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websmith
- 11th December 2006, 07:24
Hi I often see folks on this forum using baud as though it's the same as bits per second. It's not. See below from wikipedia:

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 100/1000 Mbit/s Ethernet LANs, and so on, achieve high data rates. 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. 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.