23.1 A simple example
''baud'' and ''bps'' are perhaps one of the most misused terms in the computing and telecommunications field. Many people use these terms interchangeably, when in fact they are not! bps is simply the number of bits transmitted per second. The baud rate is a measure of how many times per second a signal changes (or could change). For a typical serial port a 1-bit is -12 volts and a 0-bit is +12 v (volts). If the bps is 38,400 a sequence of 010101... would also be 38,400 baud since the voltage shifts back and forth from positive to negative to positive, etc. and there are 38,400 shifts per second. For another sequence say 111000111... there will be fewer shifts of voltage since for three 1's in sequence the voltage just stays at -12 volts yet we say that its still 38,400 baud since there is a possibility that the number of changes per second will be that high.
Looked at another way, put an imaginary tic mark separating each bit (even though the voltage may not change). 38,400 baud then means 38,400 tic marks per second. The tic marks at at the instants of permitted change and are actually marked by a synchronized clock signal generated in the hardware but not sent over the external cable.
Suppose that a "change" may have more than the two possible outcomes of the previous example (of +- 12 v). Suppose it has 4 possible outcomes, each represented by a unique voltage level. Each level may represent a pair of bits (such as 01). For example, -12v could be 00, -6v 01, +6v 10 and +12v 11. Here the bit rate is double the baud rate. For example, 3000 changes per second will generate 2 bits for each change resulting in 6000 bits per second (bps). In other words 3000 baud results in 6000 bps.
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