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Physical unit
Unit name baud
Unit symbol
Physical quantity (s) Symbols per s
In SI units
Named after Émile Baudot
Derived from second

Baud [bɔːd] , Bd is the unit for the symbol rate (step speed) in communications engineering and telecommunications technology . 1 baud is the speed if 1  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 transmission must be the same on the sending and receiving side.

If the transmitted symbol size corresponds to one bit , the symbol rate also corresponds to the value of the data transmission rate in bits per second. For larger symbols, the data transmission rate is the corresponding multiple of the symbol rate.

The unit is named after Jean-Maurice-Émile Baudot , who invented the Baudot code in 1874 .


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

Example: Gigabit Ethernet 1000BASE-T has a symbol rate of 125 Mbaud. With the multi-stage pulse amplitude modulation (PAM) in the form of 5-PAM , 2 useful data bits are transmitted per symbol, the information content is ld (5) ≈ 2.32 bits per symbol. In addition, four wire pairs are used at the same time. This achieves a data transfer rate of .

For n (combined) bits, a set of 2 n different symbols is required - i.e. H. There is a corresponding symbol for each bit combination. Due to the exponentially increasing symbol requirement, the number of bits to be transmitted per symbol is usually limited to approx. 10 bits / symbol as the upper limit. The limitation results on the receiving side from the difficulty of reliably distinguishing the individual symbols. In particular, noise as a disturbing influence plays an important role here.

Confusion with the bit rate

The baud rate is often confused with the data transfer rate, which indicates the amount of data transferred per period in bits per second as the bit rate . The baud rate, however, indicates the number of symbols per period. With a transmission duration of a symbol of z. B. 200 milliseconds the baud rate is 5 baud.

Binary modulation methods only know two states of the carrier, which corresponds to the transmission of exactly one bit per change of state or symbol, whereby bit and baud rates are the same as a special case in binary transmissions. Modulation methods with more than two states have a higher bit rate than baud rate. This is synonymous with a higher spectral efficiency . For example, the 4B3T code carries four bits per three symbols and the 2B1Q code carries two bits per symbol. The more bits are transmitted with a symbol (with the same baud rate, over the same physical channel), the more susceptible to interference the transmission.


Individual evidence

  1. ^ Fritz Tröster: Control and regulation technology for engineers: Volume 2: Control technology. Walter de Gruyter GmbH & Co KG, 2015, ISBN 978-3-110-42395-2 , p. 303. ( Definition: walking speed in the Google Book Search edition from 2015)