13:15:00
Baud means "state changes
of the line per second"
The baud rate of a data
communications system is the number of symbols per second transferred. A symbol
may have more than two states, so it may represent more than one binary bit (a
binary bit always represents exactly two states). Therefore the baud rate may
not equal the bit rate, especially in the case of recent modems, which can have
(for example) up to nine bits per symbol.
For example, a Bell 212A modem uses
Phase Shift Keying (PSK) modulation, and each symbol has one of four phase
shifts (of 0(deg), 90(deg), 180(deg), or 270(deg)). Since it requires two bits
to represent four states (00, 01, 10, and 11), the modem transmits 1,200 bits/s
of information, using a symbol rate of 600 baud.
Usually the baud rate of a modem
will not equal the bit rate and is of no interest to the end user--only the
data rate, in bits per second, is.
Therefore in referring to the data
rate of a modem, use bits/s (or kbits/s, etc.), not baud rate.
Named after J. M. Emile Baudot
(1845-1903), who was a French telegraph operator, who worked out a five-level
code (five bits per character) for telegraphs? It was standardized as
International Telegraph Alphabet Number 2, and is commonly called Baudot (and
is a predecessor to ASCII). Since 2^5 is only 32 and the uppercase letters, numbers,
and a few punctuation characters add to more than that, Baudot uses Shift In
and Shift Out characters (analogous to how the Caps Lock key on a PC keyboard
reduces the number of keys needed by enabling each letter key to represent two
characters).
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