed the first uniform-length 5-bit code for the characters of the alphabet. It was essentially the modulation rate or the number of times a line changes its state per second.
This change is not always equivalent to bits per second (BPS). As a symbol may have more than two states, it may represent more than one binary bit. Binary bits always represent precisely two states.
The baud rate and the bit rate are equivalent when connecting two serial devices with direct cables. With this setup, running at 19200 BPS means that the line is changing state 19200 times per second.
Recent modems usually do not have equal baud rates and bit rates. As modems transmit signals over a telephone line, the baud rate is limited to a maximum of 2400 baud. The use of complicated phase modulation and data compression methods augment this physical restriction of the lines.
For instance, a modem from Bell (212A modem) transmits 1,200 bps of information, using a symbol rate of 600 baud. It uses ph.ase shift keying (PSK) modulation, and each symbol features one of the four phase shifts of 270, 180, 90, and 0. As the baud rate will often not equal the bit rate, the bit per second unit is often used in referring to the data rate of a modem.