Baud [ bɔ ː d], Bd is the unit of symbol rate ( walking speed ) in communications engineering and telecommunications. 1 is the baud rate, if one symbol is transmitted per second. Each symbol corresponds to a defined, measurable signal change in the physical transmission medium. The baud rate of a data transfer must be equal to the transmitting and receiving side.

The name comes from Jean -Maurice -Émile Baudot, who invented in 1874 the Baudot code.


A symbol represents, depending on the coding different number of bits of a data stream.

Example: Gigabit Ethernet 1000BASE- T has a symbol rate of 125 Mbaud. With the multi-level pulse amplitude modulation (PAM ) in the form of 5 -PAM are transferred 2 user bits per symbol, the information content is ld ( 5) ~ 2.32 bits per symbol. In addition, the same four pairs of wires are used. This enables a data transfer rate is achieved.

For n (aggregated ) bits, a set of 2n different symbols is needed - ie per bit combination there is a corresponding symbol. Due to the exponential increase in the number of symbol needs to be transmitted bits per symbol is usually limited to about 10 bits / symbol as the upper limit. The limitation arises at the receiving end by the difficulty of being able to reliably distinguish between the individual symbols. Especially as noise interference effect plays an essential role here.

Confusion with the bit rate

The baud rate is often confused with the data transfer rate, which indicates the amount of data transmitted per unit of time in bits per second than bit rate. The baud rate is, however, the number of symbols per time unit. At a transmission time of a symbol of, for example 200 milliseconds, the baud rate is 5 baud.

Binary modulation methods have only two states of the carrier, what exactly one bit per state change or symbol corresponding to the transmission, which bit and baud rate are the same for binary transfers as a special case. Modulation scheme having more than two symbols having a higher bit or baud rate. This is equivalent to a higher spectral efficiency. For example, the 4B3T code transmits four bits per three symbols and the 2B1Q code two bits per symbol. The more bits are transmitted with one symbol (for the same baud rate, over the same physical channel ), the more susceptible to interference is the transmission.

Sources and Literature

  • Karl -Dirk Kammeyer: message transmission. 4th edition. Vieweg Teubner, 2008, ISBN 978-3-8351-0179-1.
  • Unit (data transmission)