Baud

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 symbolic changes (signalling event) made to the transmission medium per second in a digitally modulated signal. It is named after Émile Baudot, the inventor of the Baudot code for telegraphy. As each symbol may stand for more than one bit of information, the amount of information sent per second is the product of the rate in baud and the number of bits of information represented by each symbol. When channel efficiency is important, as in modems, it is the engineer's job to maximize the information (bits) conveyed by each symbol sent. This means choosing an optimal symbol set that considers channel bandwidth, desired information rate, noise characteristics and the receiver's ability to disciminate among symbols.

For example: 250 Bd means that 250 symbols are transmitted per second. If 16 different symbols are used, each can represent 4 bits of information, so in each second, 1000 bits of information are transmitted, but with having to send only 250 actual symbols. Claude Shannon proved that an optimal encoding (bits per symbol) exists for any channel. This Shannon Limit has been approached by modem developers and theorists over the years, but never fully realized, because Shannon only proved its existence, not its actual design.

Note: Baud should not be confused with data rate in "bits per second" (or bytes per second, etc.). Each symbol transmitted can carry one or more bits (for example, 8 bits in 256-QAM modulation) of information. When each symbol is binary it carries just one bit, so baud and bit rate are equal. This is a cheap, simple encoding. However, it's common to make better use of channel bandwidth by encoding multiple bits per symbol. This reduces the time required to send a given quantity of data, and it's exactly how good modems, FDDI and 100/1000Mb/s Ethernet LANs, and so on, achieve high data rates. Thus, a 2400 bit/s modem actually transmits at 600 baud (600 symbols/sec), where each quadrature amplitude modulation symbol carries four bits of information. And further, 100Mb/s Ethernet LAN cables use multiple wire pairs and multiple bits per symbol to encode their data payloads.

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, many orthogonal symbols, each conveying many bits, thus a high data rate.

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Baud

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