Baud Rate (...don’t miss the Rate!)

          The measure was named after a French engineer, Jean-Maurice-Emile Baudot, the inventor of the asynchronous telegraph printer.  It was first used to measure the speed of telegraph transmissions, and is represented in accordance with the rules for SI units
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          Baud unit symbol "Bd is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud or modulation rate; the number of distinct symbol changes (signaling events) made to the transmission medium per second in a digitally modulated signal or a line code.

          Baud is related to but should not be confused with gross bit rate expressed in bit/s. A baud is a unit of measure for analog signaling. At a minimum, one baud corresponds to one bit per second (bps) although at higher signaling speeds, multiple bits can be transferred in a single baud.

          Baud was the prevalent measure for data transmission speed until replaced by a more accurate term, bps (bits per second). One baud is one electronic state change per second. Since a single state change can involve more than a single bit of data, the term characters per second (CPS) is closer than the term bps (bits per second).

          The confusion between bps and baud started back when antique low speed modems were high speed modems, the bps rate actually did equal the baud rate. One bit would be encoded per phase change. People would use bps and baud interchangeably, because they were the same number.

           For example, a 300 bps modem also had a baud rate of 300. This all changed when faster modems came around, and the bit rate exceeded the baud rate.

          One way this problem gets resolved is to use the term "symbol rate" instead of "baud" and thus avoid using the term "baud". However, when talking about the "speeds" between the modem and the serial port (DTE speed) baud and the symbol rate are the same. And even "speed" is a misnomer since we really mean flow rate.

          Currently though bps unit of measurement has replaced it as a better expression of data transmission speeds.

          A simple example to clear any ambiguity:

           A baud of 1 kBd = 1,000 Bd is synonymous to a symbol rate of 1,000 symbols per second. In case of a modem, this corresponds to 1,000 tones per second, and in case of a line code, this corresponds to 1,000 pulses per second. The symbol duration time is 1/1,000 second = 1 millisecond.

          In digital systems (i.e., using discrete/discontinuous values) with binary code, 1 Bd = 1 bit/s. By contrast, non-digital (or analog) systems use a continuous range of values to represent information and in these systems the exact informational size of 1 Bd varies.

           In simple digital communication links use only one bit per symbol, such that binary "0" is represented by one symbol and binary “1 by another symbol. However, in advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one bit (a binary bit always represents one of exactly two states).

          At slow speeds, only one bit of information (signaling element) is encoded in each electrical change. The baud, therefore, indicates the number of bits per second that are transmitted. For example, 300 baud means that 300 bits are transmitted each second (abbreviated 300 bps).

          Assuming asynchronous communication, which requires 10 bits per character, this translates to 30 characters per second (cps). For slow rates (below 1,200 baud), you can divide the baud by 10 to see how many characters per second are sent.

At higher speeds, it is possible to encode more than one bit in each electrical change. 4,800 baud may allow 9,600 bits to be sent each second. At high data transfer speeds, therefore, data transmission rates are usually expressed in bits per second (bps) rather than baud. For example, a 9,600 bps modem may operate at only 2,400 baud.

          With relevance to data serial cables used in Hydrographic surveys, the baud rate is significant to compensate for large data strings, data requiring high updates or high resolution.

          Note the change of bite frame length in relation with time at a baud rate 4800 is 10 bits in 2.083 ms and at 19200 it is 0.560 ms. To me this signifies two things; high baud rate allows for a bigger data string to be processed & for a shorter data string it allows  higher update (…better resolution).

If you didn’t get it still… read on!

          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 it’s 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 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.

          The above mentioned example is overly simple. Real examples are somewhat complicated but based on the same idea. This explains how a modem running at 2400 baud, can send 14400 bps (or higher). The modem achieves a bps rate greater than baud rate by encoding many bits in each signal change (or transition). Thus, when 2 or more bits are encoded per baud, the bps rate exceeds the baud rate.

          I leave it to your discretion while I conclude my discussion. Hope it was of some aid.

Cheers!!!