baud

A unit used to measure the capacity of a communications channel to carry digital data; named after the French inventor, J M E Baudot. The baud rate of a communications channel is the number of signal changes per second with which the channel can cope. Using an analog link and current modem technology, up to 16 bits can be coded into each signal change; hence a 2400 baud channel can (but rarely does) carry 38 Kbps (kilobauds per second) of digital information. Even with digital transmission, the baud rate is not the same as the number of bits per second that can be transmitted. Manchester encoding requires two signal changes for each bit of data, so the maximum bit rate is only half the baud rate.

(Discuss)

(Merge proposal made 10 Nov 2006)

In telecommunications and electronics, baud (pronounced /bɔːd/, unit symbol "Bd") is a measure of the symbol rate;

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 discriminate among symbols. 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.

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. However, it's common to make better use of channel bandwidth by encoding multiple bits per symbol. Thus, a 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.

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. 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.