Bits Bauds and Symbols

There is much confusion among professionals in the telecommunication industry over terminology, especially in differentiating, bits, bauds, and symbols. The bit, a binary digit, has been defined previously.

The baud is a unit of transmission rate or modulation rate. It is a measure of transitions per second. A transition is a change of state. In binary systems, bauds and bits per second (bps) are synonymous. In higher-level systems, typically rn-ary systems, bits and bauds have different meanings. For example, we will be talking about a type of modulation called QPSK. In this case, every transition carries two bits. Thus the modulation rate in bauds is half the bit rate.

The industry often uses symbols per second and bauds interchangeably. It would be preferable, in our opinion, to use "symbols" for the output of a coder or other conditioning device. For the case of a channel coder (or encoder), bits go in and symbols come out. There are more symbols per second in the output than bits per second in the input. They differ by the coding rate. For example, a 1/2 rate coder (used in FEC) may have 4800 bps at the input and then would have 9600 symbols per second at the output. Period of a Bit, Symbol, or Baud. The period of a bit is the time duration of a bit pulse. When we use NRZ (nonreturn-to-zero) coding (discussed in Section 10.7.5), the period of a bit, baud, or symbol is simply 1/(bit rate), 1/(baud rate) or 1/(symbol rate). For example, if we were transmitting 9600 bits per second, what is the period of a bit? It is 1/9600 = 104.16 |isec. For 2400 baud = 1/2400 = 416.6 |sec; for 33.6 kbps = 1/33,600 = 0.0297 |sec, or 29.7 nsec.

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