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96 lines
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<TITLE> Modem-HOWTO: Appendix C: "baud" vs. "bps"</TITLE>
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<H2><A NAME="s23">23.</A> <A HREF="Modem-HOWTO.html#toc23">Appendix C: "baud" vs. "bps"</A></H2>
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<H2><A NAME="ss23.1">23.1</A> <A HREF="Modem-HOWTO.html#toc23.1">A simple example</A>
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</H2>
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<P> ``baud'' and ``bps'' are perhaps one of the most misused terms in
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the computing and telecommunications field. Many people use these
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terms interchangeably, when in fact they are not! bps is simply the
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number of bits transmitted per second. The baud rate is a measure of
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how many times per second a signal changes (or could change). For a
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typical serial port a 1-bit is -12 volts and a 0-bit is +12 v (volts).
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If the bps is 38,400 a sequence of 010101... would also be 38,400 baud
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since the voltage shifts back and forth from positive to negative to
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positive, etc. and there are 38,400 shifts per second. For another
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sequence say 111000111... there will be fewer shifts of voltage since
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for three 1's in sequence the voltage just stays at -12 volts yet we
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say that its still 38,400 baud since there is a possibility that the
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number of changes per second will be that high.</P>
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<P>Looked at another way, put an imaginary tic mark separating each bit
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(even though the voltage may not change). 38,400 baud then means
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38,400 tic marks per second. The tic marks at at the instants of
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permitted change and are actually marked by a synchronized clock
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signal generated in the hardware but not sent over the external cable.</P>
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<P>Suppose that a "change" may have more than the two possible outcomes
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of the previous example (of +- 12 v). Suppose it has 4 possible
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outcomes, each represented by a unique voltage level. Each level may
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represent a pair of bits (such as 01). For example, -12v could be 00,
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-6v 01, +6v 10 and +12v 11. Here the bit rate is double the baud rate.
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For example, 3000 changes per second will generate 2 bits for each
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change resulting in 6000 bits per second (bps). In other words 3000
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baud results in 6000 bps.</P>
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<H2><A NAME="ss23.2">23.2</A> <A HREF="Modem-HOWTO.html#toc23.2">Real examples</A>
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</H2>
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<P> The above example is overly simple. Real examples are more
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complicated but based on the same idea. This explains how a modem
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running at 2400 baud, can send 14400 bps (or higher). The modem
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achieves a bps rate greater than baud rate by encoding many bits in
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each signal change (or transition). Thus, when 2 or more bits are
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encoded per baud, the bps rate exceeds the baud rate. If your
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modem-to-modem connection is at 14400 bps, it's going to be sending 6
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bits per signal transition (or symbol) at 2400 baud. A speed of 28800
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bps is obtained by 3200 baud at 9 bits/baud. When people misuse the
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word baud, they may mean the modem speed (such as 33.6k).</P>
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<P>Common modem bps rates were formerly 50, 75, 110, 300, 1200,
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2400, 9600. These were also the bps rates over the
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serial_port-to-modem cables. Today the bps modem-to-modem (maximum)
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rates are 14.4k, 28.8k, 33.6k, and 56k, but the common rates over the
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serialPort-to-modem cables are not the same but are: 19.2k, 38.4k,
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57.6k, 115.2k, 230.4k. The high speed of 230.4k is (as of late 2000)
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unfortunately not provided by most new (and old) hardware. Using
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modems with V.42bis compression (max 4:1 compression), rates up to
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115.2k bps are possible for 33.6k modems. 203.2k (4 x 53.3k) is
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possible for 56k modems.</P>
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<P>Except for 56k modems, most modems run at 2400, 3000, or 3200
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baud. Even the 56k modems use these bauds for transmission and
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sometimes fall back to them for reception. Because of the bandwidth
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limitations on voice-grade phone lines, baud rates greater than 2400
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are harder to achieve, and only work under conditions of good phone
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line quality.</P>
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<P>How did this confusion between bps and baud start? Well, back when
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antique low speed modems were high speed modems, the bps rate actually
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did equal the baud rate. One bit would be encoded per phase change.
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People would use bps and baud interchangeably, because they were the
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same number. For example, a 300 bps modem also had a baud rate of
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300. This all changed when faster modems came around, and the bit rate
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exceeded the baud rate. ``baud'' is named after Emile Baudot, the
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inventor of the asynchronous telegraph printer. One way this problem
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gets resolved is to use the term "symbol rate" instead of "baud" and
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thus avoid using the term "baud". However when talking about the
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"speeds" between the modem and the serial port (DTE speed) baud and the
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symbol rate are the same. And even "speed" is a misnomer since we
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really mean flow rate.</P>
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