Baud

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For the town in France, see Baud, Morbihan.

In telecommunications and electronics, baud (pronounced /bɔːd/ unit symbol "Bd"), is a measure of the symbol rate; that is, the number of distinct symbol changes (signalling events) made to the transmission medium per second in a digitally modulated signal. The term baud rate is also commonly used to refer to the symbol rate. The baud is named after Emile Baudot, the inventor of the Baudot code for telegraphy. The unit was proposed at the International Telegraph Conference in 1927.

The baud rate (symbol rate) is distinct from the bit rate, because one symbol may carry more than one bit of information. For example, in modems, where bandwidth efficiency is important, it is commonly arranged for one symbol to carry 3 or more bits. So a 3000 bit per second modem, which is transmitting symbols that each carry 3 bits, should be described as operating at 1000 baud. Conversely, direct-sequence spread spectrum operation requires many symbols to carry only one bit.

Unfortunately, this distinction is not widely understood. Early modems operated only at one bit per symbol, and so baud rate and bit rate for those devices were equivalent. This has led many to believe the two terms to be synonymous, which they are not.

Conveying more than one bit per symbol has advantages. This reduces the time required to send a given quantity of data, and allows modern modems, FDDI and 100/1000 Mbit/s Ethernet LANs, and so on, to achieve high data rates. An optimal symbol set design must take into account channel bandwidth, desired information rate, noise characteristics of the channel and the receiver, and receiver and decoder complexity. A typical 2400 bit/s modem actually transmits at 600 baud (600 symbol/s), where each quadrature amplitude modulation symbol carries four bits of information. And further, 1000 Mbit/s Ethernet LAN cables use multiple wire pairs and multiple bits per symbol to encode their data payloads. Specifically, 1000BASE-T uses 4 wire pairs and 2 data bits per symbol to get a symbol rate of 125MBaud.

Conversely, representing one bit by many symbols has the advantage of overcoming noise, particularly jamming, hence is commonplace in military radio, despite the disadvantage of using more bandwidth to carry the same bit rate.

A clear example of the difference between baud (or signalling rate) and the data rate (or bit rate) is a man using a single semaphore flag. He can move his arm to a new position once each second, so his signalling rate (baud) is 1 symbol per second. However, the flag can be held in one of eight distinct positions: Straight up, 45 degrees left, 90 degrees left, 135 degrees left, straight down (which is the rest state, where he is sending no signal), 135 degrees right, 90 degrees right, and 45 degrees right. This means each signal carries three bits of information, as it takes 3 binary digits to encode 8 distinct states – so the data rate is 3 bits per second. In the Navy, more than one flag pattern and arm can be used at once, so the combinations of these produce many symbols, each conveying several bits, thus a higher data rate.